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Question on Op Amp Integrator

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victordion

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In the above is a exam question asking if the input wave and output wave are given, design a circuit with op-amp to realize it.

The solution finds R1 * C2 by considering period and voltage, which I can understand.

However it says we needs R2, and must make sure R2 * C2 is 100 times of the lowest frequency which is 1/T.

I am confused as why we need R2 at all? Can somebody help?
 

In theory you don't need R2. However in practice, there's always a little bit of DC offset at the input. Without R2, that would cause the output voltage to slowly drift up (or down) until it reached it's maximum (or minimum).
 

Then how in detail does R2 help to eliminate DC offset on Vin?
 

DC gain = R2/R1 so DC at output = DC at input * R2/R1

So you still get some DC offset, but it's limited to a sensible value.
 

The solution is
R1 = 1k R2 = 390k C2 = 24 nF
this way DC will be hugely amplified??
 

So if DC offset at input is 1mV, then DC offset at output is 390mV.
 

I see. So if there is no R2, R2 = infinite, this offset is infinitely amplified.
 

victordion said:
I see. So if there is no R2, R2 = infinite, this offset is infinitely amplified.
Click to expand...

No, not "infinitely" but with the open-loop opam gain Ao (which is 100 dB or even more).
Please keep in mind that each opamp designed to operate in its linear mode needs dc feedback for a stable and suitable bias point.
 

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