NPN Common Emitter Bias
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From: Andrew Holme on 5 Jan 2006 15:07 Andrew Holme wrote: > 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. They used lots of variations: always with a resistor between PNP collector and NPN base; often with a large capacitor from PNP collector to ground; only sometimes with a capacitor from PNP emitter to ground; and sometimes with a small NPN emitter resistor. None are anything like rail-to-rail output. Sometimes there's a diode in series with the potential divider - presumably for temperature compensation. I also found an example where they used an NPN instead of the PNP, but enough said. This is from a 500-900 MHz first LO drive circuit: .------------------------o------- +10V | | | .-. | | | 100-ohm | | | .-. '-' | | | | | .-------o-------o '-' | | | | | | | | |< --- | o------| --- 1n C| | |\ | C| L .-. | | C| | | | === | | | .-. GND | '-' | | o-------- out | | | 1k5 | === '-' | GND | | | |/ in --------------o-------------| |> | | === GND created by Andy?s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
From: Ian Bell on 5 Jan 2006 16:13 Jim Thompson wrote: > > AND: TC of output DC bias point = -2mV/?C * DCGAIN > Quite right, and probably the reason it is rarely used. In simple applications where temperature range is limited it may not be a problem. With a 12V supply and a 6V collector voltage the temp co is about 20mV/degree which over a 20 degree range is just 400mV. Ian
From: John Larkin on 5 Jan 2006 16:28 On Thu, 05 Jan 2006 21:13:45 +0000, Ian Bell <ruffrecords(a)yahoo.com> wrote: >Jim Thompson wrote: > >> >> AND: TC of output DC bias point = -2mV/?C * DCGAIN >> > >Quite right, and probably the reason it is rarely used. In simple >applications where temperature range is limited it may not be a problem. >With a 12V supply and a 6V collector voltage the temp co is about >20mV/degree which over a 20 degree range is just 400mV. > >Ian Well, just run the low side of the bottom resistor to a negative voltage. That's a great way to bias gaasfets, too. John
From: Ian Bell on 5 Jan 2006 17:04 John Larkin wrote: > On Thu, 05 Jan 2006 21:13:45 +0000, Ian Bell <ruffrecords(a)yahoo.com> > wrote: > >>Jim Thompson wrote: >> >>> >>> AND: TC of output DC bias point = -2mV/?C * DCGAIN >>> >> >>Quite right, and probably the reason it is rarely used. In simple >>applications where temperature range is limited it may not be a problem. >>With a 12V supply and a 6V collector voltage the temp co is about >>20mV/degree which over a 20 degree range is just 400mV. >> >>Ian > > Well, just run the low side of the bottom resistor to a negative > voltage. That's a great way to bias gaasfets, too. > > John I suspect if the OP had a negative voltage available he would want the output to swing all the way down to it ;-) Ian
From: Winfield Hill on 5 Jan 2006 17:05
Andrew Holme wrote... > > Andrew Holme wrote: >> 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. > > They used lots of variations: always with a resistor between PNP collector > and NPN base; often with a large capacitor from PNP collector to ground; > only sometimes with a capacitor from PNP emitter to ground; and sometimes > with a small NPN emitter resistor. None are anything like rail-to-rail > output. Sometimes there's a diode in series with the potential divider - > presumably for temperature compensation. I also found an example where they > used an NPN instead of the PNP, but enough said. > > This is from a 500-900 MHz first LO drive circuit: > > .------------------------o------- +10V > | | > | .-. > | | | 100-ohm > | | | > .-. '-' > | | | > | | .-------o-------o > '-' | | | > | | | | > | |< --- | > o------| --- 1n C| > | |\ | C| L > .-. | | C| > | | | === | > | | .-. GND | > '-' | | o-------- out > | | | 1k5 | > === '-' | > GND | | > | |/ > in --------------o-------------| > |> > | > | > === > GND The circuit has three problems, all easily fixed. First, the bypass cap should go across the sense resistor, etc., because the goal is to get a constant base-bias current to the RF transistor, unaffected by supply fluctuations, RF currents, etc. Second, we don't want the servo-loop gain too high at high frequencies, so some PNP emitter resistance is in order. Third, we'd like to reduce the wasted voltage drop across the sense resistor, yet avoid tempco issues with the PNP, so we need a tempco-matching transistor. Here's the result of these changes. .. .---------------------+----+------- +Vcc .. | | | .. R1 _|_ Rs .. 250mV --- 200mV .. | | | .. v\| .--/\/\--+----+ .. |---, | 50mV | .. /| | |/v | .. +-----+----| C| .. | |\ C| L .. R2. | C| .. 5mA | | .. etc | | .. | | +-------- out .. GND | | .. | |/ .. RF IN------+-----------| .. |\v .. | .. === .. GND -- Thanks, - Win |