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# Slew Rate for 2 stage op amp

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#### centaurus01

##### Newbie level 3
Have two questions regarding slew rate.

I read somewhere that slew rate is specified for unity gain buffer configuration. Any reason for that? Does closed loop gain affect slew rate in any way?
Secondly, when we design the Miller compensation for two stage op amp we place Rzero in series with Cc. Cc affects the slew rate but does the value of Rzero affect slew rate in any way?

Thanks!

I'm not sure but I don't think closed-loop gain has a significant effect on op amp slew rate.

The Rzero will affect the slew rate since it affects how fast Cc can charge when the internal circuit driving the compensation circuit saturates, which is what primarily determines slew rate.

Have two questions regarding slew rate.
I read somewhere that slew rate is specified for unity gain buffer configuration. Any reason for that? Does closed loop gain affect slew rate in any way?
Secondly, when we design the Miller compensation for two stage op amp we place Rzero in series with Cc. Cc affects the slew rate but does the value of Rzero affect slew rate in any way?
Thanks!

What is the slew rate? It is the ability of an amplifier with feedback to return to linear operation after the first stage was driven into saturation.
Example: Imagine a unity-gain amplifier (100% feedback) with an input step of 1V. The input stage immediately will saturate
and it takes some time to charge the largest capacitor within the circuit, which is the compensation capacitance. This leads to a time delay of the feedback signal.
When the feedback signal arrives at the inverting terminal the differential input voltage will reduce and the opamp returns to linear operation until the output voltage is +1V.
This return process determines the slew rate. Without any feedback the opamp never would return to linear operation.
Therefore, the feedback plays a major role in determining the slew rate, and the worst case - as far as the slew rate is concerned - is 100% feedback.
From this description it is clear that a slew rate test signal must be able to overdrive (saturate) the 1st stage but it must NOT be able to drive the output signal into saturation.

Unity gain configuration allows for large input step (max).
Slew limited part ends when the feedback takes control of the loop, so it is easy to see that the unity gain configuration would exhibit the same slew rate over a larger time/voltage that makes it easier to demonstrate and measure.
The series resistor would introduce a initial kick, but it cannot change the "slew-rate", it could change the "slew-time" slightly though.

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