I can't believe one lousy diode can dictate sensitivity

Discussion in 'General Electronics Chat' started by mik3ca, Mar 12, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    I did a little experiment on my receiver today.

    When it worked it's best, I used Harry's Homebrew power amplifier (from 3 transistors) with the bootstrapping enabled, and the two diodes set to 1N4007's. The URL to the amplifier is at:


    When I changed them to only 1 1N4148 diode, I lose remote stations.
    So I took it upon myself to CUT the diode, making that section an open circuit. So what happens? the volume is louder, which is nice, the remote station tries to pull itself in while mixing with a local station, but it fails. If only I can separate the two stations, then I would be happy.

    I'm wondering if I should go along with 3 or 4 diodes in series, or use my 1 fat 1N5408 diode instead.

    It seems that the power amplifier is what now determines the quality and the distortion of the output.

    If all else fails, I might end up reverting back to the 1N4007's.
  2. Audioguru

    New Member

    Dec 20, 2007
    Each output transistor has about 0.7V base-emitter voltage drop. So you need a total voltage of about 1.4V between the two bases or the amplifier will have severe crossover distortion.
    Crossover distortion is caused when one transistor turns off by the signal but the other transistor does not turn on when it is needed to make the output signal waveform continuous and smooth.

    Two diodes are nearly the same (1.4V) as the two base-emitter diodes so they almost completely eliminate crossover distortion.

    I simulated an audio amp in this forum with two diodes to reduce crossover distortion. I adjusted the current in the diodes so it is almost the same as the current in the two base-emitter diodes. Then the crossover distortion was zero.
    I also showed the severe crossover distortion when only one diode is used.

    I don't think an audio amplifier would work if the diodes are cut. The NPN transistor would be turned on hard and would be a short to the signal and the PNP transistor would be saturated close to ground.
    Maybe you have the pins on the transistors connected backwards?

    With more than two diodes then the transistors operate in class-A with a very high wasteful current and they and the battery or power supply will get very hot.

    Look on the datasheets for big fat diodes and skinny little ones. Their forward voltages at low currents are nearly the same.

    I think that since your radio does not have low value ceramic disc supply bypass capacitors with low impedance at high frequencies that are needed in any high frequency circuit, then the audio amplifier also amplifies RF and affects the radio.

    You keep talking about losing remote stations. A good FM radio has enough gain to pickup remote stations at the same volume as local stations. AM radios have automatic-volume-control because weak stations have much less amplitude (volume). Your super-regen is an AM radio without automatic-anything.