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.