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OPAMP OL Gain and Slope.

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satheeshvelu

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Hi everyone

I got some interesting question for you all.

In the OPAMP Open Loop Gain Frequency Response graph,

Pole & Zero are decided by the compensation (miller) capacitor and resistor inside.

I would appreciate if some someone explains how the slope of this gain plot is decided. This slope is always at -20dB/dec

Which device make this slope and responsible for this magical number -20?

Thanks in advance,

Regards,
Satheesh
 

A single pole always produces 20dB/dec. e.g. one resistor and one capacitor. It means that for a 10 times increase in frequency you get a 10 times reduction in the signal. As the reactance of a capacitor is inversely proportional frequency, that makes sense.

Keith.
 

satheeshvelu said:
...........................
This slope is always at -20dB/dec
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Click to expand...

This is true (far above the 3-dB corner frequency) only for a first order system - that means, if the slope is caused by one capacitor only. If the frequency is high enough and more capacitive effects come into the game, you will observe always a slope of -40 dB/dec and higher.
 

Hi All,

Thanks. I could not relate the Pole created by just a RC circuit and the Miller RC inside the OPAMP.

I'm clear with your explanation.

Could you also please explain how the OPAMP oscillates and how the Gain Margin and Phase margin protects the OPAMP from Oscillation?

Thanks
Satheesh
 

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