Hello,
what happens when a negative swing at the base of a NPN transistor reachs the break down voltage?
Does it turn on the transistor, and then as long as the current is limited to a safe value, it won't damage it or do other weird things?
I can see breakdown voltage specified in datasheets, but not what will happen if it gets reached. As I understand it, PN junctions do not suffer damage from breakdown like e.g. a Schottky or a MOSFET gate would.
Electron wrote:
> what happens when a negative swing at the base of a NPN transistor reachs
> the break down voltage?
Generally, a reverse-biased B-E junction will function as a (noisy) zener
diode somewhere in the range of 10-20V. Obviously, if any significant current
is allowed to flow in this mode, it will dissipate a lot of power in the
transistor. However, I have seen circuits in which a transistor was
deliberately operated in this mode as a noise source (e.g., analog music
or drum synthesizer).
> Does it turn on the transistor, and then as long as the current is limited
> to a safe value, it won't damage it or do other weird things?
No, the transistor doesn't turn on; the reverse current in the B-E junction
doesn't inject the right kind of carriers (if anyt) into the B-C junction,
which is also more strongly reverse-biased than usual.
> Hello,
> what happens when a negative swing at the base of a NPN transistor reachs the break
> down voltage?
>
> Does it turn on the transistor, and then as long as the current is limited to a safe
> value, it won't damage it or do other weird things?
>
> I can see breakdown voltage specified in datasheets, but not what will happen if it
> gets reached. As I understand it, PN junctions do not suffer damage from breakdown
> like e.g. a Schottky or a MOSFET gate would.
>
> Thanks!
> Mario
Typically the base-emitter junction does a zener action at the Vbe breakdown voltage specified. In this condition they can make a rather nice white noise source (I remember having a series of articles out of a UK originated magazine for a drum machine that used a reverse biased base-emitter junction as a white noise source). I believe in this state they generate more white noise than a proper zener diode.
>> Hello,
>> what happens when a negative swing at the base of a NPN transistor
>reachs the break
>> down voltage?
>>
>> Does it turn on the transistor, and then as long as the current is
>limited to a safe
>> value, it won't damage it or do other weird things?
>>
>> I can see breakdown voltage specified in datasheets, but not what
>will happen if it
>> gets reached. As I understand it, PN junctions do not suffer damage
>from breakdown
>> like e.g. a Schottky or a MOSFET gate would.
>>
>> Thanks!
>> Mario
>
>Typically the base-emitter junction does a zener action at the Vbe
>breakdown voltage specified. In this condition they can make a rather
>nice white noise source (I remember having a series of articles out of
>a UK originated magazine for a drum machine that used a reverse biased
>base-emitter junction as a white noise source). I believe in this
>state they generate more white noise than a proper zener diode.
Thanks for the reply (also to Dave Tweed): but in regard of damage,
should I really worry? Of course I will allow only 10-20 mA max into
the junction of these 2N3904.. not more than that.
>Hello,
>what happens when a negative swing at the base of a NPN transistor
>reachs the break down voltage?
>
>Does it turn on the transistor, and then as long as the current is
>limited to a safe value, it won't damage it or do other weird things?
>
>I can see breakdown voltage specified in datasheets, but not what
>will happen if it gets reached. As I understand it, PN junctions do
>not suffer damage from breakdown like e.g. a Schottky or a MOSFET gate would.
>
>Thanks!
>Mario
A few things that may be unexpected--
- a permanent reduction of transistor beta. This is
essentially damage to the BJT that is (practically speaking)
irreversible. You should thus avoid that region of operation
if you want to use it again as a transistor (once a noise source..)
This is why you often see an inverse-parallel switching diode.
- you can make a relaxation oscillator with just the
C & E pins if the transistor is flipped (so the base-emitter
junction breaks down). Again, permanent degradation of forward beta.
- you can get a bit of current of unexpected polarity at the
collector if you break the E-B junction down- it's a result
of IR light being emitted by the E-B junction and received by
the C-E junction! Again, permanent degradation of forward beta.
>At 10:32 AM 11/01/2012, you wrote:
>
>>Hello,
>>what happens when a negative swing at the base of a NPN transistor
>>reachs the break down voltage?
>>
>>Does it turn on the transistor, and then as long as the current is
>>limited to a safe value, it won't damage it or do other weird things?
>>
>>I can see breakdown voltage specified in datasheets, but not what
>>will happen if it gets reached. As I understand it, PN junctions do
>>not suffer damage from breakdown like e.g. a Schottky or a MOSFET gate would.
>>
>>Thanks!
>>Mario
>
>A few things that may be unexpected--
>
>- a permanent reduction of transistor beta. This is
> essentially damage to the BJT that is (practically speaking)
> irreversible. You should thus avoid that region of operation
> if you want to use it again as a transistor (once a noise source..)
> This is why you often see an inverse-parallel switching diode.
>
>- you can make a relaxation oscillator with just the
> C & E pins if the transistor is flipped (so the base-emitter
> junction breaks down). Again, permanent degradation of forward beta.
>
>- you can get a bit of current of unexpected polarity at the
> collector if you break the E-B junction down- it's a result
> of IR light being emitted by the E-B junction and received by
> the C-E junction! Again, permanent degradation of forward beta.
Thank you. It's clear that I won't use it then. But I will remember it
in case I'll ever need a white noise source better than a zener, the beta
is irrelevant for this application and it can be useful in this case.
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