NPN Common Emitter Bias
in [Design]
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From: Pooh Bear on 5 Jan 2006 02:57 John Popelish wrote: > 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. Lower value resistors reduce the beta ( current gain ) sensitivity too. Of course this configuration applies negative feedback - No idea if that's an issue for the OP. Graham
From: Pooh Bear on 5 Jan 2006 02:55 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? Not easily. > 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. Any objection to an IC if it's small signal ? What's the application ? Graham
From: Andrew Holme on 5 Jan 2006 05:08 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. I've seen a circuit where the NPN is biased using a PNP transistor. >From memory, it was something like an RC low-pass filter from the NPN collector to the PNP emitter; with a potential divider setting the PNP base voltage; and the PNP collector feeding bias to the NPN base. I think it was a wideband RF buffer stage in a Marconi signal generator. I'll check the service manual when I get home tonight.
From: Fred Bloggs on 5 Jan 2006 08:25 > 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. This can be done any number of ways but your question cannot be answered without knowing the frequency range of the signal to be amplified, the required gain of the circuit, the loading, and the source characteristics. Local feedback is not going to cut it since the transistor gain goes to zero at cutoff and saturation.
From: Winfield Hill on 5 Jan 2006 10:54
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 |