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Germaniums with 4 leads: Why

Discussion in 'Electronic Components' started by c a l a n d e, Jan 12, 2004.

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  1. I've been salvaging transistors from some old AM/FM/SW radios and have a
    number of Hitachi 2SA234 and 2SA235 types.

    My limited research has told me these are high frequency transistor used
    in the RF mixer sections. But what I don't know is what's the purpose of
    the forth lead and what the pin-out is.

    Your help would be appreciated
     
  2. Dave Platt

    Dave Platt Guest

    www.nteinc.com says that the 2SA234 crosses to an NTE160. The latter
    is a PNP germanium (!) mesa transistor, designed for mixer and
    oscillator applications up to 900 MHz. The fourth lead appears to be
    a connection to the metal case - presumably this lead would be
    grounded in most applications.

    http://www.nteinc.com/specs/100to199/NTE160.html
     
  3. And of course it's not unique to germanium transistors. I can think
    of various silicon transistors with a fourth lead connected to the case.
    (Mind you, at least some metal cased transistors have the collector
    connected to the case.) It's surely the intended application, RF
    and maybe some other crucial uses, that adds that fourth lead,
    rather than the semiconductor used in the device.

    Though, from memory it does seem that fourth leads were fairly common
    in cheap radio transistors thirty and so years ago. I have no
    idea if that's because they figured it was needed, until they
    realized otherwise, or if it simply disappeard as metal case transistors
    became less common. Obviously, many/most transistors in those
    cheap radios been plastic cased for two or three decades.

    Michael
     
  4. Not sure about that particular transistor, but most all of the RF
    tranaistors of those years had a metal package, with the fourth lead
    connected to the metal package. So that lead is grounded, to shield
    the transistor from the rest of the circuit.


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  5. Thanks for the prompt response.
     
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