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Old 30th May 2006, 02:42 PM   #1
Bazukaz is online now Bazukaz  Lithuania
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Default Transistor base negative voltage

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 ?

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Lukas.
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Old 30th May 2006, 02:49 PM   #2
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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)
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Old 30th May 2006, 02:58 PM   #3
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Thanks.

I understood that i should limit max. negative base voltage to -6V (transistor's max. Vbe is 6V).

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Lukas.
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Old 30th May 2006, 03:18 PM   #4
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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.
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Old 30th May 2006, 03:24 PM   #5
anatech is offline anatech  Canada
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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
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Old 30th May 2006, 03:27 PM   #6
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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
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Old 30th May 2006, 03:28 PM   #7
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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.
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Old 30th May 2006, 03:42 PM   #8
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Quote:
Originally posted by richie00boy
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.

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
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Old 30th May 2006, 03:47 PM   #9
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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.
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Old 30th May 2006, 03:54 PM   #10
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Quote:
Originally posted by AndrewT
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.
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
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