Substituting Germanium transistors for Silicon

Discussion in 'Digital Circuit Design' started by MountainGuy, Jul 15, 2017.

  1. MountainGuy

    Thread Starter Member

    Jan 15, 2017
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    I was wondering if anyone could help me locate a way to find substitutions for the following germanium transistors. I'd like to convert a circuit to silicon if possible. I have SK3009, 2N2428 and 2N2431 transistors. Any help would be appreciated. Thanks.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    It depends upon the circuit.
    Please post a schematic.
     
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Your problem will be the base bias for the transistors, Germanium is 200mV, whereas Silicon is 700mV, do you have a circuit diagram?
     
  4. ian field

    AAC Fanatic!

    Oct 27, 2012
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    With split secondary audio driver transformers - its too easy. Most had simple resistor divider for each base bias, but using a bias diode of the appropriate material was a worthwhile improvement - especially if you gave the diode thermal contact with the transistor heat sink.

    There are a few general circuits that accommodate the same modification, but usually the bias divider needs to be re calculated.

    Germanium transistors were usually seriously leaky - silicon; not so much. Once or twice I've had to change the resistor in the emitter self bias network to trim the operating point.
     
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  5. MrAl

    Distinguished Member

    Jun 17, 2014
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    Hi,

    One of the main concerns here is the operating voltage. If this runs off of a 1.5v battery then it maybe hard to get it to work properly with silicon unless the circuit does not have to start up at a very low voltage. Sometimes it works sometimes it doesnt.

    Back about 10 years ago i suggested looking into using a germanium transistor for a boost converter that has to start up at a little as 0.5 volts DC. That was meant for consumers that wish to have their flashlights start up even with a very low battery in a critical application such as for a caving light (cave explorers above and below water). There are some problems that come up though, but then i read that there was research being done on hybrid transistors that are part germanium and part silicon. If they are on the market by now maybe you could check into that. Otherwise perhaps a germanium to start the circuit and then bootstrap to keep the circuit going.
     
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  6. ian field

    AAC Fanatic!

    Oct 27, 2012
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    AFAICR: germanium devices have a higher dynamic resistance, Vf & VCEsat are a bit more elastic than silicon.

    There's a German manufactured boost converter disguised as a SMD transistor - there was one free on a long back issue of Elektor magazine. Its primarily for single cell white LED applications and micropower PSUs. But it demonstrates sound principle. Its something like 0.7 or 0.9V minimum start up. Very foggy distant memory thinks its a PR4401 or something like that. Generally; blocking oscillator type Joule thief type circuits are the sort of thing that's easy to find.
     
  7. MrAl

    Distinguished Member

    Jun 17, 2014
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    Hi,

    Yes the germanium transistors have some undesirable qualities, but they can allow starting a boost converter even at 0.5 volts. Once the converter starts, the germanium transistor is no longer needed so it goes back to working on silicon. So it's used to start the circuit and then bootstrapping keeps it running without the need for the germanium transistor.
     
  8. recklessrog

    Member

    May 23, 2013
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    In the early days of replacing Germanium transistors with Silicon, as a rule of thumb, was to double the value of the emitter resistor to bias them correctly. Works 9 times out of ten.
     
  9. MountainGuy

    Thread Starter Member

    Jan 15, 2017
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    That would be worth trying. So I guess I could just try to get transistors that could handle the voltage and swap.
     
  10. ian field

    AAC Fanatic!

    Oct 27, 2012
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    If you're bootstrapping a silicon topology switcher - the most likely is a blocking oscillator Joule thief. They usually work better with no emitter resistor at all.
     
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