V5 should be removed. Presently the amplifier output is shorted.
I only wanted to point out the posted circuit doesn't implement negative feedback as frankrose suggested but a shorted amplifier output. It's a different question how the complete circuit will be connected. Surely neither with voltage sources at both inputs.however if I removed v5, when I remove the feedback wire after setting the dc operating points and my gm value wouldnt the circuit be incomplete as the differential pair are not identical ?
You don't show the measurement setup generating the magnitude curve but I doubt that you see gm in the plot. It looks more like capacitive feedforward with zero Id (= zero gm).if the value of gm is increasing through my bandwidth as follows, how can I make it constant over my required bandwidth?
I only wanted to point out the posted circuit doesn't implement negative feedback as frankrose suggested but a shorted amplifier output. It's a different question how the complete circuit will be connected. Surely neither with voltage sources at both inputs.
You don't show the measurement setup generating the magnitude curve but I doubt that you see gm in the plot. It looks more like capacitive feedforward with zero Id (= zero gm).
Use this test setup to simulate the Gm:
Set input Vcm to 0.6V, the series AC source to 1V AC magnitude. Choose Rbias to 10MOhm, C1 and C2 to 1uF.
Calculate the Gm with this expression: 1/(mag(IF("/C2/PLUS")))
I think you have problems with basics of electronics, you cannot short the output to ground with a 0 Ohm resistor or 0V voltage source. Read more about biasing, operating point, etc. Forum cannot replace a school.
Totally good the 4.71u. And the point is not to disappoint you, to help you. I have read hundreds of pages then I got experience, not I invented the solutions above, everything is written in good books, free online materials. If you read them you can find out solutions by yourself. It is not easier to get the basics from this forum, but if something is not written or hard to find, then feel yourself free to ask anything. This is just my advice, to spend more time with the mentioned topics by reading books instead of learning from any forum. This is not guaranteed here that you get correct answer from me for example. I am sure wikipedia is better many times. So that was just an advice.
I didn't advise you common mode feedback, this is not a fully differential circuit. I just mentioned negative feedback, and common mode voltage, don't mix these! In your circuit negative feedback is used to set the output voltage close to the common mode voltage, where your non-ideal circuit can operates similar as the ideal equivalent on your last figure.
After you have put together your designed Gm cells, use this setup:
Connect Vi1 and Vi3 to 0.6V DC voltage without AC magnitude, and connect Vi2 to 0.6V DC voltage with 1V AC magnitude.
If you simulate the whole 3 stage circuit in DC you should see that every node voltage on top level is close to 0.6V because of the negative feedback.
If you simulate the whole 3 stage circuit in AC you should get a transfer function which corresponds to 2nd input, beacuse only at the Vi2 you have set AC magnitude to 1V.
Your last circuit is not fully differential! Fully differential means the input is differential and the output is differential. On your last circuit only the input is differential, the output is asymmetric or non-differential.
Here is a fully differential OTA:
https://file.scirp.org/Html/10-7600157/58c58d2b-f15a-4e50-b424-614ab0dc0ce8.jpg
As you can see it has got 5 terminal, 2 inputs and 2 outputs and common-mode feedback input, CMFB. You have got 2 inputs and 1 outputs.
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