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https://designers-guide.org/forum/YaBB.pl 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 6th, 2013, 12:41am |
Title: question on the loop gain of this circut Post by RaDex on Sep 6th, 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 6th, 2013, 1:26am Test bench for open loop gain. |
Title: Re: question on the loop gain of this circut Post by RaDex on Sep 6th, 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 6th, 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 6th, 2013, 1:37am loop gain and phase margin |
Title: Re: question on the loop gain of this circut Post by Frank Wiedmann on Sep 6th, 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 6th, 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 6th, 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 6th, 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:) ) |
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