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Old 3rd February 2017, 09:40 PM   #651
RobLK is offline RobLK  United States
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I ran the source in the feedback loop test that Lynn showed with this circuit. I hope I did it right.
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Old 3rd February 2017, 10:17 PM   #652
lhquam is offline lhquam  United States
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Both sets of measurements look plausible, but I am surprised that the -3dB frequency is so high with the input and actually lower for the feedback loop. Is this with a CFA input stage or something else? In my simulations with a jfet gate stopper of 33K is get -3dB at around 105kHz from the input, and about 800kHz for the feedback loop.
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Old 3rd February 2017, 10:27 PM   #653
RobLK is offline RobLK  United States
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If you put the circuit above on the second stage of a three stage amp to drive another source follower you could fun the feedback for the offset from the final output stage back to the op amp. Then it should keep the output offset at zero and the second stage bias constant. You would need to have the voltage gain in the first stage though.
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Old 3rd February 2017, 10:29 PM   #654
RobLK is offline RobLK  United States
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Lynn, I used the same front end as you did.
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Old 3rd February 2017, 10:34 PM   #655
RobLK is offline RobLK  United States
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I don't know the answer to that Lynn. I posted it for you to look at with your big brain.
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Old 3rd February 2017, 10:38 PM   #656
RobLK is offline RobLK  United States
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In my other posts I was talking about the gate resistors on the mosfets that changes the frequency response.
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Old 3rd February 2017, 11:21 PM   #657
lhquam is offline lhquam  United States
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Quote:
Originally Posted by RobLK View Post
Lynn, I used the same front end as you did.
Sorry, I was addressing Generg's measurements.
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Old 3rd February 2017, 11:49 PM   #658
RobLK is offline RobLK  United States
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Oh ok Lynn, yes I thought something didn't sound right. I didn't have results like that.
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Old 4th February 2017, 06:56 PM   #659
generg is offline generg  Germany
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i have to correct something, data will come...
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Old 4th February 2017, 07:09 PM   #660
generg is offline generg  Germany
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Indeed Lynn the first data had considerable CFB, now with out only global FB

input AC

magvout: mag(v(out))=(21.1412dB,0) at 1000
f3db: mag(v(out))=magvout/sqrt(2) AT 188336
phase_margin: atan(im(v(out))/re(v(out)))=(33.6045dB,0) at 1.76034e+06
gain_margin: -20*log10(mag(v(out)))=(26.3862dB,0) at 1.04495e+07

feedback loop AC

magvout: mag(v(out))=(21.1507dB,0) at 1000
f3db: mag(v(out))=magvout/sqrt(2) AT 187676
phase_margin: atan(im(v(out))/re(v(out)))=(35.353dB,0) at 1.81407e+06
gain_margin: -20*log10(mag(v(out)))=(27.3796dB,0) at 1.42083e+07

both -3dB are very similar
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