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What's the deal with old Ge transistors?

Discussion in 'Electronic Design' started by [email protected], Jul 20, 2003.

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  1. Guest

    Anyone have any tips or websites for biasing these things for audio
    use? I recently got a bag of old PNP Ge transistors and began playing
    around with them. I was expecting to be able to just pop them into the
    old standard PNP circuits I'm used to. But these things demand some
    unusual biasing to run in linear range.

    Not only that, but they seem to drift like crazy with temp.

    OK, I know these things are old and obsolete, etc. I'm not a EE or
    elect. tech, just a hobbyist, wondering how in the world people ever
    were able to design with these things. I'm sure this may be obvious to
    many of you, and lots of you could even recall using them. But these
    thing have me stumped. Any insights, websites, sources of knowledge to
    help me understand why these things work the way they do?
     
  2. Robert Baer

    Robert Baer Guest

    1) The Vbe of Ge transistors is about 350mV or roughly half of Si
    transistors at about 680mV.
    2) Ge transistors are rather "leaky", similar to putting a resistor from
    emitter to collector in a Si circuit. And every Ge transistor is
    different.
    3) For a stable, linear DC operating point (Ge or Si for that matter),
    use biasing so emitter resistor and collector resistor drop about 1/3 of
    the total supply voltage - making the Vce about 1/3 of the total supply
    voltage.
    Item #3 is a crude "rule of thumb" that can give a good starting
    point.
    But like any rule, it can be violated to good effect in special
    applications.
     
  3. That's why general purpose Ge transistors are obsolete. They're leaky
    and temperature sensitive. Circuits usually had capacitors on the
    emitter to reduce the DC gain.

    (-)
    |
    R
    |
    +------
    |
    C
    ----B
    E
    |
    +---
    | |
    | R
    | |
    R C
    | |
    +---
    |
    (+)

    The BE junction doesn't have the sharp half-volt conduction knee of a
    silicon junction. It's a gradual curve. A low voltage, low impedance
    signal that drives a silicon transistor will not work the same way with
    a germanium transistor.

    Another feature is that big germanium transistors are slow. The ECG27
    specification shows an hFE of 120 that degrades to unity at only 2000
    Hz. Audio power transistors max out in the 10KHz to 22KHz range.
     
  4. ISTR that the other major difference with Ge's is that their thermal
    instability is mediated by the increase in temperature of the
    collector junction; whereas with Si's, the thermal problems arise from
    the contraction of the b/e barrier height potential. In both types the
    effects are self-reinforcing and give rise to destructive thermal
    runaway if not compensated for.
     
  5. Jim Thompson

    Jim Thompson Guest

    That is basically correct. Silicon's sensitivity is due to the
    sharpness/exponential characteristic of the B-E junction; Germanium
    has a grotesque *beta* sensitivity.

    I had a course in transistors around 1961 where the dominant
    discussion was about stable biasing of Germanium transistors.

    ...Jim Thompson
     
  6. Robert Baer wrote:
    (snip)
    A better model is a negative temperature coefficient thermistor
    between collector and base, because the gain of the transistor
    amplifies this temperature dependent leakage.

    (snip)
     
  7. N. Thornton

    N. Thornton Guest

    Hi

    And finally they're very temperature sensitive in another way too.
    IIRC the germanium can survive upto about 90C, then its gone. So
    direct soldering is a bit iffy if you use the long long leads they
    come with, and if you solder them like silicons they dont have much
    hope of working. The standard advice is long long leads plus a
    heatshunt, aka pliers.

    They do have their uses and are still sold, but certainly are not
    common.

    Regards, NT
     
  8. James Meyer

    James Meyer Guest

    Zeners are used (usually) in a reverse biased manner. There is no
    reasonable scale that you can chose that will convert a zener's reverse curve
    into a straight line.

    Jim
     
  9. Jim Thompson

    Jim Thompson Guest

    I was at Motorola SPD in the '60s so my view is biased... I built
    audio amps using RF grade power transistors ;-)

    But I think Ge power held its own against Si until the early '70s.

    I was still using Ge diodes in the mid '80s for low drop situations,
    instead of (then) expensive Schottkys.

    ...Jim Thompson
     
  10. Ban

    Ban Guest

    As others have said, there are very pronounced thermal dependencies. At
    constant temperatures we get kinda these curves. Same with transistors. that
    is why a Si-semiconductor makes a bad switch, because you always have an
    offset.
    ^
    Log(I) | /Ge /Si
    | / /
    | / /
    |/ /
    100nA/ /
    | /
    = =
    0--/------------>
    35-200mV Vf(25°)
    right, but there must be a knee in the curve in the 3rd quadrant, where
    avalanche/zener effects are occurring.

    ciao Ban
     
  11. Once I needed a good low voltage zener function.
    I measured and compared different alternatives by plotting them in a
    logaritmic diagram.

    I used a low voltage zener (2V at 1mA), red and green leds, and 2 Si
    diodes in series. So all the components were in the 1-2 Volt range.

    All of the alternatives became straight lines in the diagram.

    The best low voltage zener functions were the leds, the lines for them
    had the least angle, that is they had the most constant voltage over
    several decades of current.

    So you are wrong, the zener in reverse mode also has a logaritmic
    response, which becomes a straight line in a diagram like the one
    shown in the earlier message in this thread, only the angle of the
    straight line is different.

    Try it yourself if you don't believe it, plot log I against voltage
    like in the diagram Mouget showed earlier in the thread.

    I used 4 decades of current, logaritmically spaced as equal distances
    for 10uA, 100uA, 1mA, 10mA.
    The other scale is linear, 1 volt, 2 volt, 3 volt, etc..
     
  12. Robert Baer

    Robert Baer Guest

    Excuse me, but a capacitor *cannot* change DC gain!
    Furthermore the capacitor would *increase* the AC gain above a certain
    frequency.
     
  13. Give me plotted values and I will believe you.

    Give me the voltages for 5uA, 50uA, 500uA and 5mA.
     
  14. Very crude, and a bad starting point which is never required in a common
    emitter amplifier. I have never even seen a circuit with this much bias
    across the emitter resistor.
    In most cases this rule is never used. Never seen it in around 30 years.

    What matters is the *absolute* emitter voltage, not relative to the
    supply. There is usually no need in a common emitter amplifier to run
    say, 10V supply * .333 = 3.3V. Sure, it will work, but you will lose
    headroom.

    A volt across the emitter resistor is usually enough. Suppose the base
    voltage is fixed at say, 1.7V. This would give a nominal 1 volt across
    RE for vbe=0.7. Now suppose, that the vbe was 0.6, this would give 1.1V
    across RE. This is only a 10% change in the emitter current, in contrast
    to no emitter resistor whaer the same change would give 50 times more
    current.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  15. Very crude, and a bad starting point which is never required in a common
    emitter amplifier. I have never even seen a circuit with this much bias
    across the emitter resistor.
    In most cases this rule is never used. Never seen it in around 30 years.

    What matters is the *absolute* emitter voltage, not relative to the
    supply. There is usually no need in a common emitter amplifier to run
    say, 10V supply * .333 = 3.3V. Sure, it will work, but you will lose
    headroom.

    A volt across the emitter resistor is usually enough. Suppose the base
    voltage is fixed at say, 1.7V. This would give a nominal 1 volt across
    RE for vbe=0.7. Now suppose, that the vbe was 0.6, this would give 1.1V
    across RE. This is only a 10% change in the emitter current, in contrast
    to no emitter resistor whaer the same change would give 50 times more
    current.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  16. N. Thornton

    N. Thornton Guest

    re geranium transistors... (yes they're made from geranium leaves :)


    So what sort of temps can trannies like OC71 and AC127 take?

    And what apps are they still used in? I think I've only used them twice in design.

    Regards, NT
     
  17. Rob Judd

    Rob Judd Guest

    Not only that, but if you get the ones painted in black and scrape it
    off, they make a great ligh-dependent transistor!

    OC26's from distant vague memory.

    Rob
     
  18. Then there are no good zeners for voltages below about 4.8 volts.
    Some other technology is needed for the characteristic you describe at
    those voltages.
     
  19. Jim Thompson

    Jim Thompson Guest

    Below ~5V is where the true Zener effect occurs.

    Between ~5V and ~7V you have both Zener effect and avalanche
    multiplication... producing a relatively low TC.

    Above ~7V it's all avalanche multiplication.

    See, for example:

    http://www.wia.org.au/armag/2002/AR_Oct02_Zener_diodes.pdf

    ...Jim Thompson
     
  20. Rob Judd

    Rob Judd Guest

    Ergh, my memory finally came back. OA70's of course. The OC26 had a tin
    hat.
     
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