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From: dgc on 4 Jan 2006 18:52
Can an NPN transistor in a common emitter configuration be safely biased for
standing DC current WITHOUT having an emitter resistor in the circuit, i.e
emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
swing on the collector with only a 12 volt supply available. Any emitter
resistor obviously lowers the peak voltage the collector can handle, while
remaining linear, so I am trying to avoid using one.

Thanks.


From: John Popelish on 4 Jan 2006 19:34
dgc wrote:
> Can an NPN transistor in a common emitter configuration be safely biased for
> standing DC current WITHOUT having an emitter resistor in the circuit, i.e
> emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
> swing on the collector with only a 12 volt supply available. Any emitter
> resistor obviously lowers the peak voltage the collector can handle, while
> remaining linear, so I am trying to avoid using one.
>
> Thanks.
>
>
You can use collector voltage feedback, instead.
From: dgc on 4 Jan 2006 20:42

"John Popelish" <jpopelish(a)rica.net> wrote in message
news:nY2dne2sVLcD9SHenZ2dnUVZ_s-dnZ2d(a)adelphia.com...
> dgc wrote:
>> Can an NPN transistor in a common emitter configuration be safely biased
>> for standing DC current WITHOUT having an emitter resistor in the
>> circuit, i.e emitter directly tied to ground? I'm trying to accomodate a
>> 12 volt peak swing on the collector with only a 12 volt supply
>> available. Any emitter resistor obviously lowers the peak voltage the
>> collector can handle, while remaining linear, so I am trying to avoid
>> using one.
>>
>> Thanks.
> You can use collector voltage feedback, instead.

Thanks John. I have seen collector voltage feedback modeled in a text I am
reading. However, just for my education, could a transistor be biased for
standing DC current with a direct ground connection on the emitter, without
damaging the transistor.


From: Tim Wescott on 4 Jan 2006 21:03
dgc wrote:

> Can an NPN transistor in a common emitter configuration be safely biased for
> standing DC current WITHOUT having an emitter resistor in the circuit, i.e
> emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
> swing on the collector with only a 12 volt supply available. Any emitter
> resistor obviously lowers the peak voltage the collector can handle, while
> remaining linear, so I am trying to avoid using one.
>
> Thanks.
>
>
It can be done, but it gets complex. "Radio Frequency Transistors --
Principals and Practical Applications" by Dye and Granberg has some
examples.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
From: John Popelish on 4 Jan 2006 22:27
dgc wrote:
> "John Popelish" <jpopelish(a)rica.net> wrote in message
> news:nY2dne2sVLcD9SHenZ2dnUVZ_s-dnZ2d(a)adelphia.com...
>
>>dgc wrote:
>>
>>>Can an NPN transistor in a common emitter configuration be safely biased
>>>for standing DC current WITHOUT having an emitter resistor in the
>>>circuit, i.e emitter directly tied to ground? I'm trying to accomodate a
>>>12 volt peak swing on the collector with only a 12 volt supply
>>>available. Any emitter resistor obviously lowers the peak voltage the
>>>collector can handle, while remaining linear, so I am trying to avoid
>>>using one.
>>>
>>>Thanks.
>>
>>You can use collector voltage feedback, instead.
>
>
> Thanks John. I have seen collector voltage feedback modeled in a text I am
> reading. However, just for my education, could a transistor be biased for
> standing DC current with a direct ground connection on the emitter, without
> damaging the transistor.

If the transistor is kept at a fairly constant temperature, and the
bias is carefully set, the transistor is unlikely to be damaged, since
in a class A operation, the worst case power dissipation occurs at
zero signal. The real problem is whether the bias point will stay
centered well enough for the required signal swing without clipping.

If the temperature changes more than a few degrees, all bets are off.
Transistors are pretty good thermometers.

A simple collector voltage bias involves connecting the base divider
to the collector, instead of to the supply, with the top resistor
value dropped to account for the lower voltage across it.
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