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RaDex

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question on the loop gain of this circut
Sep 06^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

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gain_bandwidth.png

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RaDex Junior Member Offline Posts: 10	Re: question on the loop gain of this circut Reply #1 - Sep 6^th, 2013, 1:26am Test bench for open loop gain.
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RaDex Junior Member Offline Posts: 10	Re: question on the loop gain of this circut Reply #2 - Sep 6^th, 2013, 1:31am folded cascode fully differential amplifier with cmfb
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RaDex Junior Member Offline Posts: 10	Re: question on the loop gain of this circut Reply #3 - Sep 6^th, 2013, 1:35am testbench for loop gain simulation. R1=R2=1Kohms.
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RaDex Junior Member Offline Posts: 10	Re: question on the loop gain of this circut Reply #4 - Sep 6^th, 2013, 1:37am loop gain and phase margin
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Frank Wiedmann

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Munich, Germany

Re: question on the loop gain of this circut
Reply #5 - 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.

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RaDex Junior Member Offline Posts: 10	Re: question on the loop gain of this circut Reply #6 - Sep 6^th, 2013, 10:09am i used cmdmprobe but still got th same result
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RaDex Junior Member Offline Posts: 10	Re: question on the loop gain of this circut Reply #7 - Sep 6^th, 2013, 11:11am i used two back to back baluns with iprobes in between but no change Can someone help?
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nat New Member Offline Posts: 8 Warsaw	Re: question on the loop gain of this circut Reply #8 - 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:) )
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