Quote:
Originally Posted by Radio Wrangler
Extra caution is needed. Reverse bias avalanche of a PN junction - especially a base-emitter junction can do damage even before the smoke release point. For transistors, you'll find a progressive reduction in gain and an increase in noise factor. Not as dramatic, but still worth avoiding. Maybe the smoke stays in, but gets contaminated?
David
|
One thing about this the effects are seldom seen but in two places I know of.
The audiophiles complain that in their amplifiers with BJT based IC's, with coupling capacitors between stages, that sometimes with power cycling the IC's get degraded over time as from time to time the B-E junctions of the input devices in the IC get degraded. Sometime they recommend replacing all the IC's in an amp for this reason. I have never been able to hear the effect of this myself.
The other example that happens is in the simple common 2 transistor multivibrator circuit. If it is powered by more than 7V the B-E junctions Zener on each half cycle , it's easy to see on a scope on the capacitor's charging waveform at the base of the transistor. This does relentlessly damage the transistor and after some time the multivibrator will not start as the transistor's hfe gets progressively lower. This is an effect I have seen in an instrument.
Motorola wrote a paper on the effect of hfe degradation of transistors after zenering their B-E junctions, as I recall the exact mechanism of it remained speculative.
I think there is an application for it though. If two transistors require an exactly matched hfe, the one with the higher hfe could be treated with some pulses of breakdown to lower it until it exactly matched the other transistor.
Of course it doesn't matter if an LED or some diode gets zenered, as long as the dissipation is low, because there is no hfe issue to contend with. The big evil of it is for a power supply rectifier because the circuit impedances are low, the breakdown voltages are high and the reverse breakdown currents are high, so the device goes boom when a voltage spike comes along and its piv gets exceeded.