Principle of Operation
An OTA is a voltage controlled current source, more specifically the term “operational” comes
from the fact that it takes the difference of two voltages as the input for the current conversion.
The ideal transfer characteristic is therefore
IOut = gm(VIn+ − VIn−) (1)
or, by taking the pre-computed difference as the input,
IOut = gmVIn (2)
with the ideally constant transconductance gm as the proportionality factor between the two. In
reality the transconductance1
is also a function of the input differential voltage and dependent on
temperature, as we will later see.
To summarize, an ideal OTA has two voltage inputs with infinite impedance (i.e. there is no
input current). The common mode input range is also infinite, while the differential signal between
these two inputs is used to control an ideal current source (i.e. the output current does not depend
on the output voltage) that functions as an output. The proportionality factor between output
current and input differential voltage is called transconductance.