An equal control voltage (CV) is applied to the base of each BJT (I guess it wouldn't be proper to call this a dif. pair in this case due to how they're biased? More like parallel CE amplifiers?)
You are absolutely correct. This is not a "diff pair". In my head, I've been calling these two npn transistors a "differential amplifier" because the output is differential.
If this CV goes up, the voltage output of each BJT CE amp goes down equally. If this CV goes down, the output voltage of each BJT CE amp goes up equally. (Assuming everything is nicely matched).
Correct. I am confidant the transistors are fairly well matched (both in their manufacturing process and in their temperature at any given time) because they are on the same piece of silicon. See CA3046.
Regardless, this output is fed differentially to the differential amplifier, and so this common mode signal is subtracted away, leaving any differential mode signal (the audio signal). However, when CV goes up/down, gm also goes up and so the gain of what is basically a common emitter amplifier goes up as well... so you can effectively control the gain by modulating the gm of a CE stage.
Again, you are correct in your analysis of the circuit. The gain is modulated by the control voltage exponentially (as the relationship between current through the transistor and voltage at the base is exponential). While this might not be desirable in all applications, I love the exponential relationship. Here is a recording I made of the VCA modulating a low-frequency sine wave. It sounds sort of like a helicopter :3
Can I ask how you arrived at this design? I would have never thought to do this. I think it is cool :)
Hmmm... That is a most difficult question. I'll try to answer it as coherently as I can.
In the beginning, I knew I wanted something to control the amplitude of an audio signal. I thought that an exponential control would be fantastic (for example, "let the gain of the amplifier rise by 20 dB per volt on the CV input"). My mind immediately went to BJTs; they are good at converting linear voltages into exponential currents.
Take a look at this schematic of a typical common emitter amplifier with emitter degeneration. The "DC bias" of the amplifier is set up by the resistors. The AC voltage signal is coupled into the base of the transistor through the capacitor.
In that standard CE amplifier, the gain is set up by the emitter and collector resistors. So to make an amplifier where the gain is controlled by voltage, I thought it would be a nifty trick to remove the emitter resistor and just work with a true CE amplifier. Now the current through the device is determined by the voltage at the base.
In removing the emitter , the AC audio signal coupled into this "true" CE amplifier must have a very small amplitude to retain its linearity. Without the resistor, the audio signal can be said to be adjusting the bias in exactly the same way the CV is adjusting the bias). A change in audio signal voltage creates an exponential change in the base current.
I realized that to ensure the distortion of the audio signal was low, the audio signal's presence on the base of the transistor would have to be much smaller than that of the CV. The CV must affect the base voltage in a range hundreds of millivolts to modulate the gain through multiple decades. But the audio signal should appear on the transistor's base with an amplitude of 5 mV to ensure the signal is not excessively distorted.
At this point, I realized that as the CV wildly changes the current in the transistor, the voltage drop across the collector resistor will also wildly change. To remedy this, I thought it would be a good idea to use a second transistor (matched to the first one) and only apply the CV to its base. That would give me a reference for how much the CV is affecting the voltage bias of the amplifier. Then just subtract the two signals.
I realize that may have been a bit of a rant.
To summarize,
The CV sets up the bias and gain of the transistor. It determines to what degree small voltage input perturbations (like a small audio signal) will affect the current output (and ultimately the voltage across the resistors).
I like to think of the audio signal as being "AC coupled" into this amplifier, even though it is not.
Here is the circuit as it currently stands. It is not finished, but it is starting to work acceptably.
Final Fun Fact:
Because the control voltage can very quickly change the gain of the two parallel CE amplifiers, the audio signal can be modulated by another audio signal. Frequencies can be mixed together in this VCA, and other non-harmonics spewed out.
