Saturday, 14 September 2019

capacitor - Understanding this AC coupled circuit


I'm having a little trouble understanding this circuit and could use some help.


So far I understand that this is an AC coupled circuit, and the capacitor is a blocking capacitor. I also know that this is a high pass filter, but if I was to find the cutoff frequency, what resistance would I use? Would I simply use the 2k lower resistor or combine both resistors for a 4k resistor here?


Also what does this circuit do to an input signal? Is everything offset by +10V here, or does the capacitor block this so any signal from V(t) have an average voltage centered around 0?


enter image description here



Answer



DC circuit:


$V_{out}$ is just the output of a voltage divider, so you have 5V out. The $1 \mu F$ capacitor behaves as an open circuit, so you can ignore V(t) because it's unconnected.


AC circuit:


The 10V DC voltage source is shorted and the 1uF capacitor behaves like a "wire" (short-circuit or low impedance). So you have $V(t)$ in parallel with a 1k resistor: $V_{out} = V(t)$.



Thus, when considering DC+AC:


Vout=5V+V(t)


EDIT:


For the expression above to be an approximation good enough, you must check that the frequency of $V(t)$ is well above the cut-off frequency of the high pass section formed by the capacitor and the parallel of the 2k resistors, thus:


f12π1000106159.15Hz


(thanks go to Vladimir for pointing this to me in the comments)


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