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.
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.