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Very simple AF amplifier for simple radio

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neazoi

neazoi

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Hi I have built the top right preamplifier (2 transistors and a resistor) https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/Fun_Radio_W6WTU.gif for my receiver **broken link removed**
The amplification is low when the RX reflex switch is switched to off, but oscillation happens when the switch is switched on.

The AF amplifier is designed for 1.5v and the author uses a Germanium transistor for the final one. I have used a bc549c and a 2n2907 for the final and I power it from 12v.

Can you suggest me how can I stop the oscillation? I believe the input signal is too much with the reflex switched to on, but I would like to correct the amplifier rather than include an input attenuator.
 

That 'preamplifier' is actually the audio power amplifier but it will only drive a high impedance load and it has no DC stabilization. With 1.5K headphones and 1.5V supply it probably isn't a problem but the bias conditions are entirely depending on the transistor gains as the supply increases. I wouldn't recommend using that circuit but the LM386 equivalent looks OK.

Brian.
 

betwixt said:
That 'preamplifier' is actually the audio power amplifier but it will only drive a high impedance load and it has no DC stabilization. With 1.5K headphones and 1.5V supply it probably isn't a problem but the bias conditions are entirely depending on the transistor gains as the supply increases. I wouldn't recommend using that circuit but the LM386 equivalent looks OK.

Brian.
Click to expand...

Thanks Brian,
How about this one with the single 2n7000? It has a gain of 50 and with an input of 2mv it can output 108mv.
Isn't the mosfet going to give more power and driven by less?

I am looking for a discrete small amplifier for headphones preferably "standard" 32R or so.
simplicity must match the simplicity of my RX so you get an idea.

Another option is the second attachment with 100db of gain and the three BJTs. This is from the solid state design for the radio amateur book. However, it has far more components.
 

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  • 2mV to 108mV Gain-50 af amplifier.gif
    2mV to 108mV Gain-50 af amplifier.gif
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  • ampi100db.PNG
    ampi100db.PNG
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