FM Transmitter

Discussion in 'Wireless & RF Design' started by kazafken, Mar 19, 2009.

  1. kazafken

    Thread Starter New Member

    Jun 19, 2008
    My groupmates and I need help regarding a little problem we faced in building the amplifier part of the FM Transmitter.
    Actually, we don't really understand how does the bypass capacitor and coupling capacitor works.
    1. Accepts voice signal (300Hz - 3000Hz)
    2. Voltage gain = 100
    3. Our input = 20m V peak
    4. Desired output = around 2 V peak

    Here's the problem,
    Before (given value form the assignement):
    1. C2 = 10 uF
    2. C3 = 10 uF
    3. The output we get is not 2 Vp and the amplified is not constant from 300Hz to 3000Hz. When it's at 300Hz,the ouput is lower,vice versa for 3000Hz.

    1. We changed C3 = 1000 uF, C2 = 1uF
    2. The output is around 1.6 Vp and it's contant from 300 Hz - 3000Hz.
    3. this is the output we desred.

    Now,we don't understand why izit the the bypass and coupling (C2 & C3) affects the output??

    Below is our circuit (after):


    Vcc = 9V

    All of your help is much aprreciated,thank you. :)
    Last edited: Mar 19, 2009
  2. beenthere

    Retired Moderator

    Apr 20, 2004
  3. kazafken

    Thread Starter New Member

    Jun 19, 2008
    Thx,bro for the info.

    And now i faced a problem.

    I soldered the amplifier circuit into the PCB and then after a lot of testing with the oscillator(breadboard),it fails to transmit. anyway, we test the amplifier circuit again to check if it's working. unfortunately, the ampliefier circuit only amplify to 1 Vp and not 2 Vp when input frequency is 3k Hz.

    I was really frustrated as i'm back to square one again, and i can't move on to my oscillator circuit which is another headache.

    What cause this problem? Is it the transistor became faulty after soldering it to PCB?
  4. Audioguru


    Dec 20, 2007
    If the collector of the audio transistor has a capacitor to ground (which is used to ground the base of the common-base oscillator) then the capacitor reduces the high frequency response. Maybe the value of this capacitor is too high.

    An FM transmitter operates at a very high frequency where the parts layout and wiring must be very compact. Stray capacitance must be minimum. It won't work if it is built on a breadboard.
  5. PRS

    Well-Known Member

    Aug 24, 2008
    The coupling capacitor, C2, sees an equivalent resistance made up of your source's output resistance in series with the input resistance of the amplifier. f=1/(2*pi*R*C2). Since the source's resistance is usually small, R, in this case will be just the Rin of your amp. This f contributes to your f(low) 3db point. If you don't know how to calculate Rin, I'll let you know.

    The other contributer to your f(low) 3dB point is the bypass capacitor at the emitter, C3. Here again you use the same equation, this time you plug in C3 and the resistance it sees. This is the parallel combination of Re and R looking into the emitter. Use the reflected resistance technique to calculate the latter.

    Use R looking into the emitter to calculate your gain. Av=Total R the collector sees divided by the total R the signal sees at the emitter. Since the emitter resistor is bypassed by C3, the denominator will be R looking into the emitter.

    Probably one of the two f(low) points will be dominant. If not take the square root of the sum of the squares of both frequencies and that's f(low).

    Your amp's high frequency roll off is dictated by the transistor's internal capacitance plus stray capacitance. More on that if you want. You have very little control on this point, though. Just keep your leads short, don't daisy chain power leads.
    Last edited: Mar 24, 2009