# FM Transmitter explanation

Discussion in 'Electronic Basics' started by Default Name, Jul 16, 2004.

1. ### Default NameGuest

Hi everyone,

I have been breaking my head over some simple circuit for a couple of
days now. The circuit can be found at
http://www.zen22142.zen.co.uk/Circuits/rf/2bjttx.htm.

It is a simple, two-stage FM transmitter. What I would like is some
simple explanation on how the circuit works in the oscillating section
(2nd stage). I have 2 years of study in EE but I can't figure it out
by myself (teachers aren't too keen on actual real-life circuits so I
feel like a beginner here).

Here's what I understand (correct me if I'm wrong):

The first stage is simply a common emitter amplifier. The second stage
is a Colpitts oscillator. I understand how the inductance and
capacitors oscillate to the modulation frequency but I have trouble
seeing how the frequency will change in proportion to the audio signal
applied at the base of the transistor.

I read it is because of variable base-emitter capacitance at high
frequencies but I would like more precise explanations.

Could anyone help?

Thanks!

Richard

2. ### Michael BlackGuest

The audio stage is directly coupled to the base of the oscillator stage.
Hence the bias of the oscillator is varied at an audio rate. Since any
shift of the bias on that oscillator will shift it's frequency, this means
that you get FM when you talk into the microphone.

Michael

3. ### Kevin AylwardGuest

To clarify.

There is an internal base emitter diffusion capacitance. This
capacitance is a function of the emitter current, i.e. Cbe =
40.Ic/2.pi.ft. This capacitance directly effects the oscillation
frequency.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

4. ### Watson A.Name \Watt Sun - the Dark Remover\Guest

Er, you mean, 40 . Ic / (2 . pi . ft) ????

5. ### Kevin AylwardGuest

Well, yes. It needs the brackets.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.