Electronic Circuits and Projects Forum

I'm having trouble with the analysis of this circuit

I understand (going from left to right) C1 takes the DC decoupled voltage from the Microphone, amplifies this via Q1, C2 decouples it again, then R6 and C3 Low Pass Filter it, but once we get to Q2, I am completly lost.

My question is: How does this circuit act as a colpitt FM oscillator. Most of them I've seen are directly attached to the base.

Secondly how does this circut actually accomplish frequency modulation: Please use math on me!!! (I know that it should change the delta F term in the equation cos(2*pi*fc*t + δF) to achieve FM, or more correctly: cos(2*pi*fc*t + δf*∫(messageSignal))

How can I determine the Frequency deviation of this circuit?

Please use Engineering terminology and Mathematical equations in your analysis: Im a Senior year EE.

Originally Posted by colin55

You may want a mathematical analysis of the circuit but can you explain the purpose of the 4p7 and 10n capacitors?

the 10n is prolly part of a LPF and the 4p7 is prolly part of the colpitt oscillator at the end.


I realize this is a horrible design becuase the Qpoints are terrible (biasing Q1 at .9 volts doesnt allow for much negative voltage)

Can you explain how it changes the capacitance, my semiconductor theory about BJTs is a bit rusty.

My goal is just to build a simple transistor level FM transmitter for my own amusement. More of an exercise on what I've learned as an engineer because I want my masters degree to be in Communications Systems and Signal Processing

I would also like to actually build it, for fairly cheap. So I'm looking at what I can get.

Could you provide me with places that have such clearly explained and defined schematics? It becomes rather difficult to find such high level designs. I would really rather work at the transistor level (i.e., no OPAMPs or ICs, including multipliers)

As u know that u can provide arbitory signal from microphone, when this signal reaches to the base of Q1, it triggers your output which you will get at its collector and the thus the voltage at the base of Q2 will change and according to this voltage variation Q2 will generate the output at its collector.

finally you can see as your base voltage varies your fm frequency varies and hence you get the frequency deviation.....

To see it clearly you should go for its simulation...