Hi guys
I am designing a most basic amplifier: nmos differential input, pmos current mirror loading. The output terminal is only planning to drive a cap.

My simulation output is in the picture attached. The red is the output without a cap and the green is with a cap. We can see the green curve is way less ideal compare to red. My analysis is as follow: I believe that the green are curves, instead of lines, suggests that it is not slew rate limited (also agree with my hand calculation). So I suspect that the reason of of the bad performance was due to the RC constant: huge output resistance make the RC constant really bad.

I have read that if your amplifier only need to drive a cap, no additional low-impedance output stage is needed. Is this statement contradict my simulation result?
Thanks
Allen

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If you need to charge/discharge a capacitor very fast and you don't want huge operating current you will need a low output impedance driving stage. But capacitance load cannot cause DC error, I think this is the reason why you read no additional stage is required.
A big load capacitor slows down the circuit, but big time constant is not a "really bad" thing, sometimes it is necessary, especially if you want to implement feedback and make a stable circuit without oscillation.

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A Differenatial Pair has pretty high Output Impedance therefore it cannot drive a large capacitive/inductor loads.
That's why there is an Output Stage ( AB-Class or Totem-Pole ) in OpAmps.An Source Follower will also work well ( try and see )

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A differential pair with capacitive load can be used as a Gm-C filter because its output resistance is quite high. So it can drive. Point is the charge/discharge speed matters for somebody or its used for something else.

Hi frankrose
I was referring to that the difference between an OTA and an opamp was the output stage. Opamp=OTA+output stage. An opamp is used to drive a small resistive load because the output stage have a low output resistance. And OTA is used in, for example, switched capacitor circuit, is only to drive a capacitive load and no resistive load. (so the high output resistance won't be reduced by paralleling a small resistor )

Then how does the OTA with a capacitive load situation handle the big output time constant problem, which lead to the slow charging time?
Thanks
Allen

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Hi BigBoss
I was referring to that the difference between an OTA and an opamp was the output stage. Opamp=OTA+output stage. An opamp is used to drive a small resistive load because the output stage have a low output resistance. And OTA is used in, for example, switched capacitor circuit, is only to drive a capacitive load and no resistive load. (so the high output resistance won't be reduced by paralleling a small resistor )

Then how does the OTA with a capacitive load situation handle the big output time constant problem, which lead to the slow charging time?
Thanks
Allen

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