How to multiply currents: Inside a counterfeit analog multiplier

A recent Twitter thread about a counterfeit analog multiplier chip attracted my attention since I'm interested in both counterfeit integrated circuits and how analog computers multiply. In the thread, John McMaster decapped a suspicious AD633 analog multiplier chip and found an entirely different Rockwell RC4200 die inside. Why would someone do this? Probably because the RC4200 (1978) currently sells for about 85 cents, while the more modern laser-trimmed1 AD633 (1989) sells for about $7.2

Die of the RC4200 analog multiplier with functional blocks labeled. Die photo courtesy of John McMaster.

Analog multiplication

Analog multiplication has many uses such as mixers, modulators, and phase detectors, but analog computers are how I encountered analog multiplication. A typical analog computer uses voltages to represent values and is wired up through a plugboard to solve a particular equation. Adding or subtracting two values is easy with an op amp, as is multiplying by a constant. Integration seems like it would be difficult, but it's almost trivial with a capacitor; analog computers excelled at solving differential equations.

Multiplying two values, however, was surprisingly difficult; multiplication techniques were slow, inaccurate, noisy, or expensive. One accurate but slow multiplier used the Rube-Goldberg configuration of servo motors turning potentiometers.3 A 1969 multiplier circuit uses a light bulb and photocells. A fast and accurate approach was the "parabolic multiplier", built from numerous expensive high-precision resistors.4 The approach I'll discuss is to multiply by adding the logarithms and taking the exponential. Inconveniently, this approach magnifies even small differences between the transistors. It is also very sensitive to temperature. As a result, this approach was simple but inaccurate.

The Model 240 analog computer from Simulators, Inc. includes analog multipliers using the parabolic multiplier approach.

However, the development of analog integrated circuits created new opportunities for analog multiplication circuits. In particular, since the transistors in an integrated circuit were created together, they have nearly-identical properties. And the components on a tiny silicon die are all at nearly the same temperature.5

The first analog multiplier integrated circuit I could find is a television demodulator from 1967. The Gilbert cell technique was introduced by Barrie Gilbert in 1968 and is used in most analog multipliers today.6 The AD530 was introduced around 1970, and became an industry standard, but required external adjustments for accuracy. Laser-trimming the resistors inside the integrated circuit during manufacturing greatly improved the accuracy, an approach used in the AD633, the integrated circuit that was counterfeited.

Before explaining the circuitry of the RC4200 (the multiplier inside the counterfeit chip), I'll discuss the components that it is constructed from, and how they appear in an integrated circuit. This will help you recognize these structures in the die photo.

Transistors

Transistors are the key components in a chip. The photo below shows an NPN transistor in the RC4200 as it appears on the chip. The different blue colors are regions of silicon that have been doped differently, forming N and P regions. The white lines are the metal layer of the chip on top of the silicon—these form the wires connecting to the emitter (E), base (B), and collector (C).

An NPN transistor on the RC4200 die. The emitter is embedded in the base, with the collector underneath.

You might expect PNP transistors to be similar to NPN transistors, just swapping the roles of N and P silicon. But for a variety of reasons, PNP transistors have an entirely different construction. They consist of a circular emitter (P), surrounded by a ring-shaped base (N), which is surrounded by the collector (P). This forms a P-N-P sandwich horizontally (laterally), unlike the vertical structure of the NPN transistors. The diagram below shows one of the PNP transistors in the RC4200.

A PNP transistor has a circular structure.

The input and output transistors in the RC4200 are larger than the other transistors and have a different structure to support higher currents. The photo below shows one of the output transistors. Note the multiple interdigitated "fingers" of the emitter and base.

A larger output transistor with parallel emitters and bases.

Capacitors

Capacitors are important in op amps to provide stability. A capacitor can be built in an integrated circuit as a large metal plate separated from the silicon by an insulating oxide layer. The main drawback of capacitors on ICs is they are physically very large. The 15pF capacitors in the RC4200 have a very small capacitance but take up a large fraction of the die area. In the photo below, the red arrows indicate the connection to the capacitor's metal layer and to the capacitor's underlying silicon layer.

The large metal area on the upper left is a capacitor.

Resistors

Resistors are a key component of analog chips. Unfortunately, resistors in ICs are very inaccurate; the resistances can vary by 50% from chip to chip. The photo below shows four resistors, formed using different techniques. The first resistor is the zig-zagging blue region on the left. It is formed from a strip of P silicon, with metal wiring (white) attached on the left and right. Its resistance is 3320 Ω. The resistor in the upper right is much shorter, so it is only 511Ω (long, narrow resistors have higher resistance than short, wide resistors). The remaining resistors are 20KΩ despite their small size because they are "pinch resistors". In the pinch resistor, the square layer of brownish N silicon on top makes the conductive region much thinner (i.e. pinches it). This allows a much higher resistance for a given size. (Otherwise, a 20 KΩ resistor would be 6 times as long as the first resistor, taking up excessive space.) The tradeoff is the pinch resistor is much less accurate.

Four resistors, one on the left and three on the right.

Multiplying with logs and exponentials

This integrated circuit multiplies using the log-antilog technique. The idea is that if you take the log of two numbers, add the logs together, and then take the antilog (i.e. exponential), you get the product of the two numbers. Conveniently, transistors have a logarithmic / exponential characteristic: the current through the transistor is an exponential of the voltage on the base. Specifically, if V_BE is the voltage between the transistor's base and emitter, the current through the collector (I_C) is an exponential of that voltage, as shown in the graph below. The analog multiplier takes advantage of this property.

Ic vs Vbe curve for a transistor, showing the exponential relationship. Generated by LTspice.

The main complication with this approach is that the curve above is very sensitive to the temperature and to the manufacturing characteristics of the transistor. Because the curve is exponential, even a small shift in the curve will radically change the current. This was a serious difficulty when building a multiplier from discrete transistors, since the properties varied from transistor to transistor. To stabilize the temperature, some multipliers used a temperature-controlled oven. However, using an integrated circuit mostly solved these problems. The transistors in an integrated circuit are well-matched since they were built from the same piece of silicon under the same conditions. And the transistors in an integrated circuit die will be at almost the same temperature. Thus, integrated circuits made transistor-log circuits much more practical.

The diagram below shows the structure of the RC4200 multiplier chip. The user provides three current inputs (I1, I2, and I4) and the chip computes the output current I3, where I3 = I1×I2÷I4. (The use of current inputs and outputs is a bit inconvenient compared to other multipliers, such as the AD633, that use voltages.)

Structure of the RC4200 multiplier, from the datasheet. Note that the supply voltage (pin 3) is negative. V_OS1 and V_OS2 are offset adjustment pins to improve accuracy.

The four transistors in the middle of the diagram are the multiplier core, the key to the IC's operation. The transistors are configured so their base-emitter voltages sum: V_BE3 = V_BE1+V_BE2-V_BE4. Because the transistor current is related exponentially to the voltage, the result is that I₃ = I₁×I₂÷I₄.

In more detail, first note that the voltages V_BE1 through V_BE4 control the collector currents I_C1 through I_C4 through the transistors (below). The op amps adjust the base-emitter voltages so the input currents match the transistor currents, i.e. I₁ = I_C1 and so forth. (This is accomplished by op amp feedback.) Now, if you go through the loop of base-emitter voltages starting at the base of Q1 and ending at the base of Q4 (red arrows), you find that V_BE1+V_BE2-V_BE3-V_BE4 = 0. (The voltages must sum to zero since you start at ground and end at ground.7) Now, because I_C is related to exp(V_BE), taking the exponential of the equation yields I_C1×I_C2÷I_C3÷I_C4 = 1. (Details in footnote8.)

Traveling around the loop indicated by the arrows, the voltages must sum to 0.

Next, I'll explain how the V_BE voltages are generated. Each current input has an op amp associated with it that produces the "correct" V_BE voltage for the current using a feedback loop9 For example, suppose I_C is too low so not all the input current flows through the transistor. The excess current will raise the voltage on the op amp's negative input, causing it to reduce its output voltage and thus the transistor's emitter voltage. This raises V_BE (since the base will now be higher compared to the emitter), causing more collector current to flow through the transistor. Similarly, if too much current is flowing through the transistor, the op amp's input will be pulled lower, reducing V_BE. Thus, the feedback loop causes the op amp to find the exact V_BE for the current input.10

Correcting for emitter resistance

The above circuit works reasonably well, but there's a complication: the transistors have a small emitter resistance R. The voltage drop across this resistance will increase V_BE by I_CR, disturbing the nice exponential behavior. This creates a nonlinearity that reduces the accuracy of the result. The datasheet says that "Raytheon has developed a unique and proprietary means of inherently compensating for this undesired term." They don't explain this further, but by studying the die I have figured out how it works.

In the compensation circuit, each of the four multiplier transistors is paired with an identical "mirror" transistor with the corresponding emitters and corresponding bases connected, as shown below. These connections give the paired transistors the same base and emitter voltages, so they have the same collector currents. In other words, they form a current mirror. The mirrored currents are fed into special correction resistors that match the undesired emitter resistance, 0.1 Ω according to the datasheet.11 The voltage across the correction resistors will be the same as the excess voltage that needs to be compensated (since the resistance and current are the same). The final step is the correction resistors are connected to the base of the multiplication transistors, replacing the connection to ground. This will shrink V_BE by the amount it was erroneously increased, fixing the computation.

The main multiplier consists of four transistors. Each transistor has a mirror transistor generating the same current, used to correct for emitter resistance.

Why are there two correction resistors? Recall that the multiplier has two transistors adding and two transistors subtracting (i.e. V_BE1+V_BE2-V_BE3-V_BE4 = 0). To handle this, the correction circuit is split in two. The left half sums I_C1 and I_C2 and applies this current to a correction resistor on the Q3/Q4 side, while the right half sums I_C3 and I_C4 and applies this to a correction resistor on the Q1/Q2 side. The addition and subtraction work out to yield the desired net correction.

Schematic

The schematic below shows the complete circuitry of the RC4200; I've highlighted the main functional blocks. (Inconveniently, I didn't find this schematic until after I'd traced out the circuitry from the die photo.) The multiplier core and the correction resistors were discussed above The op amps circuits are fairly similar to the 741 op amp, which I've written about. They lack the output stage of typical op amps; the output transistor (Q112/Q212/Q412) corresponds to the intermediate gain state in a typical op amp. The bias circuit (orange, lower right) provides a fixed bias voltage for the op amps.12

Schematic from the datasheet, with main functional groups labeled.

Conclusion

Before integrated circuits, analog multiplication was difficult to implement. However, integrated circuits made it easy to create matched transistors, leading to fast, inexpensive analog multiplication integrated circuits. Unfortunately, analog multiplier integrated circuits were introduced just as analog computers were dying out, killed by inexpensive digital microprocessors, so analog computing missed most of the benefit of these chips.

While most analog multipliers use a circuit called the Gilbert cell, the Raytheon RC4200 analog multiplier uses a different technique to multiply and divide values represented by currents. Although, it includes a special error compensation circuit to improve its accuracy, it is obsolete compared to accurate, laser-trimmed multipliers. Now, counterfeiters re-label RC4200 chips and sell them as the more-expensive AD633 multiplier.

Die photo of the RC4200, courtesy of John McMaster.

I announce my latest blog posts on Twitter, so follow me at kenshirriff for updates. I also have an RSS feed. Thank you to John McMaster for the die photos used in this blog post; the photos are here.

Notes and references

1. One reason that the AD633 multiplier is so expensive is that the resistors on the die are laser-trimmed resistors for accuracy. To get an accurate result, an analog multiplier requires exactly-tuned resistances. The older RC4200 requires adjustable external resistors, which is much less convenient. ↩

2. I'm a bit puzzled by this counterfeit chip. Sometimes people will label a cheap op amp as an expensive op amp, as explained by Zeptobars. At first glance, that's what's going on here: a cheap multiplier repackaged as an expensive one. However, the two multipilers are so different that I can't imagine one working at all in place of the other. Specifically, the AD633 takes differential voltage inputs and outputs two currents (a differential current), and it computes A×B+C. The RC4200, on the other hand, takes current inputs and outputs a single current, and it computes A×B÷C. ↩

3. An example of a servo multiplier is the Solartron Servo Multiplier from the late 1950s. This 17-pound unit contained a potentiometer controlled by a servo motor, allowing it to multiply numbers represented by +/- 100 volts. It's surprisingly fast considering its mechanical operation, responding in under 30 milliseconds. Power consumption was high: 70 watts, cooled by a fan. (In comparison, the RC4200 chip uses 40 milliwatts of power.)

   ↩

   This photo shows the Solartron TJ961 Servo Resolver. This implements multiplication as well as sine/cosine computation. Photo from manual via Analog Museum.

4. The 1969 analog computer I'm restoring uses a parabolic multiplier, a technique used for high-accuracy multiplication. The idea is that to compute A×B, you compute ((A+B)^2 - (A-B)^2)/4, which has the same value. That equation looks much more complex than the original product, but is easier to implement on an analog computer because op amps can perform the sums, subtraction, and division by four. Squaring is easier than multiplication because it is a function of a single variable, so it can be implemented by an "arbitrary function generator".

   

   Parabolic multiplier circuit board from a Simulators, Inc. 2400 analog computer.

   The photo above shows a function board from an analog computer that computes the square, i.e. a parabola. The board approximates the function by multiple piecewise-linear segments, each defined by resistors. (Note the extremely accurate 0.01% resistors on the left.) The metal block in the center holds diodes, temperature-balanced by the metal. Each diode is biased to turn on at a particular voltage; the diodes act as switches, selecting the appropriate resistors for each linear segment. Note the large amount of precision hardware required for multiplication; a single product requires two of these parabolic function boards as well as multiple op amps. ↩

5. To minimize the effect of temperature on the integrated circuit, the critical multiplier transistors are placed close together in the center of the chip. If there is a thermal gradient across the chip, this will minimize the temperature difference between the transistors. (Compared to putting the transistors in the corners, for instance.) To reduce temperature gradients even more, the datasheet specifies a "thermal symmetry line". Putting a temperature source on this line ensures that the hotter transistors will tend to cancel each other out.

   ↩

   The datasheet shows the IC's thermal symmetry line.

6. Barrie Gilbert, inventor of the Gilbert cell, has a video explaining translinear circuit, circuits based on the exponential current-voltage relationship of a bipolar transistor. This video explains translinear analog multipliers in detail, discussing two approaches> The first approach, used by the RC4200, is the "log-antilog" approach, where op-amps force and sense the collector currents. The second, used in the AD633 and many other multipliers, is the "integrated" approach, built from voltage-to-current conversion, a differential current-mode core, and current-to-voltage conversion. ↩

7. I should mention that the chip uses a -15 V supply, so ground is the highest voltage and the other internal voltages are all negative. Just a warning since this makes things confusing and backward compared to circuits where ground is the low voltage. ↩

8. The relationship between the base voltage and the collector current is given by the Ebers-Moll model. This equation (below) is filled with interesting constants: α: a gain factor (almost 1), k: the Boltzmann constant, I_S: the saturation current (extremely small, order of 10^-15 A), T: the absolute temperature, q: the charge on the electron. (The temperature in the exponential term reflects the importance of temperature stability for the multiplier.)

   

   Substituting the thermal voltage V_T (about 26 mV) for kT/q, making some minor approximations, and taking the log yields:

   

   Substituting that into the multiplier's V_BE loop equation yields

   

   Taking the exponential and assuming the transistors all have the same temperature and saturation current yields the desired equation relating the four currents:

   

   This equation shows how the four currents are related by multiplication and division. See the datasheet for more details. ↩

9. In a sense, the op amps compute the inverse of the transistor's exponential function. The transistor takes V_BE as an input and produces the exponential current as an output. However, we have the current as the input and want the logarithmic voltage as the output. By using the op amp with a function in its feedback loop, we can find the inverse of a function, in this case giving us the logarithm. That is, the op amp will converge on the output X where f(X) equals the input, i.e. X = f^{-1</sup(input). The same technique can be used to generate a square root from a multiplier chip: use the multiplier to square its input, and then use an op amp to compute the inverse function, i.e. the square root. ↩}

10. You might wonder why the op amp finds the "correct" value and doesn't overshoot and oscillate. Handwaving away all the theory, the idea is that the capacitor on the op amp input stabilizes it and prevents oscillation. Even so, the datasheet warns that the circuits become unstable as the input currents approach 0. This corresponds to dividing by zero, so it's not surprising that the circuitry doesn't handle it well. Mathematically, the op amp is trying to find ln(0), which isn't going to work. If you want to multiply by zero or negative values, the datasheet describes how the inputs can be biased with resistors to keep the inputs positive but still get the correct answer. ↩

11. The two resistors below are used for the emitter correction; they have unusual construction and a very small resistance, 0.1 Ω. Each resistor consists of the two vertical stripes, connected together at the bottom; the vertical region in the center is connected to the ground pin, forming the other side of each resistor. These resistors improve the accuracy of the product by correcting for the emitter resistances. Based on their purple color, which doesn't appear elsewhere on the die, they appear to be specially doped. The metal contacts at the bottom cover part of the resistor; I believe that by adjusting the size of the metal contacts, the resistor values can be tuned. I believe that the thick and thin regions allow for coarse and fine tuning.

    

    Precise small-valued resistors provide a correction factor.

     ↩

12. The bias voltage circuit generates a stable voltage of one diode drop (about 800 mV) from Q4's collector; this voltage biases the op amps. The tricky part is how to keep the power supply voltage from influencing this voltage or the Zener voltage.

    

    The bias generation circuit, from the datasheet.

    The idea is that the Zener diode puts 5.5 volts on the base of Q13. The voltage across R3 will be two diode drops lower (2.8 V) due to Q13 and Q12. This yields a fixed current of 2.8 V / 1430 Ω = 2 mA through Q4, resulting in a stable voltage drop across Q12 and a stable output. But a Zener's voltage fluctuates a bit with current, so the clever part is how the Zener's current is kept stable. Transistors Q14, Q15, and Q16 form a current mirror, so the current through the Zener will match the current through the resistor, which is 2 mA. Thus, the Zener voltage keeps the resistor current and output voltage stable, while the resistor current keeps the Zener stable. The final piece of the puzzle is the FET Q17, which provides a tiny current through the Zener to start the feedback cycle. ↩

Understanding and repairing the power supply from a 1969 analog computer

We recently started restoring a vintage1 analog computer. Unlike a digital computer that represents numbers with discrete binary values, an analog computer performs computations using physical, continuously changeable values such as voltages. Since the accuracy of the results depends on the accuracy of these voltages, a precision power supply is critical in an analog computer. This blog post discusses how this computer's power supply works, and how we fixed a problem with it. This is the second post in the series; the first post discussed the precision op amps in the computer.

The Model 240 analog computer from Simulators Inc. was a "precision general purpose analog computer" for the desk top, with up to 24 op amps. (This one has 20 op amps.)

Analog computers used to be popular for fast scientific computation, especially differential equations, but pretty much died out in the 1970s as digital computers became more powerful. They were typically programmed by plugging cables into a patch panel, yielding a spaghetti-like tangle of wires. In the photo above, the colorful patch panel is in the middle. Above the patch panel, 18 potentiometers set voltage levels to input different parameters. A smaller patch panel for the digital logic is in the upper right.

The power supply

The computer uses two reference voltages: +10 V and -10 V, which the power supply must generate with high accuracy. (Older, tube-based analog computers typically used +/- 100 V references.) The power supply also provides regulated +/- 15 V to power the op amps, power for the various relays in the computer, and power for the lamps.

The power supply in the bottom section of the analog computer. The transformer/rectifier section is on the left and the regulator card cage is on the right. Wiring harnesses on top of the power supply connect it to the rest of the computer.

The photo above shows the power supply in the lower back section of the analog computer. The power supply is more complex than I expected. The section on the left converts line-voltage AC into low-voltage AC and DC. These outputs go to the card cage on the right, which has 8 circuit boards that regulate the voltages. The complex wiring harnesses on top of the power supply provide power to the five analog computation modules above the power supply as well as the rest of the computer.

With a vintage computer, it's important to make sure the power supply is working properly, since if it is generating the wrong voltages, the results could be catastrophic. So we proceed methodically, first checking the components in the power supply, then testing the power supply outputs while disconnected from the rest of the computer, and finally powering up the whole computer.

The transformer / rectifier section

We started by removing the power supply from the computer, and disconnecting the two halves. The left half of the power supply (below) produces four unregulated DC outputs and a low-voltage AC output. In contains two large power transformers, four large filter capacitors, stud rectifiers (upper back), smaller diodes (front right), and fuses. This is a large and very heavy module because of the transformers.2 The smaller transformer powers the lamps and relays, while the larger transformer powers the +15 and -15 volt supplies as well as the oscillator. Presumably, using separate transformers prevents noise and fluctuations from the lamps and relays from affecting the precision reference supplies.

This section of the power supply reduces the line-voltage AC to low-voltage DC and AC.

One concern with old power supplies is that the electrolytic capacitors can dry out and fail over time. (These capacitors are the large cylinders above.) We measured the capacitance and resistance of the large capacitors (using Marc's vintage HP LCR meter) and they tested okay. We also checked the input resistance of the power supply to make sure there weren't any obvious shorts; everything seemed fine.

We removed all the cards from the card cage, cautiously plugged in the power supply, and... nothing at all happened. For some reason, no AC voltage was getting to the power supply. The fuse was an obvious suspect, but it was fine. Carl asked about the power switch on the control panel, and we figured out that the switch was connected to the power supply via the socket labeled "CP" (below). We added a jumper, powered up the supply, and this time found the expected DC voltages from the module.

The side of the power supply has three twist-lock AC sockets labeled "FAN", "DVM-LOGIC", and "CP" (control panel). The "DVM-LOGIC" socket powers a 5-volt supply for the digital logic, which we still need to repair.

The regulator cards

Next, we tested the power supply's various cards individually. The power supply has four regulator cards generating "lamp voltage", "+15", "-15", and "relay voltage". The purpose of a regulator card is to take an unregulated DC voltage from the transformer module and reduce it to the desired output voltage.

We hooked up the regulator cards using a bench power supply as input to make sure they were working properly. We tweaked the potentiometer on the +15 V regulator to get exactly 15 V output. The -15 V regulator seemed temperamental and the voltage jumped around when we adjusted it. I suspected a dirty potentiometer, but it settled down to a stable output (narrator: this is foreshadowing). We don't know what the lamp and relay voltages are supposed to be, and they're not critical, so we left those boards unadjusted.

One of the voltage regulator cards. A large power transistor is attached to the heat sink.

The photo above shows one of the regulator cards; you might think it has a lot of components just to regulate a voltage. The first voltage regulator chip was created in 1966, so this computer uses a linear regulator built from individual components instead. The large metal transistor on the heat sink is the heart of the voltage regulator; it acts kind of like a variable resistor to control the output. The rest of the components provide the control signal to this transistor to produce the desired output. A Zener diode (yellow and green stripes on the right) acts as the voltage reference, and the output is compared to this reference. A smaller transistor generates the control signal for the power transistors. In the lower right, a multi-turn potentiometer is used to adjust the voltage output. The larger capacitors (metal cylinders) filter the voltage, while the smaller capacitors ensure stability. Most power supply of just a few years later would replace all of these components (except the filter capacitors) with a voltage regulator IC.

The chopper oscillator

The precision op amps in the analog computer use a chopper circuit for better DC performance, and the chopper requires 400 Hertz pulses. These pulses are generated by the oscillator board in the power supply (called the gate for some reason). We powered up the board separately to test it, and found it produced 370 Hz, which seemed close enough.

The gate card provides 400 Hertz oscillations to control the op amp choppers.

The circuitry of this card is somewhat bizarre, and not what I was expecting on an oscillator card. The left side has three large capacitors and three diodes, powered by low-voltage AC from the transformer. After puzzling over this for a bit, I determined it was a full-wave voltage doubler, producing DC at twice the voltage of the AC input. I assume that the chopper pulses needed to be higher voltage than the computer's +15 volt supply, so they used this voltage doubler to get enough voltage swing.

The oscillator itself (right side of the card), uses one NPN transistor as an oscillator, and another NPN transistor as a buffer. It took me a while to figure out how a single-transistor oscillator works. It turns out to be a phase-shift oscillator; the three white capacitors in the middle of the board shift the signal 180°; inverting it causes oscillation.

The op amps

Calculations in the analog computer are referenced to +10 volt and -10 volt reference voltages, so these voltages need to be very accurate. The regulator cards produce fairly stable voltages, but not good enough. (While testing the regulator cards, I noticed that the output voltage shifted noticeably as I changed the input voltage.) To achieve this accuracy, the reference voltages are generated by op amp circuits, built from two op amp boards and a feedback network card.

An op amp card. This card has a single input on the right. It uses a round metal-can op amp IC, but the chopper circuitry improves performance.

Somewhat surprisingly, the op amp cards used in the power supply are exactly the same as the precision op amps used in the analog computer itself. Back in 1969, op amp integrated circuits weren't accurate enough for the analog computer, so the designers of this analog computer combined an op amp chip with a chopper circuit and many other parts to create a high-performance op ap card. I described the op amp cards in detail in the first post, so I won't go into more detail here.

The network card

The network card has two jobs. First, it has precision resistors to create the feedback networks for the power supply op amps. Second, it has two power transistors (circular metal components below) that buffer the reference voltages from the op amp for use by the rest of the computer.

The network card. The two connectors on the left are attached to the op amp inputs.

One of the problems with an analog computer is that the results are only as accurate as the components. In other words, if the 10 volt reference is off by 1%, your answers will be off by 1%. The result is that analog computers need expensive, high-precision resistors. (In contrast, the voltages in a digital computer can drift a lot, as long as a 0 and a 1 can be distinguished. This is one reason why digital computers replaced analog computers.) Typical resistors have a tolerance of 20%, which means the resistance can be up to 20% different from the indicated value. More expensive resistors have tolerance of 10%, 5%, or even 1%. But the resistors on this board have tolerance of 0.01%! (These resistors are the pink cylinders.) The two large resistors on the left are 15Ω "Brown Devil" power resistors. They protect the voltage outputs in case someone plugs the wrong wire into the patch panel and shorts an output, which would be easy to do.

The network card receives an adjustment voltage from the control panel, and also has multi-turn potentiometers on the right for adjustment (like the regulator cards). The green connectors are used to connect the network card to the op amp cards. (The op amps have a separate connector for the input, to reduce electrical noise.)

Powering it up and fixing a problem

Finally, we put all the power supply boards back in the cabinet, put the power supply back in the computer, and powered up the chassis (but not the analog computer modules). Some of the indicator lights on the control panel lit up and the +15 V supply showed up on the meter. However, the -15 V supply wasn't giving any voltage, and the op amp overload lights were illuminated on the front panel, and the reference voltages from the op amps weren't there. The bad -15 V supply looked like the first thing to investigate, since without it, the op amp boards wouldn't work.

I removed the working +15 regulator and failing -15 regulator from the card cage and tested them on the bench. Conveniently, both boards are identical, so I could easily compare signals on the two boards. (Modern circuits typically use special regulators for negative voltage outputs, but this power supply used the same regulator for both.) The output transistor on the bad board wasn't getting any control signal on its base, so it wasn't producing any output. Tracing the signals back, I found the transistor generating this signal wasn't getting any voltage. This transistor was powered directly from the connector, so why wasn't any voltage getting to the transistor?

A regulator board was failing due to loose screws (red arrows). The circuit was powered through the thick bottom PCB trace and then current passed through the heat sink from the lower screw to the upper screw.

I studied the printed circuit board and noticed that there wasn't a PCB trace between the transistor and the connector! Instead, part of the current path was through the heat sink. The heat sink was screwed down to the PCB, making a connection between the two red arrows above. After I tightened all the screws, the board worked fine.

The analog computer with the plugboard and sides removed to show the internal circuitry. The power supply is in the lower back section. One module has been removed and placed in front of the computer.

We put the boards back in, powered up the chassis, and this time the voltages all seemed to be correct. The op amp overload warning lights remained off; the warning light went on before because the op amps couldn't operate with one voltage missing. The next step is to power up the analog circuitry modules and test them. We also need to repair the separate 5-volt power supply used by the digital logic since we found some bad capacitors that will need to be replaced. So those are tasks for the next sessions.

Follow me on Twitter @kenshirriff to stay informed of future articles. I also have an RSS feed.

Notes and references

1. The computer's integrated circuits have 1968 and 1969 date codes on them, so I think the computer was manufactured in 1969. ↩

2. Most modern power supplies are switching power supplies, so they are much smaller and lighter than linear power supplies like the one in the analog computer. (Your laptop charger, for instance, is a switching power supply.) Back in this era, switching power supplies were fairly exotic. However, linear power supplies are still sometimes used since they have less noise than switching power supplies. ↩

Op amp on the Moon: Reverse-engineering a hybrid op amp module

I recently obtained a mysterious electronic component in a metal can, flatter and slightly larger than a typical integrated circuit.1 After opening it up and reverse engineering the circuit, I determined that this was an op amp built for NASA in the 1960s using hybrid technology. It turns out that the development of this component ties connected several important people in the history of semiconductors, and one of these op amps is on the Moon.

The module was packaged inside a TO-8 metal can, which is wider and flatter than a typical metal can IC. It is just a bit narrower than a dime.

To determine what this component did and how it worked, I sawed the top off the metal can with a jeweler's saw, revealing the circuitry inside. There wasn't an integrated circuit inside but a larger hybrid module, built from tiny individual transistors on a ceramic substrate. In the photo below, the ceramic wafer has grayish conductive traces printed on it, similar to a printed circuit board. Individual silicon transistors (the smaller shiny squares) are attached to the traces on the ceramic. Thin gold wires connect the components together, and connect the circuit to the external pins.

Sawing off the top of the metal can reveals the hybrid circuitry inside. For scale, the package is slightly smaller than a dime.

Hybrid circuitry was widely used in the 1960s before complex circuits could be put on an integrated circuit. (The popular IBM System/360 computers (1964), for instance, were built from hybrid modules rather than ICs.) Although integrated circuit op amps were first produced in 1963, hybrids could avoid limitations of IC manufacturing and produce better performance, so hybrids remained popular in the 1970s and even 1980s.

At first, I couldn't identify this part, so I asked op amp expert Walt Jung for help. He identified the "a" on the package for Amelco, which helped me track down the rather obscure 2404BG op amp manufactured by the now-forgotten company Amelco.2 This part sold in 1969 for $58.50 each (equivalent to about $300 today). In comparison, you can get a modern JFET quad op amp for under 25 cents.

Some op amp history

The op amp is one of the most popular components of analog circuits because of its flexibility and versatility. An op amp takes two input voltages, subtracts them, multiplies the difference by a huge value (100,000 or more), and outputs the result as a voltage. In practice, a feedback circuit forces the inputs to be nearly equal; with an appropriate feedback circuit, an op amp can be used as an amplifier, a filter, integrator, differentiator, or buffer, for instance. A key figure in the early development of op amps was George Philbrick who started a company of the same name. The commercial history of the op amp started in 1952 when Philbrick introduced the K2-W op amp, a two-tube module that made op amps popular.3

I'll now jump to Jean Hoerni, who founded Amelco. One of the key events in the history of Silicon Valley was the 1957 departure from Shockley Semiconductor of eight employees, known as the "traitorous eight". They founded Fairchild, which led to dozens of startups and the growth of Silicon Valley. (Moore and Noyce, two of the eight, later left Fairchild to found Intel.) Physicist Jean Hoerni, of the traitorous eight, worked at Fairchild to improve transistors and succeeded beyond anyone's expectations. In 1959, he invented the planar transistor in 1959, which revolutionized semiconductor fabrication. (The planar process is essentially the technique used in modern transistors and ICs, using masks and diffusion on a flat silicon die.) Interestingly, the transistors in the op amp module (below) look identical to Hoerni's original teardrop-shaped planar transistors. Transistors from the 1970s and later look entirely different, so it was a bit surprising to find Hoerni's original design in use in this module.

An NPN transistor inside the hybrid module. Tiny bond wires are connected to the base and emitter, while the collector is on the underside.

Hoerni left Fairchild in 1961 and helped found a company called Amelco. It focused on semiconductors for space applications, avoiding direct competition with Fairchild. Linear (analog) integrated circuits were a major product for Amelco, with Amelco building op amps for Philbrick (the pioneering op amp company). Amelco also manufactured discrete transistors using Hoerni's planar process. At Amelco, Hoerni developed a technique to built a type of transistor called a JFET using his planar process, and these transistors became one of Amelco's most popular products. The key benefit of a JFET is that the input current to the transistor's gate is extraordinarily small, an advantage for applications such as op amps. Amelco used Hoerni's JFET in the industry's first JFET op amp, producing a high-performance op amp.

Bob Pease,4 a famous analog circuit designer, ties these threads together. In the 1960s, Bob Pease designed op amps for Philbrick, including the Q25AH hybrid FET op amp (1965). Amelco manufactured this op amp for Philbrick, so Bob Pease visited Amelco to help them with some problems. The story (here and here) is that during his visit Bob Pease got in a discussion with some Amelco engineers about NASA's requirements for a new low-power, low-noise amplifier. Bob Pease proceeded to design an op amp during his coffee break that met NASA's stringent requirements. This op amp was used in a seismic probe that Apollo 12 left on the Moon in 1969, so there's one of these op amps on the Moon now. Amelco marketed this op amp as the 2401BG.

As for the 2404BG I disassembled, its circuitry is very similar to Bob Pease's 2401BG design5, so I suspect he designed both parts. The 2404BG op amp also made it to the Moon; it was used in the high voltage power supply of the Lunar Atmosphere Composition Experiment (LACE). LACE was a mass spectrometer left on the Moon by the Apollo 17 mission in 1972. (LACE determined that even though the moon has almost no atmosphere, it does has some helium, argon, and possibly neon, ammonia, methane and carbon dioxide.)

In 1966 Amelco merged with Philbrick, forming Teledyne Philbrick Nexus which after some twists and turns was eventually acquired by Microchip Technology in 2000. (Among other things, Microchip produces the AVR microcontrollers used in the Arduino.)

Inside the hybrid op amp

In this section, I'll describe the construction and circuitry of the 2404BG op amp in more detail. The photo below shows a closeup of the ceramic wafer and the components on it. The grayish lines on the ceramic are conductive circuit traces. Most of the squares are NPN and PNP transistors, each on a separate silicon die. The underside of the die is the transistor's collector, connected to a trace on the ceramic. Tiny gold wires are attached to the emitter and base of the transistor, wiring it into the circuit. The two rectangular transistors in the lower right are the JFETs. The large square in the middle is a collection of resistors, and a single resistor is in the upper right. Note that unlike integrated circuits that can be mass-produced on a wafer, hybrid modules required a large amount of expensive mechanical processing and wiring to mount and connect the individual components.

A closeup of the hybrid module.

I reverse engineered the circuitry of the op amp module and generated the schematic below.6 This circuit is fairly simple as op amps go, with about half the components of the classic 741 op amp. The inputs are buffered by the JFETs (green). The differential pair (blue), amplifies the input, directing current down one side of the pair or the other. The current source (red) generates a tiny fixed current for the differential pair using a current mirror circuit. The second stage amplifier (orange) provides additional amplification. The output transistors (purple) are set up in a class AB configuration to drive the output. The remaining components (uncolored) bias the output transistors. External capacitors on the compensation pins (8 and 9) prevent the op amp from oscillating.

Schematic of the 2404BG op amp.

Most of the resistors are on the single die in the middle of the module; this die is 1.7mm (1/16") on a side. The zig-zag shapes are thin-film resistors constructed from tantalum deposited on an oxide-coated silicon wafer. (One advantage of hybrid circuitry over integrated circuits was more accurate and better quality resistors.) The resistance is proportional to the length, so the meandering shapes allowed larger resistors to fit on the die. Around the outside of the die are metal pads; the bond wires attached to the pads connected the resistors to other parts of the circuit. Note the small circle to the left of the upper right pad; one innovation at Amelco was "mark-in-mark" targets to align the masks used for different layers of a chip.

The die in the middle of the module contains multiple resistors.

The current source circuit needed a very high-valued resistor, so it used a separate resistor die (below). This resistor used a long, thinner trace to produce a higher resistance than the resistors on the previous die. Note the circular alignment target in the lower right. The die for this resistor is 0.8mm on a side.

This resistor controlled current through the op amp. The bond wire in the upper left was knocked off the pad during photography.

The photo below shows one of the junction FET transistors used in the op amp. The metal fingers connect to source and drain regions. The gate (green) is connected underneath. This design is almost identical to the first planar JFET that Hoerni invented in 1963. It was initially difficult to produce high-quality JFETs on an integrated circuit, which motivated the production of hybrid JFET op amps. It wasn't until 1974 that National Semiconductor engineers developed the ion implantation technique for fabricating consistent, high-quality JFETs and used this "BIFET" technique to build better JFET op amp integrated circuits.

The diagram below compares the structure of the NPN and PNP transistors in the module, with photos at top and a cross-section diagram below.

A FET transistor inside the module. The die is are 0.6×0.3mm.

Each transistor starts with a square die of silicon, which is doped with impurities to form N and P regions with different properties. The N and P doped silicon show up as different colors under the microscope. The shiny metal layer on top is visible, with one bond wire attached to the central emitter. A second bond wire is attached to the base region surrounding the emitter; the "teardrop" shape provides a wider area to attach the base wire. The underside of the die is the collector, which makes contact with the wiring on the ceramic wafer. The NPN transistor follows the straightforward planar structure. The PNP transistor, however, required an extra "annular ring" to operate at the op amp's higher voltages.7

Comparison of NPN and PNP transistors in the module. Each transistor is 0.5mm on a side. Approximate cross-sections are shown below.

Conclusions

This random component that I opened up turned out to have a more interesting history than I expected. It ties together the early days of op amps with Philbrick, Bob Pease's analog circuit development, now-forgotten Amelco, and NASA's scientific experiments on the Moon. The transistors inside this module were built using Hoerni's original planar designs, providing a glimpse into the development of the planar process that revolutionized semiconductors. Finally, this op amp shows the capabilities of hybrid technology, now almost completely eliminated by integrated circuits.

If you enjoyed this look inside a hybrid op amp, you may also like my analysis of another JFET op amp and the famous 741 op amp. I announce my latest blog posts on Twitter, so follow me at @kenshirriff. I also have an RSS feed. Thanks to op amp guru Walt Jung for help identifying the module.

Notes and references

1. The module was packaged in a standard 12-pin TO-8 package. Most metal can integrated circuits are in the smaller TO-5 package, but the larger hybrid circuits require more room. ↩

2. The "15818" on the package is a CAGE code, a NATO identifier used to track suppliers. Originally, 15818 was assigned to Amelco; due to mergers, this number now shows up as TelCom Semiconductor. ↩

3. Several sources provided much of the information for this blog post. The book History of Semiconductor Engineering discusses in great detail the history of various semiconductor companies and the people involved. For an extremely detailed history of op amps, including the development of JFET op amps in the 1970s, see Op Amp History by Walt Jung, along with his Op Amp Applications Handbook. IC Op-Amps Through the Ages also has a history of op amps. ↩

4. Bob Pease wrote a popular column "Pease Porridge" on analog circuits. He also wrote books such as Troubleshooting Analog Circuits. ↩

5. Bob Pease's article What’s All This 2401BG Stuff, Anyhow? (page 54) provides a schematic of the 2401BG (below). Comparing the schematics, the 2401BG is very similar to the 2404BG that I examined. (I've colored the functional blocks to match my 2404BG schematic to make comparison easier.)

   

   Bob Pease's schematic of the 2401BG hybrid op amp that he designed for NASA.

   The main difference is the output stage: the 2401BG takes the output directly from the second amplifying pair (with a current mirror at the bottom to sink current), while the 2404BG adds a class AB output stage. The 2401BG also has a separate current mirror for the bases of the input NPN transistors. ↩

6. After I reverse-engineered the op amp schematic, I found a 1968 databook with a schematic for an Amelco hybrid op amp. The two schematics are almost identical, except the databook schematic includes two compensation capacitors, which are external on the 2404BG.

   

   Photo of an Amelco hybrid op amp.

   The databook provided the above photo of the hybrid op amp, which is completely different from the 2404BG I examined. The databook did not give a part number (which is unusual for a databook), so I suspect this was a version of the 2404BG under development at the time. ↩

7. You'd expect NPN and PNP transistors to be symmetrical, but the PNP transistors needed to be different to support high-voltage operation. The problem was that an interaction between the P region and the silicon dioxide on top caused N-type properties in a thin layer of the weakly-doped P region. At higher voltages, this could cause the transistor to short out. The solution was to create a strongly-doped P+ "annular ring" to interrupt this unwanted N behavior. Details in Jack Haenichen oral history and patent 3226611. ↩