Old transistor circuits

Discussion in 'General Electronics Chat' started by Llamarama, Jul 21, 2013.

  1. Llamarama

    Thread Starter Active Member

    Feb 1, 2010
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    Hello everyone,

    I've just been flicking through a few old transistor applications books and noticed most if not all used PNP transistors and negative power supplys, however the vast majority of circuits I've seen in British electronics books from the 70s to 90s have almost always used NPN transistors and standard power supplies.

    Why did early designs and circuits rely so heavily on PNP transistors, were they easier to manufacture of higher quality than early NPN devices? Of was this style (negative voltages with a positive ground) of circuit carried over from valve technology?

    Just noticed it when reading the GE Transistor Manual (2nd edition)

    Thanks for any insights - Llamarama :)
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I would imagine it involved the profit the General Electric Corporation could expect from the 2N187, 188A, and 190 PNP devices being greater then their equivalent NPN devices.
     
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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    ....Ahem you must have later books than mine, Mullard Reference Manual 1961.
    Almost all the designs include OC71, OC72, OC81 all NPN!.
    Here is one explanation, (not verified).

    'NPN is mostly used because electron current is greater than hole current in NPN transistor. In PNP transistor hole current is greater than the electron current.'

    Max.
     
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  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Well the melting point of silicon at 1414 is nearly 500 degrees hotter than that of germanium at 938.
    So there is significant extra cost processing silicon. So most early transistors were made of germanium.

    Early germanium transistors were made by the alloy or point contact methods, which produced pnp transistors. this was a lower temperature (=cheaper) non melting process which used diffusion.

    npn germanium transistors (and there were not many of these) were produced by melting the germanium and then successively doping it in the liquid to 'grow' transistors as the melt was allowed to crystallise.

    Attaching the leads created a manufacturing difficulty in the npn case and introduced an unwanted resistance into the 'ohmic contacts' at the collector, if it was n type.

    p type germanium material already forms the contact so does not suffer from this problem.

    So it was easier and cheaper to manufacture pnp then npn in the early days.

    All this changed round with the introduction of the planar (epitaxial) silicon transistor, and the world went npn.
     
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  5. MaxHeadRoom

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    Jul 18, 2013
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    Here's a bit of trivia on the OC71, I had one acquaintance that brought me his transistor radio and he mentioned that when he opened the back to replace the battery, the volume went right up.
    If you are familiar at all with the OC71 it has a black painted outer.
    The paint had flaked off or whatever?
    The transistor was now acting as a photo electric light source and which would increase the gain.
    Restoring the black covering eliminated it!
    Max.
     
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  6. Llamarama

    Thread Starter Active Member

    Feb 1, 2010
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    That certainly clears things up! I've heard about the OC71 trick, The story I heard was the early devices were exactly the same as the phototransistors, but in later ones Mullard filled them with a milky white goo (I'm guessing petroleum jelly or something) so the effect couldn't be taken advantage of as they charged more for the phototransistor. I did accidentally break one and can confirm the later ones are full of goo.

    Thanks for the information, I can now sleep soundly :)
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Right.. I had forgotten about that!
    Different century.
    Max.
     
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