Electronics > Beginners
AC Coupling cap
ricko_uk:
Hi,
when AC coupling an amplifier output to the input of a second amplifier, how do you choose the AC coupling capacitor size? What parameters need to be considered in the choice? Maybe inpute impedance of the following amp or not? What else?
And does the capacitor introduct any noise at all (even a microscopic amount)? If so how can it be kept to a minimum?
Thanks
TimFox:
For a “normal” amplifier output driving a “normal” input, the coupling capacitance determines the low-frequency cutoff. The response will fall by 3 dB (0.707 V/V) at the frequency where the capacitive reactance equals the sum of the output resistance of the first amplifier and the input resistance of the second amplifier.
ricko_uk:
Thank you Tim, :)
why also the first amplifier output resistance (and not just the second amp input resistance)?
I looked at few datasheets including the classic 741 but can only find the input resistance and nowhere mentioning of the output resistance. How do I find the output resistance?
Thank you
EEEnthusiast:
Output resistance would depend upon the feedback circuit used. That's why most of them do not specify this parameter.
ebastler:
In nearly all practical cases, especially in the audio domain, I would expect the driver's output resistance to be negligibly small vs. the following stage's input resistance.
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