Here's an amplifier with some unique properties that may be of interest to amplifier buffs, but first a little history, at least as best we can remember
Back in the 90s, the late Barrie Gilbert developed and interesting type mixer he called the MicroMixer. This mixer utilized the cross-coupled quad LO switching he made famous with his analog Multiplier/Mixer, however he utilized a very unique transconductor totally different than the ones seen in the earlier Multiplier/Mixer below the LO switching devices.
We're not sure if Barrie was the originator of this unique transconductor amplifier topology, and likely utilized before the MicroMixer. Whether used as a amplifier or mixer this circuit has some very unique and useful properties.
First off the amp is a single ending bi-polarity input with a pseudo-differential output, but operates from a single supply rail. This is accomplished by steering the input current between a pair of output transistors similar to a class-B stage but with two different current outputs. These currents are theoretically unlimited and only constrained by the transistors. The pseudo-differential type is because the output currents are only sunk, and when the input signal is sufficient amplitude, one side is completely cut-off similar to a class-B amp with the crossover current set by a steady state bias level, and the output current is "steered" to the active side inverted or non-inverted.
Another interesting circuit feature is the input impedance is symmetrically controlled about the steady state bias condition and plateaus with large input signal levels of either polarity. This property allows the control of harmonic generation and has been utilized to create harmonic nulls as the input power is swept.
We've included some notes created from memory, and utilized these with 200~300GHz SiGe bipolar devices in some of our custom IC work way back, so hopefully they are somewhat close. Also conjured up an LTspice circuit to revel some of the unique properties (just used 2N3904s tho, so obviously completely different devices & parasitics from what we employed).
Here's how it operates, at least a very simplistic view. The + input current signal causes additional current (above Ibias) to flow in Q1 causing it's collector potential to rise and begins to shut off Q3. Q1s current is mirrored by Q2 and becomes the + Output Current. As Q3 begins to cut off its' effect along with R3 on the input impedance diminishes and the input Z is now dictated by R2, dynamic 1/gm of Q2 and R1. In the other input current direction, Input current is pulled from Q1 collector and Q3 emitter until Q1 cuts off due to the voltage drop at Q1's base/collector, and Q3 supplies all the negative outgoing current thru R3 and it's collector sinks the inverted Io_bar Output Current. At large negative input currents, the result is symmetrical as the input Z approaches R3, the dynamic 1/gm of Q3 and R1.
The input impedance is tricky to revel in simulation, and requires the proper use of the derivative function {Zin = d(Vin)/d(Iin)} as well as removing the input offset with the offset bias source V1. Note this circuit is self biasing and an AC coupling capacitor would be used in series in actual use.
A quick analysis of the input shows that the input impedance at no signal level should be:
Zin = R1 + (R2 + 1/gm)/2, or ~ 57 ohm with value shown, and for simulation:
Zin = d(Vin)/V(Iin) as shown.
1/gm is (kT/q)/Ic or ~50.5 ohms at Ibias = 512ua.
With large input signal of either polarity then:
Zin approaches R1 + R2 (or R3) as 1/gm -> 0 for large currents, or 41 ohms.
Note that Q4 and R4 are optional in the notes, and not used in the LTspice simulations. The higher CE voltage for Q2 helps compensate for the finite beta of Q2.
We've used the DC sweep function (1VPP, 20VPP) from 50 ohm source to show the Input Z and how the Output Currents behave, note at zero input the outputs are both at Ibias. We also used a 20VPP sinewave input from 50 ohm source and you can see the differential output current and individual output currents.
The analysis of this topology quickly becomes transcendental for just about every important metric and requires some serious computing power and simulation software. We had the benefit of both back when employed but not so today after retirement, so very limited. A couple interesting topics wrt to this, was long ago recall a note about a Post Doctoral student being "employed" to run analysis and simulations to find the somewhat unique "Harmonic Nulls" of this topology, this required optimization of the bipolar device characteristics, resistor values and bias conditions, an enormous task indeed. One result that came out of this efforts was not only the means to achieve the nulls, but also the use of resistor-inductors, these being the series resistor with purposeful series ESL inductance. If you note the arrangement of Q1 and Q2 one might spot the use of the emitter degeneration inductance to achieve input matching popular with low noise RF amplifier designs, works well here also!!
The values we've included are from past memory and were for a specific result, so may be in error regarding optimizitation (especially wrt Harmonic Nulls). Speaking of the Nulls, these were simulated by sweeping in the input power levels and plotting the fundamental and 3rd harmonic as a function of input power level, recall a very time consuming process. The differential output currents can be utilized in various ways, one of which is with a differential transformer.
Anyway, hope some folks find this topology interesting, we certainly did a few decades ago
Best,