# Circuit Analysis for tuned amplifier

#### Casablanca

Joined Oct 27, 2009
11
Hello ,,

I have this tuned amplifier (the one attached below )
the question is how to find V tank using circuit analysis ..
thie thing which is confusing is the varying current !

i'll appriciate any hint or help ..

thnx

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#### PRS

Joined Aug 24, 2008
989
I've got to confess, right out, that I've never seen a varying current in a circuit like this. I suspect it chops the output. To find out use Spice to simulate the circuit.

As far as the rest of the circuit, you've got a tuned common base amplifier. The RLC circuit at the collector is first solved for its center frequency. This is given by:

fc = 1/(2*Pi*sqrt(R*C))

Check if this is the same frequency as your sine wave input. It should match, otherwise the solution is even more complicated than it appears. The impedence seen by the collector should be the parallel combinaton of the R, L and C in parallel with ro of the transistor and this ro should be around 100 Kohms. Once you've found this impedance, Z, the gain should be Av = gm*Z.

This will give an amplitude for Vout. And, if it were not for the current source at the emitter, this would be the straightforward solution. However, the amplifier seems to be turned on and off by this current source. Subsequently, just draw vertical lines though your output diagram and erase any output for the times when the transistor is off.

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