[SOLVED] OTA vs telescopic OTA for grounded resistor implementation

[Basil1402]

Hello.

I am designing a tunable high pass filter : the signal goes through a cap then into a grounded resistor. To make it tunable, the res is actually a differential inputs/single-ended output OTA, fed through the invert input, and with a feedback from the output to the invert input (like this)

I am using a OTA designed by a colleague, and the results are fine.

The only thing I don't really understand is the fact that the OTA is a (single stage) telescopic one (like p. 213 in Sansen). I understand it gives a higher output resistance, but I don't understand the point here.

Can someone enlight me ?

Thanks in advance.

 

[erikl]

... the OTA is a (single stage) telescopic one. I understand it gives a higher output resistance

The OTA's gain is approximately gm*rout, so the telescopic OTA has a higher gain than the simple OTA, hence provides better accuracy.

 

[Basil1402]

Yes, but, in the configuration I am refering to, the voltage gain is not of interest. The circuit is used to emulate a resistor, so what is important is the transconductance gm (=1/R).

What I understand is : the higher Rout, the smaller the current in the feedback branch, and, as a consequence, the smaller the chances that the feedback unbalances my differential pairs. Is this somehow correct ?

Thanks.

 

[Basil1402]

Ok. I answer to my own question.

I read Gray & Meyer. It's a series-shunt feedback configuration, with a feedback transfer function of 1. So the impedance of my circuit is Rout/(1+gmRout). As a consequence, my circuit acts as a resistance if Rout is very high. And therefore, a telescopic cascode OTA, with higher Rout than simple OTA, is better.

QED:grin: