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Design >> Analog Design >> question on the loop gain of this circut
https://designers-guide.org/forum/YaBB.pl?num=1378453275

Message started by RaDex on Sep 6^th, 2013, 12:41am

Title: question on the loop gain of this circut
Post by RaDex on Sep 6^th, 2013, 12:41am

Hi , I recently joined this forum.
Iam working on the design of a fully differential folded cascode ota to be used in a switched capacitor integrator block of a sigma-delta modulator. Iam using Cadence IC614 for circuit design and simulation. I did some simulation to check the open loop gain, GBW shown below.
Next I made a testbench to measure the loop gain and phase margin by Stb analysis.
Question is on the loop gain as its around -12db which is not correct. Am I doing something wrong here ? Plz suggest

Title: Re: question on the loop gain of this circut
Post by RaDex on Sep 6^th, 2013, 1:26am

Test bench for open loop gain.

Title: Re: question on the loop gain of this circut
Post by RaDex on Sep 6^th, 2013, 1:31am

folded cascode fully differential amplifier with cmfb

Title: Re: question on the loop gain of this circut
Post by RaDex on Sep 6^th, 2013, 1:35am

testbench for loop gain simulation. R1=R2=1Kohms.

Title: Re: question on the loop gain of this circut
Post by RaDex on Sep 6^th, 2013, 1:37am

loop gain and phase margin

Title: Re: question on the loop gain of this circut
Post by Frank Wiedmann on Sep 6^th, 2013, 4:57am

Something strange seems to be happening in your simulation setup. The annotation of the DC operating points shows 897.3 mV at the input of the diffstbprobe and 0 V at its output. You should of course have the same voltage on both sides. You might want to try if the obsolete cmdmprobe works better for you or build your own diffstbprobe from two ideal baluns connected back-to-back with iprobes between them.

Title: Re: question on the loop gain of this circut
Post by RaDex on Sep 6^th, 2013, 10:09am

i used cmdmprobe but still got th same result

Title: Re: question on the loop gain of this circut
Post by RaDex on Sep 6^th, 2013, 11:11am

i used two back to back baluns with iprobes in between but no change

Can someone help?

Title: Re: question on the loop gain of this circut
Post by nat on Sep 6^th, 2013, 4:34pm

The load of an OTA should be purely capacitive. By closing the loop the way you did (with small resistors), you have loaded your amplifier with resistance, which have caused OTA to fail.

If you want to to that you should add output buffer to an OTA (make an opamp out of it:) )