I don't know if questions can be asked like this here but if they can I want this question. In an exam paper I saw a question to determine the parallel value of the R1 and R2 resistors in the image below but I am not sure how to do this. I tried using the bias equations for a common emitter amplifier but I have not been able to get the answer. I put the exact question in the image below so you can see how exactly it was asked. I am only asking for question c as I have done the first two(I may or not be correct). You do not necessarily need to solve it but if you at least know formula or method I could use it would be very helpful.

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I think the question is confusing because of the way "X" is indicated in the schematic.

What they likely want to ask about is the input resistance seen by the source \$V_{in}\$.

First, assume the frequency is high enough that capacitor C1 can be ignored. Then, the input resistance of the circuit is essentially the resistance at "point X", if X were drawn pointing to the same node of the circuit, but somewhere to the left of where R1 and R2 connect to it.

That means the the input resistance of the amplifier is given by $$R_1 || R_2 || R_{in}$$ where \$R_{in}\$ is the input resistance of the emitter-degenerate common-emitter amplifier.

So you just have to work out what \$R_{in}\$ is (you have enough information in your schematic), and then find what needs to be put in parallel with that to get 10 kOhms.


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