[sdiy] Operational Transconductance Amplifiers

[Antti Huovilainen]

On Tue, 9 Mar 2010, David Moylan wrote:

> Is the tanh response the reason why the differential input voltage has a 
> very small (linear) range?

Yes. When the signal amplitude goes above Vt or so you start to get 
noticeable distortion.

It's not that bad in practise in many lowpass filters that use OTAs 
because 1) they only apply the difference between filter input and filter 
output to the OTA input and 2) the inherent feedback below cutoff reduces 
the distortion further (and above cutoff there's the lowpass effect so the 
distorted signal is small in amplitude compared to passband).

> Is there a certain transistor arrangement in the amp that gives it this 
> response or is it an innate transistor phenomenon?

The response is due to the differential pair and is fundamental for BJT 
transistors. It can be somewhat linearized by predistorting the input 
and perhaps some other tricks.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova