I would like to build a FM radio reciever with PNP germanium transistors but I can not find a simple yet efiicient schematic for this. Does anyone know of a schematic I can use?

I have found one schematic that I've built, but it doesn't work. Maybe it's because I have used other transistors?

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  • \$\begingroup\$ Not "maybe" but very likely. FM is in the very high-frequency range, and using the right transistors is critical. Which models did you use instead of those indicated in the schematic? \$\endgroup\$ Mar 25 at 7:32
  • \$\begingroup\$ P423 old soviet transistor that I have pulled out of the oscillator stage of a radio. \$\endgroup\$ Mar 25 at 7:34
  • \$\begingroup\$ None of the transistors on that schematic are PNP Germanium transistors, and in any case few Ge transistors are any good at FM (Band II) frequencies. If you have Ge transistors to hand, I recommend building an AM band receiver first. \$\endgroup\$
    – user16324
    Mar 25 at 9:29
  • \$\begingroup\$ Do you want that old circuit that in not sensitive and has poor selectivity? It picks up all the interference and static as an AM radio because its detector is AM and it is missing audio pre-emphasis so it will even sound like an old AM radio. \$\endgroup\$
    – Audioguru
    Mar 25 at 20:19

1 Answer 1


Your "replacement" transistor would not work.

The P423 transistor has the following characteristics:

  • Material: Germanium
  • Transition Frequency (ft): 100 MHz
  • Collector Capacitance (Cc): 10 pF
  • Forward Current Transfer Ratio (hFE), MIN: 24

The BF324 from the schematic:

  • Material: Silicon
  • Transition Frequency (ft): 350 MHz
  • Collector Capacitance (Cc): O.1 pF
  • Forward Current Transfer Ratio (hFE), MIN: 25

As you can see, not only are they made of a different material which makes their base-emitter bias voltage different, but more importantly they have significantly different maximum frequency.
The transition frequency is the frequency at which a transistor has a gain of 1, meaning the frequency at which it stops acting as an amplifier, and after which it actually reduces the input signal.
Another important factor is a transistor's own inter-electrode (between its own electrodes/pins) capacitance, which can significantly weaken a high-frequency signal.
The original, BF324 has only 0.1pF collector capacitance, while your replacement has 10pF, which is 100 times higher! That will definitely make a difference at very high frequencies.

Your replacement transistor can't amplify FM signals because their frequency is too high for it, but the BF324 definitely can.
In high-frequency circuits, you should always check to see if the replacement transistor's maximum supported operating frequency is as high as, or higher than, the original, or at least close to it.

  • \$\begingroup\$ What do you mean by "collector capacitance"? Would that be Coes? \$\endgroup\$
    – Hearth
    Mar 25 at 14:41
  • \$\begingroup\$ @Hearth: It is hard to figure out exactly because various datasheets name it differently, but I believe it's the Cob, collector output capacitance. I have just quickly copied these specs from the All Transistors website, and that's how it was stated there. Checking the actual datasheets show Coe, so I'm not sure what to tell you. I couldn't find datasheet for the Russian transistor. \$\endgroup\$ Mar 25 at 17:02

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