Hi,
I would like to ask , is it bad if a transistor's base is driven with negative voltages(up to -10V) in amplifier circuit , when amp is clipping ?
What maximum negative base voltage is allowed for BC547 ?
Best Regards,
Lukas.
I would like to ask , is it bad if a transistor's base is driven with negative voltages(up to -10V) in amplifier circuit , when amp is clipping ?
What maximum negative base voltage is allowed for BC547 ?
Best Regards,
Lukas.
If you look at the datasheet, you have 3 voltage ratings:
Vce (collector-emitter) - basically the max device voltage
Vcb (collector-base)
Veb (emitter-base)
Vce (collector-emitter) - basically the max device voltage
Vcb (collector-base)
Veb (emitter-base)
Thanks.
I understood that i should limit max. negative base voltage to -6V (transistor's max. Vbe is 6V).
Regards,
Lukas.
I understood that i should limit max. negative base voltage to -6V (transistor's max. Vbe is 6V).
Regards,
Lukas.
I am not sure. Just because the limit of base referred to emitter is 6V, does not mean that it can also withstand that much in reverse bias. At least I think.
In general, you don't want to allow the base emitter to go into reverse breakdown. I'd clamp it lower than the expected reverse breakdown voltage.
That's why the 2SC2878 muting transistor is special. It is designed for high reverse emitter base voltages, and they still fail. They tend to get leaking in funny ways that test good.
-Chris
That's why the 2SC2878 muting transistor is special. It is designed for high reverse emitter base voltages, and they still fail. They tend to get leaking in funny ways that test good.
-Chris
The B-E junction will zener at about 7V (in an NPN) if driven negative. When that happens, the current needs to be limited to a few mA or damage will occur.
Jan Didden
Jan Didden
Interesting. What about the EF Type II output stage where the driver can reverse bias the output? They seem to survive OK - lots of Leach amps and AKSA amps and others and you never hear of them breaking.
richie00boy said:
Well, first of all, drivers and output transistors are more robust so probably they can survive higher reverse currents - the outputs for sure. Also, if they are driven by a Vas stage, the current is also inherently limited which may save them.
Jan Didden
Hi,
Vbe max is the limit when driven into saturation in the normal direction.
I would treat that as an absolute limit unless the manufacturer is more forthcoming. Maybe 70% of absolute for normal operation when saturated.
The reverse (-Vbe for NPN and +Vbe for PNP) voltage will be different and could be much smaller.
How about adding a parallel diode to short the reverse current past the junction and thus produce a volts drop across the preceeding resistor.
Vbe max is the limit when driven into saturation in the normal direction.
I would treat that as an absolute limit unless the manufacturer is more forthcoming. Maybe 70% of absolute for normal operation when saturated.
The reverse (-Vbe for NPN and +Vbe for PNP) voltage will be different and could be much smaller.
How about adding a parallel diode to short the reverse current past the junction and thus produce a volts drop across the preceeding resistor.
AndrewT said:
The reverse is normally around -7 (NPN) for zenering to occur. But it is a very noisy zener.
There was a time when reverse biased B-E junctions were used as noise sources in test equipment.....
The forward is normally not a problem as long as the base current is not exceeded. What will happen is if you increase normal Vbe is that the Vc will get lower and lower until at saturation Vce will be just a couple of mV (in a small signal transistor). Actually, base current will then start to flow directly into the collector because the B-C junction becomes forward biased.
Jan Didden
The B-E junction of bipolar transistors acts like a zener diode when it's reverse biased. This means that it also has its own power rating and pulsed SOA curves like zeners, that unfortunately are not published in datasheets. In other words, it's excess dissipation or excess peak current what will damage a reverse biased B-E junction and not just the negative voltage itself.
By the way, I have tested some NPN switching power transistors to survive up to -1A of pulsed B-E breakdown current or -100mA DC without damage (no changes in Vbe or hFE happened). Small signal transistors seem to be quite fragile and easy to damage, though.
By the way, I have tested some NPN switching power transistors to survive up to -1A of pulsed B-E breakdown current or -100mA DC without damage (no changes in Vbe or hFE happened). Small signal transistors seem to be quite fragile and easy to damage, though.
So , as i understood , it is the most important to keep base current low ; the negative voltage itself is not a problem ?
Hi Bazukaz,
I wouldn't place money on that. Avoid reverse bias if you can.
I have seen many signal and muting transistors go bad over the years. So there may be a percentage that may become defective.
-Chris
I wouldn't place money on that. Avoid reverse bias if you can.
I have seen many signal and muting transistors go bad over the years. So there may be a percentage that may become defective.
-Chris
Bad Ju Ju...
You can't limit the current; because you don't know what to limit it to. There are some BE junctions (gold doped maybe?... i can't remember) that are more fragile then others.
Manufcacturers don't provide data because you're not supposed to do it. Limit the voltage you apply... a 3.3V zener in series with silcon rectifier does the trick. You need the silicon rectifier to burn some voltage so you don't forward bias the zener during normal transistor operation.
😉
You can't limit the current; because you don't know what to limit it to. There are some BE junctions (gold doped maybe?... i can't remember) that are more fragile then others.
Manufcacturers don't provide data because you're not supposed to do it. Limit the voltage you apply... a 3.3V zener in series with silcon rectifier does the trick. You need the silicon rectifier to burn some voltage so you don't forward bias the zener during normal transistor operation.
😉
Hi,
Thanks for support.
I have two choices : (1) add a diode from emitter to base ; or (2) add a voltage divider before base , so base negative voltage would be limited to around -5V.
In the first case, i'd need additional resistor to prevent previous stage(opamp) from drawing excess current.
Regards,
Lukas.
Thanks for support.
I have two choices : (1) add a diode from emitter to base ; or (2) add a voltage divider before base , so base negative voltage would be limited to around -5V.
In the first case, i'd need additional resistor to prevent previous stage(opamp) from drawing excess current.
Regards,
Lukas.
Bazukaz said:
A diagram might help. Maybe it is already self-limiting? You have a base series resistor?
Jan Didden
I would delete R34, it wastes half your drive voltage. A diode from E to B probably is best.
You could also put a diode in series with the protection zener on U1 so that the transistor E can swing down to the - supply which would avoid the problem altogether, but I don't know what the allowed neg voltage on the FET gate is.
If you can increase R33 to limit opamp saturation that would also be good.
Have you simulated this? How do the curves look for Vbe?
Jan Didden
You could also put a diode in series with the protection zener on U1 so that the transistor E can swing down to the - supply which would avoid the problem altogether, but I don't know what the allowed neg voltage on the FET gate is.
If you can increase R33 to limit opamp saturation that would also be good.
Have you simulated this? How do the curves look for Vbe?
Jan Didden
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