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Simulators >> Circuit Simulators >> slew rate of fully differential OpAmp
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Message started by IvoryDesigner on Mar 11^th, 2010, 1:32pm

Title: slew rate of fully differential OpAmp
Post by IvoryDesigner on Mar 11^th, 2010, 1:32pm

Hi,
I'm simulating performances of a fully differential OpAmp, and I would like to know How to simulate the Slew rate. I know how to do it with a single ended OpAmp: Set the OpAmp into follower configuration and enter a pulse at the input and look at the output.

Is it the same with a fully differential OpAmp??

Thank you for your help! :)

Title: Re: slew rate of fully differential OpAmp
Post by AdrianSN on Mar 27^th, 2010, 3:26pm

The output of a fully differential amplifier is a differential signal. You would need a differential component at its output to see the slew rate behavior. What do you use for simulation?

In SPICE, you could use a differential-to-single-ended component like an ideal VCVS. This does not alter the slew-rate behavior and, at the same time, allows you to plot the output, which is single ended this time.

Title: Re: slew rate of fully differential OpAmp
Post by IvoryDesigner on Mar 29^th, 2010, 5:39am

Hi Adrian,

Thank's for your answer, I use at the output an ideal_balun to do the diff-to-single conversion. My question was especially about the OpAmp configuration for the measurement. is the follower configuration right? like measuring slewrate for a single ended OpAmp.

Thank You! ;-)

Title: Re: slew rate of fully differential OpAmp
Post by AdrianSN on Mar 30^th, 2010, 10:38am

Then you are doing the right thing. Fully differential amplifiers work the same as single ended ones. The difference is that, in the case of fully differential amplifiers, the differential voltage is carried all the way to the output. By contrast, in single ended ones, the differential voltage is converted in a single ended voltage.

Both types of amplifiers have to be compensated so that they are stable. Even the so called uncompensated ones, or decompensated, which are not stable at unity-gain, have a gain-bandwidth product which is constant at higher gains. Therefore, there is an integrated capacitance which is charged by a voltage controlled current source (gm) which results in a rate of charge. This rate of charge is the slew-rate.

So, both amplifiers (single ended and differential ended) behave the same from the slew-rate point of view. Test the differential amplifier the same way as you would the single ended one. Just make sure you look at the differential output, and you confirmed that you are doing that.

Hope this helps.

Title: Re: slew rate of fully differential OpAmp
Post by IvoryDesigner on Mar 31^st, 2010, 6:47am

Hi Adrian,

Thanks, this confirm what I thought before.

have a nice day

best regards!

IvoryDesigner