Transistor Amp

Discussion in 'The Projects Forum' started by kd5vmf, May 12, 2014.

  1. kd5vmf

    Thread Starter New Member

    Apr 25, 2014
    This circuit is my first attempt at a four transistor MW RF preamp or transmitter amp. I'm not sure where I would put my low pass filter and if R5 is correct to control gain.
  2. DickCappels


    Aug 21, 2008
    To reduce intermodulation (when one signal affects another one on a different frequency) you should put a filter directly on the input. An simple LC low pass filter might be what you want.

    Your amplifier will also be a low pass filter,you can just place small capacitors across the feedback resistors. The -3db frequency will be when Xc = R. You will get -3db/octave X number of stages.

    If you don't add capacitors collector to base capacitance will make your amp roll off at less than 200 kHz.

    R5 will act as a gain control by varying the emitter current and thus the emitter resistance if you eliminate the AC feedback between the base and collector (split the 220k into two 100k resistors and bypass the point where the transistors connect to each other with a RF bypass capacitor large enough to greatly attenuate the frequencies you want to pass.

    By the way, given that this is a low frequency amplifiers, you might find that you will get much more gain with inter-stage coupling capacitors larger than the 10 pf shown on the schematic.