Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

[SOLVED] OTA vs telescopic OTA for grounded resistor implementation

Status
Not open for further replies.
B

Basil1402

Newbie level 4
Joined
Apr 3, 2014
Messages
7
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
57
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.
 

Basil1402 said:
... the OTA is a (single stage) telescopic one. I understand it gives a higher output resistance
Click to expand...

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

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.
 

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:
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top