AC Coupling cap - Page 1
Forum section: electronics / beginners

author: ricko_uk
author_rank: Super Contributor
timestamp: January 02, 2020, 03:43:45 am
content: 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

author: TimFox
author_rank: Super Contributor
timestamp: January 02, 2020, 03:57:08 am
content: 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.

author: ricko_uk
author_rank: Super Contributor
timestamp: January 02, 2020, 04:12:35 am
content: 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

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: January 02, 2020, 06:10:54 am
content: Output resistance would depend upon the feedback circuit used. That's why most of them do not specify this parameter.

author: ebastler
author_rank: Super Contributor
timestamp: January 02, 2020, 10:57:15 am
content: 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.

author: Circlotron
author_rank: Super Contributor
timestamp: January 02, 2020, 11:31:49 am
content: Quote from: ricko_uk on January 02, 2020, 03:43:45 amhttps://en.wikipedia.org/wiki/Ceramic_capacitor#Microphony

author: TimFox
author_rank: Super Contributor
timestamp: January 02, 2020, 02:24:29 pm
content: Quote from: ebastler on January 02, 2020, 10:57:15 amThis is true for op-amp feedback amplifiers driving each other. In simpler circuits, such as capacitor-coupled common-emitter (or vacuum tube) stages, the output resistance of stage 1 may be important. In general, both resistances enter into the equation for low-frequency cutoff.

author: David Hess
author_rank: Super Contributor
timestamp: January 02, 2020, 10:28:01 pm
content: Besides setting the low frequency cutoff, a low value of capacitance will introduce distortion if a ceramic capacitor with high voltage coefficient of capacitance is used or an electrolytic capacitor. This is because when the voltage across these capacitors changes significantly, then the capacitance does also. So to minimize distortion if these capacitor types are used, the capacitance must be much larger than required for the low frequency cutoff.Some electrolytic capacitors can be noisy. Poorly made metalized paper or film capacitors can be noisy because of short clearing. Sometimes circuits which must have guarantied low noise use metal foil capacitors. But in general noise is not something to worry about in capacitors.

author: TimFox
author_rank: Super Contributor
timestamp: January 02, 2020, 10:33:52 pm
content: Also, electrolytic capacitors can have substantial leakage current into a high load resistance, which can shift the dc operating point of the second stage, and may itself be noisy.