# Engineering Small-signal input and output resistances of the circuit

#### stevo2393

1. With the circuit provided determine the small-signal input and output resistances of the circuit.

2. I was either given or found all the following info for the circuit:

VA = 110V

β = 115

T = 300k

quiescent base current is 12.0 microA

R1 = 12 kOhms, R2 = 6 kOhms and Rc = 2 kOhms

Ic = 1.38 mA, IE = 1.392 mA and IB = .012 mA

3. Now I am stuck on how to find the small signal info. This is what I am thinking though:

Would I change it to a small signal circuit to find the info? Then find input current and divide it by Vi?

Am I going down the right path?

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#### rude man

Homework Helper
Gold Member
1. Would I change it to a small signal circuit to find the info? Then find input current and divide it by Vi?
That would be one way to do it.

You realize this is a terrible design since it's overwhelmingly dependent on beta. For example, Rf is not a feedbak resistor. It should have gone from the collector to the base. Also, the computed Vcc is only about 0.84V to get ib = 12uA. I have no idea what you meant by VA = 110V.

I have computed the approximate input & output impedances but you probably don't want those. I'll keep tabs on the thread to see what others come up with & possibly offer comments.

#### NascentOxygen

Mentor
Would I change it to a small signal circuit to find the info? Then find input current and divide it by Vi?

Am I going down the right path?
After determining Zin of the transistor model, you have R2 in series with it. Then R1 in parallel.

(Ditto for query on VA?)

I would have expected R1 to be some MΩ, and rude man is right to be suspicious about the figures you provide.

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