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Design >> Analog Design >> Maximize ICMR
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Message started by extremis on Oct 19^th, 2010, 7:36am

Title: Maximize ICMR
Post by extremis on Oct 19^th, 2010, 7:36am

I am designing an OPAMP with 0.6μm technology (Vtp=-0.9V, Vtn=0.7V) and i have a specification of ICMR=0.5-2.5V to meet. The power supply is 3V (single). I have considered the clasic 2 stage OPAMP with Miller compensation, the cascode and the folded cascode OPAMP, but i don't see how the ICMR requirement could be satisfied.

The folded cascode OPAMP seems to meet either the upper or the lower specification depending whether it has a NMOS or PMOS input differential pair, but not both. Could anyone suggest a simple topology that could meet the ICMR specification?

Thanks in advance.

Title: Re: Maximize ICMR
Post by AnalogDE on Oct 19^th, 2010, 9:29am

I think you need an input stage that contains both N and P diff pairs. Search for "rail-to-rail" OTAs/op-amps.

Title: Re: Maximize ICMR
Post by extremis on Oct 22^nd, 2010, 2:56am

Thanks for the advice. I have found some rail to rail topologies, but they are too complicated and might even not satisfy other requirements. I was wondering if there is some kind of trick to increase ICMR using a simple topology.

Title: Re: Maximize ICMR
Post by Dan Clement on Oct 23^rd, 2010, 1:03pm

Level shifters... Like resistor dividers or source followers.

Rail to rail amps can be a real pain because now you have two input stages to lead to distortion, offset, noise, etc...

Good luck.

Title: Re: Maximize ICMR
Post by extremis on Nov 23^rd, 2010, 8:14am

Dan, i tried both approaches: source followers work just fine until Vcm~0.8V but after that the gain is lowered and resistor dividers seem to degrade performance, especially slew rate. I guess it has something to do with input capacitances but still i cannot find any way around it.