VCA the OTA way

To recap, an OTA has a voltage input, control current input, and output current. An obvious synth application would be as a VCA. The challenge is a VCA is voltage-controlled, not current-controlled. Also the output current must turn into an output voltage, which isn't a huge deal since that really just requires a resistor. When I talk about VCAs, I am referring to the amplifier that is controlled by the envelope to shape the sound, creating the illusion of a note playing. Final level controls especially in polysynths can also be VCAs and use OTAs, but those are boring.

The following is a list of synths using the 3080 or 3280 OTAs for the VCA:

CA3080: Moog Taurus (I & II), Prodigy, Rogue, Opus 3, (Realistic) MG-1, ARP Odyssey, Octave CAT
CA3280: Sequential Circuits Pro-One

Not all synths use OTA chips for the VCA. Roland mostly used the BA662 which is a VCA rather than OTA. Also many later synths (especially polysynths) used SSM or CEM chips for the VCA. Korg synths (at least the MS-10 & MS-20) use a different solution. In conclusion, if you don't like OTAs, you don't like America.

OTA Synopsis

Here are the important points about OTAs that set them apart from opamps:

1. The output is a current, not a voltage.
2. The output level is determined by a control current I_ABC, not fixed by feedback resistors.
3. The input is a small differential voltage, but cannot be assumed to be 0V.

The equation for the output current is the following:

I_{out} = V_{in}\frac{\alpha I_{ABC}}{2V_T}

V_T = kT/Q and is known as thermal voltage. It frequently shows up in semiconductor equations. For normal conditions, this equation simplfies to:

I_{out} = V_{in} I_{ABC} \cdot 19.2

Another important point from the application notes is the voltage of the control current pin. It is one diode drop above the negative supply voltage. So we know roughly what the voltage is, but it will vary with varying control currents. For this reason a current buffer is usually placed before the OTA control current input. In the next post I'll track down some real life applications of OTAs in synthesizers.

OTA, how can I explain it?

OTAs or operational transconductance amplifiers are commonly used in analog synths for a variety of purposes. Most commonly they are used as VCAs. After all, how would you make a VCA with standard op-amps? I graduated with an electrical engineering degree without ever hearing about OTAs. Then again, I could say that about a lot of things. I digress. We must teach ourselves about these triangles.

I was first shown the ways of the triangle by Prof. Aaron Lanterman's online videos of his Electronics for Music Synthesis class at Georgia Tech. See session 6 and 14. That would be a good place to start if you're like me and you long for the days of sitting in a classroom watching somebody solve a math problem. If only those problems were about synthesizers. Well now they are!

If you're more hardcore, check out the application notes for the CA3080. These are filled with all kinds of good information if you can actually stand to read them. When you're finished, put down your pencils and I will continue.