Hallo,

I am building a variable gain amplifier using a simple OTA (5 MOSFETS) with a current mirror load. The gm of the OTA is changed by changing the bias current of the OTA. It is working fine (gain varies) but the problem is that the output DC point of the amplifier changes with the bias current. My question is how can I correct the output DC level and make it independent from the bias current? I tried a CMFB circuit to bias the load transistors but It doesn't correct the problem?

Thanks in advance

Best regards,

aamar

Dear Paul,

The tuning range is about 100mV in my case, do you consider it narrow or it would be enough?, I am using a 3.3V supply. I am still using an ideal source for the gain control voltage, but later I have to generate this signal in the design.

Do you think generating such a signal will be difficult? It is an optical receiver and this VGA would be next to the TIA and will be followed by a next stage of post amplifier.

If you can suggest me any reference for other topologies I would be thankful also.

Thanks and best regards,

aamar