Transistor beta drops very low

Thread Starter

sjgallagher2

Joined Feb 6, 2013
131
Hello all,
I was testing an old amplifier circuit I'd built a few years ago, since I'd never characterized it and was curious how it performed. It was seemingly normal, a bootstrapped common emitter amplifier with a 2N3904. But that was only looking at the gain and phase between input and output. I hadn't thought to remeasure the operating point, but when I simulated with SPICE, the gain in the amplifier was markedly higher. So I checked, and sure enough, the DC operating point was way out of wack. There is a voltage divider, with a series resistor going to the base of the transistor, and this series resistor (10k) was seeing a current of 46uA, as compared to the emitter current of 300uA. Yes, that's an h_FE of six. That would explain why the amplifier wasn't behaving as expected, but here's my question: Why would a transistor behave normally, but see such a dramatic drop in beta? The circuit still functioned, everything was normal, it was just the base current that was too high and was dropping the gain by a few dB. Is this a normal failure mode for a BJT? I don't think I've come across such a thing before. If anyone really wants to see the circuit to make sure it's not my own error, I'll attach a schematic.

Thanks in advance for any insight. Sam.
 

dl324

Joined Mar 30, 2015
10,713
Why would a transistor behave normally, but see such a dramatic drop in beta? The circuit still functioned, everything was normal, it was just the base current that was too high and was dropping the gain by a few dB. Is this a normal failure mode for a BJT? I don't think I've come across such a thing before. If anyone really wants to see the circuit to make sure it's not my own error, I'll attach a schematic.
It sounds like you have the collector and emitter terminals reversed and the transistor is operating in inverted mode instead of active.

Post a schematic and check the transistor connections.
 

GopherT

Joined Nov 23, 2012
8,012
Hello all,
I was testing an old amplifier circuit I'd built a few years ago, since I'd never characterized it and was curious how it performed. It was seemingly normal, a bootstrapped common emitter amplifier with a 2N3904. But that was only looking at the gain and phase between input and output. I hadn't thought to remeasure the operating point, but when I simulated with SPICE, the gain in the amplifier was markedly higher. So I checked, and sure enough, the DC operating point was way out of wack. There is a voltage divider, with a series resistor going to the base of the transistor, and this series resistor (10k) was seeing a current of 46uA, as compared to the emitter current of 300uA. Yes, that's an h_FE of six. That would explain why the amplifier wasn't behaving as expected, but here's my question: Why would a transistor behave normally, but see such a dramatic drop in beta? The circuit still functioned, everything was normal, it was just the base current that was too high and was dropping the gain by a few dB. Is this a normal failure mode for a BJT? I don't think I've come across such a thing before. If anyone really wants to see the circuit to make sure it's not my own error, I'll attach a schematic.

Thanks in advance for any insight. Sam.
In addition to the flipped connection mentioned above, check the DATASHEET of your output transistors. The beta on many transistors drop slightly at higher current(and temp).
 

Thread Starter

sjgallagher2

Joined Feb 6, 2013
131
Ah I would believe that the transistor had been flipped, it would seem to make the most sense. Still operating, but not operating as expected. As well, it was assembled some time ago, and I hadn't checked the pinout today assuming I had assembled it properly; now I doubt it. The transistor was replaced in any case, so it is no longer a problem, and I can't verify whether or not it was flipped as it went right in the bin. Thanks dl324 for determining the solution.
 
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