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opamp simulation
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Jay_Diddy_B:

--- Quote from: promach on April 14, 2019, 01:25:12 am ---In your opamp_JDB_bode_plt.asc , why DC 0.6V input stimulus for Vin+   ?

And why are you not using large LC for DC feedback ?


--- End quote ---

The op-amp will only work properly if both inputs are within the common mode range and the output is between the supply rails.

If I step the input voltage and measure the Bode plot:



I have changed the feedback to be a voltage follower, that is a gain of +1. A negative input would require the output to be negative. It can't go negative because we only have a positive supply. So the output would be near zero and there would be no gain.

If the input is 0V, the circuit will work for positive half-cycles, but not negative half-cycles. If the input is positive, the circuit works properly.

I get these results:



If I look in the time domain, transient analysis, and I step the voltage and apply a 100mV peak 1kHz sinewave, is see this. You can see that this op-amp doesn't work if the output is required to go negative.



You can also see that this op-amp has a 'defect' called phase reversal when the inputs are near or just below zero.

The choice of using an LC or positioning the voltage source like I did is preference.

Jay_Diddy_B


Jay_Diddy_B:
Hi further to my last post, consider these circuits:



In one I am just using the voltage source and in the other I am using the large inductor and capacitor to close the FB loop.


If I look at the results:



In both cases I get 80dB which is 10000, the gain of the amplifier being tested. The phase is different. In one case I am measuring the inverting input, so I get -180 in the other the non-inverting input so I get 0 degrees.

Regards,
Jay_Diddy_B
promach:

--- Quote ---You can also see that this op-amp has a 'defect' called phase reversal when the inputs are near or just below zero.
--- End quote ---

Why is that so for the "near zero" case ?

See this updated schematics.

Wimberleytech:
I tweaked your design a little bit.  Improved the cascode stack.
promach:
here is your opamp3.asc

Why are you using small LC (L1, C3) ?

Could you also explain analytically on cascn, cascp and biascn ?



I have done some modification to your opamp3.asc and put it side-by-side to my latest opamp.asc
I am bit curious on how to improve the phase margin of your opamp3.asc (left)

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