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# Simple RC circuit interview questions

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#### bsrivastava

##### Member level 2
All,
I am not clear in solving such types of questions which are usually asked in the interview-
1) unit step input given to RC circuit
2) what if another R is added in parallel to R
3) what if R is added in parallel to C
4) instead of unit step if a pulse is given when the swing is -V to V volts

I don't want any Laplace type of solution.

rc circuit

Hi, well i don't have all yourquestions but what i know i would like to share

2) what if another R is added in parallel to R
ANS: The resistance will drop below the least resistance, to calculate the equivalent resistance you can use (R1xR2)/(R1+R2)

3) what if R is added in parallel to C
ANS: for capacitance youo'll have to first calculate the impedance z usnig the formula 1/2piFC and then deal with this impedence like a resistance.

Best Regards

bode plot rc circuit

the answer to Q1 depends how R and C are connected. You'll either get a spike at the rising edge if the circuit is set up as a HPF (high pass filter). Or if they are connected as an LPF the rising edge will of the step will appear as a charging curve on the o/p. The RC time constant will dictate the charging time.

Q4 you'll see the same as Q1, except that the plots will start at -V instead of 0V. If connected as a HPF the DC on the o/p will not be present, thus instead of the voltage swinging from -1 to 1V it will swing from 0V to 2V.

rc circuit bode plot

you can refer to Razavi's books, it is basic question.

stabble rc circuit

I understand that standard RC low pass and high pass circuits are discussed in many text books.
One we discuss the variant of the same as adding R in parallel to C or C in parallel to R, I am not sure of answer in time domain.
As I mentioned, I don't want LT solution because it doesn't give good feel as in time domain and also, once the question is asked, no one gives the time to calculate the LT.
Thanks

simple hpf circuit

I f it is LPF, o/p levels will change fromQ1 to Q3.

rc circuit initial

you should feel these circuit, an analog designer needs many intuitive knowledge, also need to know many rule of thumbs.
For example: R parallel with R equals with R/2, the step respone is exponential type for the RC (for the other RC 1-exp), from the slope and the final value you can see the tau etc.
very important to feel these STC circuits, after the LC, RLC circuits, after the intuition to make from pole-zero chart a Bode plot,a time domain signal...
and from a complicated analog circuit (20-30 transistors) sketch the pole-zero map.
Believe me these things are very important.

an rc circuit in parallel solution

Yes its true to intuitvely think about these circuits, therefore I avoided any laplace transformation type of approach. But sometimes in intricate R and C combination it becomes difficult.
On digging it further, I came across OTC ( open ckt time const) is best way to simplify the problem. As you rightly mentioned, it will help in understanding pole-zero location and pessimistic BW of the amplifier.
The method has following simple steps;
1. Short the voltage source, open the current source
2.Open the other capacitors which have independent initial conditions
3.Look the Thevinin equivalent of Resistance sitting at the capacitor you are calculating TC.

I think, though the problem is simplified, but one has to intutively look at it like there will be zero, if there is direct path from input to output.

-Bharat

drabos said:
you should feel these circuit, an analog designer needs many intuitive knowledge, also need to know many rule of thumbs.
For example: R parallel with R equals with R/2, the step respone is exponential type for the RC (for the other RC 1-exp), from the slope and the final value you can see the tau etc.
very important to feel these STC circuits, after the LC, RLC circuits, after the intuition to make from pole-zero chart a Bode plot,a time domain signal...
and from a complicated analog circuit (20-30 transistors) sketch the pole-zero map.
Believe me these things are very important.

best book rc circuit

many of the problems can be solved when considering that an RC circuit reaches its stable solution after 4* RC. so if for example R is added in parallel, well, R decreases which means less time constant which means it takes more time for the circuit to become stable.

Cheers

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