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Germanium Transistor Gain?

B

Bill Bowden

Jan 1, 1970
0
I'm trying to fix the oscillator/mixer section of an
old AM transistor radio. All the voltages and parts
look ok, but it won't oscillate. It amplifies pretty well
since a generator signal can be seen across the oscillator
coil and tuned to a sharp peak. If I play with the bias
voltage, it will oscillate near the top of the band, but dies
out when the frequency is lowered. I took out the germanium
transistor and measured the gain at 600 on a DMM. Is this
reasonable for a germanium transistor, or is the DMM being
fooled by the low junction voltage?

-Bill
 
J

John Larkin

Jan 1, 1970
0
I'm trying to fix the oscillator/mixer section of an
old AM transistor radio. All the voltages and parts
look ok, but it won't oscillate. It amplifies pretty well
since a generator signal can be seen across the oscillator
coil and tuned to a sharp peak. If I play with the bias
voltage, it will oscillate near the top of the band, but dies
out when the frequency is lowered. I took out the germanium
transistor and measured the gain at 600 on a DMM. Is this
reasonable for a germanium transistor, or is the DMM being
fooled by the low junction voltage?

-Bill

It's more likely being fooled by the high leakage current
characteristic of germaniums.

John
 
B

Bill Bowden

Jan 1, 1970
0
John Larkin said:
It's more likely being fooled by the high leakage current
characteristic of germaniums.

John

Yes, hadn't thought of that. Leakage measures about
2 mA at 9 volts and base open. The leakage drops
to about 1/2 mA with 10K resistor added from emitter to base.
If I remove the 10K and use 250K from base to supply,
the current increases to about 10 mA, so looks like the gain
is about 8mA/36uA = 222

-Bill
 
F

Fred Abse

Jan 1, 1970
0
Not really true. Small signal ac, and large signal dc current gain are
both useful, and are indeed used all the time.

Measuring Ic/Ib at one point is neither

If the transistor is
putting out large currents, as in a push pull output stage, you want to
know its large signal hfe. How else will you know how much base current
you need to drive it with.
Since the OP was writing about a self-oscillating mixer, small signal
characteristics seemed appropriate.
 
W

Watson A.Name - Watt Sun

Jan 1, 1970
0
Measuring Ic/Ib at one point is neither

If the transistor is
Since the OP was writing about a self-oscillating mixer, small signal
characteristics seemed appropriate.

He also said the Iceo was 2 mA. That seems excessive, more like what
you'd see on a Ge power transistor. I would think a small signal Ge
would be at least ten times lower than that, max. Maybe try another
lower leakage Ge? And of course check the tother things mentioned.


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P

Pieter Hoeben

Jan 1, 1970
0
I'm trying to fix the oscillator/mixer section of an
old AM transistor radio. All the voltages and parts
look ok, but it won't oscillate. It amplifies pretty well
since a generator signal can be seen across the oscillator
coil and tuned to a sharp peak. If I play with the bias
voltage, it will oscillate near the top of the band, but dies
out when the frequency is lowered.
Hi, if the oscillation dies out at lower frequency's, I would check
the capacitors. Especially old elko's (only used for lower frequency's
of course) get bad. They probably aren't in your signal path, but
could be the decoupling. What frequency's are you talking about?

Regards,

Pieter Hoeben
 
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