NPN Common Emitter Bias
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Posted by dgc on January 4, 2006, 5:52 pm
Please log in for more thread options 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. | ||||
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Posted by Tim Wescott on January 4, 2006, 6:03 pm
Please log in for more thread options dgc wrote: Principals and Practical Applications" by Dye and Granberg has some examples. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com | ||||
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Posted by Winfield Hill on January 5, 2006, 7:54 am
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Tim Wescott wrote... >
> 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. >
> It can be done, but it gets complex. "Radio Frequency Transistors -- > Principals and Practical Applications" by Dye and Granberg has some > examples. I don't have that book, but many wideband linear RF amplifiers I've worked on used a current-sensing resistor in the collector RF path, with a little servo circuit to establish the average value of the base current. This is easy to do with just a few transistors. The sense resistor need not drop more than 200 to 400mV. -- Thanks, - Win | ||||
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Posted by Tim Wescott on January 5, 2006, 9:54 pm
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Winfield Hill wrote: > Tim Wescott wrote...
> >>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. >>
>>It can be done, but it gets complex. "Radio Frequency Transistors -- >>Principals and Practical Applications" by Dye and Granberg has some >>examples. >
If you're building a class AB linear the average collector current can
> > I don't have that book, but many wideband linear RF amplifiers I've > worked on used a current-sensing resistor in the collector RF path, > with a little servo circuit to establish the average value of the > base current. This is easy to do with just a few transistors. The > sense resistor need not drop more than 200 to 400mV. > > (and should!) vary quite a bit -- for these you use a diode that's in close thermal contact with the transistors to establish the bias voltage to the base. The circuit that I have seen came out of an old Motorola app note (can't remember which one) and uses a 723 for the actual regulation chore. I _have not_ built one of these for a production system, so I can't comment on how much care and feeding it would demand. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com | ||||
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Posted by John Popelish on January 4, 2006, 7:34 pm
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dgc wrote: > Can an NPN transistor in a common emitter configuration be safely biased for
You can use collector voltage feedback, instead.
> 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. > > | ||||
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> 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.
>
>