Hi all,

i need to design a simple differential op amp which is able to drive a resistance of about 2K with a unity gain bandwidth of at least 100 MHz and as low power as possible. Any suggestions on a topology?

thanks,
Aaron

Hi Aaron,

I would use some kind of a class-AB topology. There are several out there, some using 2 stages + an AB, some purely AB. If you do not need
very large gain and noise is not very critical, try out: "Lopez-Martin et al" from JSSC, May 2005. Of course, there are several other possible
solutions in AB, all very good. I just know these ones because I was playing around with them and was able to make these very simply. They also
don't need any overhead circuitry. If you look carefully, you will find ways to reduce the power consumption even further, particularly with
Topology # 1 in this paper where 2 branches are redundant. Just search around for more if you like.

I would keep it simple.

Regards
Vivek

Hi,

if you have enough supply you could try to boost the gain of the first stage with a telescopic configuration and use a second stage like in Miller opamps. If the gain is enough you could let the resulting OTA drive the 2K resistance w/o output stage.

Hope this helps.

Regards,

frank

aaron_do wrote on Mar 8^th, 2008, 4:10am:


Hi Aaron,

Why do you need to drive a resistive load, especially since you mention that you are able to choose it freely (based on noise requirements)? Are you making
the next stage as a resistive differential amp? Surely you have considered all the tradeoffs involved to size the R-load to a maximum.

If possible, I would recommend using a class-AB output stage with your 2-stage opamp. That might get you what you need.

Regards
Vivek

Hi,

I'm designing an active-RC filter. Increasing the 2 kohm resistor increases the noise while reducing the requirements on the opamp. Anyway i ended up designing two different opamps. The first stage opamp is very power hungry but provides good gain. That allows it to drive the 2kohm and reduce the effect of the noise of the following stages. So the following stages all use very low power op amps.

I think i will end up going with a telescopic op amp and i'll look into the output stage. Thanks for all the help,

cheers,
Aaron