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Fully-Differential Amplifiers and the Leading Edge in Electronics

Semiconductor companies have now had fully-differential amplifiers in their product lines for a few years, though these amplifiers have been in leading-edge electronics for decades. These diff-amps are differential not only at their input but also at their output, doubling output range. Their input and output ports have closed paths without a common ground node as one of the port terminals. The isolation from ground improves waveform quality. By keeping both input and output circuit loops complete in themselves, ground is important only for static analysis and common-mode range.

The Leading Edge in Electronics

Although the electronics industry lacks a glamorous event like the Indy 500 or the Grand Prix, where race cars perform field trials of new automotive concepts, electronic developments occur in other venues. For instance, during WW II, the Radiation Laboratory at MIT turned out some excellent results in the development of radar. One can admire, even to this day, the set of “Rad Lab” volumes from that era, containing solid presentation of theory backed by implemented electronics. An earlier example is that of Vladimir Zworykin, who developed television at RCA. This was an impressive breakthrough entirely unlike the incremental improvements of next-generation commodity products of today.

In the 1950s, development of the laboratory-quality cathode-ray oscilloscope at Tektronix by Howard Vollum, Jack Murdock, Cliff Moulton, John Kobbe (to whom one Tek legend attributes the invention of the JK flip-flop), Bill Polits and other highly creative engineers led to a most desirable technical environment for the motivated designer. With over 70 % of the oscilloscope market – a market driven by technological advances – and with founders that were inventive engineers themselves, Tek was an engineering-driven company, an idea-advancing enterprise.

Although Tek and H-P (now Keysight) are outstanding examples of test and measurement (T&M) instrument companies, it is generally the case that high-performance measurement instruments are the “race track” of the electronics industry. After all, to measure the behavior of circuits under development, measurement instrument circuitry must be that much better. Consequently, in T&M equipment, interesting circuits are found. And this leads to diff-amps.

Diff-Amps Found in Scopes

Oscilloscopes have used fully-differential amplifiers for decades. (Why did it take so long for them to appear as commercial ICs?) They are usually found in vertical amplifiers which amplify the probe voltage by precise gains before applying the amplified waveform to the vertical deflection plates of the CRT. And except for the first stage, which is typically driven by a ground-referenced probe, they are fully differential amplifiers, through and through.

To demonstrate, look at part of the vertical amplifier of a Tek T935A 35 MHz scope – now obsolete, 1970s-vintage, and low-cost.

The input buffer amplifier stage has been scanned from the manual. (And by the way, the older Tek “instruction manuals” as they were called, contained circuit diagrams which were works of art, unparalleled by CAD drawings of today – the price of progress!)

The very first stage consists of JFETs Q4222A and B. The probe waveform is input to the gate of Q4222B. With the other JFET below it, a x1 buffer amplifier is formed, with near-zero offset voltage between input and output. This is accomplished by using matched JFETs, and using the lower one as a current source. Its gate is connected to the –8V supply, and the VGS that results from the drop across R4225 (the 20 Ω resistor in its source) corresponds to a drain current that flows through the JFET above it. The JFETs are matched, and with the same current, the upper JFET will have the same VGS . The corresponding 20 Ω resistor, R4224, lower-terminal voltage is consequently the same as the input gate voltage and biases the JFETs at their zero-TC operating-points – that is, zero ΔVGS for ΔIs . Current of the upper JFET is increased slightly as base current of Q4232, but it is minor, and the matching is adequate.

This amplifier drives a full-diff-amp at the second stage, consisting of Q4232 and Q4234. Only the upper BJT (Q4232) is driven by the waveform to be amplified, while the lower input – at the base of Q4234 – is dynamically (ac) grounded to the scope probe circuit ground, thus completing the return of the input circuit. Because vertical amplifiers (as for all amplifiers) have input offset error, the otherwise unused input is used for input offset-error adjustment, which in oscilloscope language is dc balance . The word balance is a hint that scope amplifiers are heavily differential and that the two sides of the amplifier must be made to operate with the same static (dc) conditions.

The output of stage 2 is also differential. This stage is only an emitter-follower, with no voltage gain, but it is needed to present a high input impedance to the JFET buffer while driving stage 3 with a low impedance. In other words, it presents a voltage source to the next stage. At its differential output, however, the input waveform is not yet differentially balanced because the emitter-followers have no gain interaction between them and no splitting of the input waveform between them occurs. Stage 2 is differential only in that it has 2 inputs and two outputs. With no voltage gain, the input difference voltage is the output difference voltage.

The succeeding three stages to the delay line are shown below, a continuation of the same amplifier.

Q4258 and Q4268 form a fully-differential amplifier stage, with shared emitter resistance R4254, a 63.4 Ω, 1 % resistor. Resistors R4257 and R4267 connect to the –8V supply and, being much larger than R4254, approximate current sources to the BJT emitters. (In the Tektronix tradition, these are called long-tail current sources, approximated by a large resistance to a voltage source.)

The waveform at the base of the upper Q4258 BJT is divided through the emitter circuit and shared (nearly) equally with the lower Q4268 BJT so that at the load resistors, balanced waveforms appear, having equal magnitude and opposite polarity. If R4254, or RE , were split into two series resistors of value RE /2 each, then their midpoint would be a “virtual ground” null node for a balanced-input diff-amp. For this stage, half the magnitude of the input waveform, which is only applied to the upper BJT, would appear at the null node instead.

The next stage (Q4274, Q4284) is the second half – the common-base stage – of a complementary cascode amplifier. It is fully differential, as is the final common-collector stage (Q4276, Q4286).

Stage Gain

To calculate the differential voltage gain of the complementary cascode stage, note that the emitter incremental (or small-signal) resistances of Q4274, Q4284, which shunt the resistors R4271, R4281 (both 825 Ω), are much smaller, so that most of ΔIC , the incremental current from Q4258, Q4268, flows through Q4274, Q4284 to develop a voltage across load resistors R4273, R4283 (both 499 Ω). The purpose of the 825 Ω resistors is to provide emitter bias current to the common-base stage. The stage gain is determined largely by the collector load resistors and the emitter resistor R4254:

where upper and lower voltages are denoted by subscripts u and l . Their differences are the input and output differential voltages. The upper and lower sides of the amplifier both contribute to the total gain; hence the x 2 before the BJT gain in Av . Because RE (R4254) is so close to the value of the dynamic emitter resistance of the BJT re , a better gain approximation adds 2 x re to RE in the denominator of the gain equation, where

Then Av ≅ –12.9, with 3.72 mA of emitter current for each BJT. Neglected is the loading of the input impedance of the next stage on the load resistors and α current loss of both cascode BJTs. Do you suppose the amplifier designer was trying to achieve a gain of –10?

Closure

Fully-differential, monolithic amplifiers have been available for a few years, such as the ADI AD8138, to drive high-resolution ADCs and for other high-performance (high speed and precision) amplifier applications. Their predecessors have been in oscilloscopes for a few decades. Might there be some other monolithic amplifier product ideas to be found by semiconductor companies in measurement instrument circuits?

2 comments on “Fully-Differential Amplifiers and the Leading Edge in Electronics

  1. Tucson_Mike
    September 12, 2018

    Nice post Dennis, but to clarify, what has come to be called the Fully Differential Amplifier has been available commercially since I think about 1999 – almost 20years. We were working on one inside BurrBrown starting in 1998 while ADI beat us out with the first one in 1999 – I believe it was the AD8138 or AD8132. TI was also aggressively developing FDA's in this time period, where I think the THS4130 might have been the first released. With the TI acquisition of BurrBrown in 2001, we stopped our FDA development for a few years picking it up again with the THS4521 typed device around 2008. The distinquishing characteristic for this class of device is the seperate output common mode control loop. Diff I/O stages can be done very easily with dual op amps, but the 2nd loop makes a whole new creature. Looking at my high speed amplifier parametric data base, I now see somewhere around 75 devices that are called FDA's where most are from ADI, LTC, and TI. This is not including a whole range of internally fixed gain FDA topologies mainly from LTC. 

    Analog Signal chain seems to change at a glacial pace, I am not sure we can call these FDA's recent or new any more. 

  2. D Feucht
    September 12, 2018

    Mike,

    Your chronology about FDAs is right, of course, and it reveals my age. The main point is that FDAs have been around in oscilloscope vertical amplifiers since at least the late 1940s. Because CRT deflection plates are inherently differential, there was an impetus for them back then, but my point of amazement is that they didn't show up in IC amplifiers in the 1970s, in other than in-house Tektronix ICs. “Glacial pace” puts it accurately.

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