# How to measure the output impedance of this cct

1. Aug 5, 2011

### zak8000

hi

i want to find the output impedance of the following cct so i can perform pi matching on it but i dont know how to find it using lt spice please help!

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2. Aug 6, 2011

### zak8000

ugh well if i do it analytically and perform small signal analysis would it be correct to short cct the DC source and short circuit the coupling capacitors and also the capacitor in parallel with the base resistor and inductor. then the output impedance will be completely reactive and given by XL=2*pi*f*L1 which equals around +57j if i put in the values, is this correct?

Last edited: Aug 6, 2011
3. Aug 7, 2011

### yungman

It really depends on what level of difficulty you expect. You need to give a lot more info in what is your expectation. I am not familiar with SPICE, I do matching simulations with Microwave Office. But I am sure you need to input transistor parameters, input the parasitic components of the inductor and capacitor to get the answer.

Looking at your second, I assume you are looking for the more simple way. assume V2 is ideal voltage with zero output impedance, the output reactance is L1 parallel to C2 if you assume the collector impedance is very high and C4 is short circuited at operating frequency. Theoractically, the output impedance is very very high at the center frequency, but practical components and collector impedance is finite, then you need more info on the transistor( data sheet), the inductor(coil resistance) and capacitor( conductance or loss tangent) to determine.

At the more detail and rf level, this involve like half the book to answer, the output impedance mostly depends on the collector impedance instead of the network that supply the power.

This is a tuned amplified with max gain at $\;f=\frac 1 {2\pi\sqrt{L_1C_2}}$.

Last edited: Aug 7, 2011