NPN Common Emitter Bias
in [Design]
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From: dagmargoodboat on 5 Jan 2006 17:52 Winfield Hill wrote: > 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------| Q2 --- 1n C| > > | |\ | C| L > > .-. | | C| > > | | | === | > > | | .-. GND | > > '-' | | o-------- out > > | | | 1k5 | > > === '-' | > > GND | | > > | |/ > > in --------------o-------------| Q1 > > |> > > | > > | > > === > > 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\| Q3 .--/\/\--+----+ > . |---, | 50mV | > . /| | |/v | > . +-----+----| C| > . | Q2 |\ C| L > . R2. | C| > . 5mA | | > . etc | | > . | | +-------- out > . GND | | > . | Q1 |/ > . RF IN------+-----------| > . |\v > . | > . === > . GND > > > -- > Thanks, > - Win Gee, I thought I'd invented this scheme, used in a 920MHz RF power amp way back when ;-) But heck, I was a noob, hacking together some stuff for the project of the day. Two quibbles: the inductor must be bypassed, else the added series resistance can spoil its 'Q', and, more importantly, you'll get undesired feedback around Q1, collector-to-base through the current-sensing transistor Q2. Secondly, Andrew's resistor in the collector of biasing/current sensing transistor Q2 is a good idea, to avoid loading the base of RF amplifier tranny Q1. Cheers, James Arthur
From: dagmargoodboat on 5 Jan 2006 18:14 Winfield Hill wrote: <snip> > The circuit has three problems, all easily fixed. [...] Here's a version that makes sure the inductor is bypassed at RF frequencies, yet, per Win, avoids AM modulation of the bias from power supply noise: .. .---------------------+----------+------- +Vcc .. | C1 | | .. R1 100n _|_ Rs .. 300mV --- 200mV .. | R3 | R4 | .. v\| .--/\/\--+--/\/\----+--------. .. |---, | 50mV 50mV | | .. /| | |/v | --- C2 .. +-----+----| C| --- 1n .. | Q2 |\ C| L1 | .. R2. | C| === .. 5mA .-. | GND .. etc | | R5 | .. | | | 1k +------------ out .. GND '-' | .. | Q1 |/ .. RF IN------+-----------------| .. |\v .. | .. === .. GND Regards, James Arthur
From: Winfield Hill on 5 Jan 2006 21:00 dagmargoodboat(a)yahoo.com wrote... > C2 = 1nF ?? -- Thanks, - Win
From: dagmargoodboat on 5 Jan 2006 22:30 Winfield Hill wrote: > dagmargoodboat(a)yahoo.com wrote... > > > C2 = 1nF ?? Sure. Xc(1nF) < 0.2 ohms at 920 MHz. Actually, 100pF might be more apropos to minimize stray inductance in the larger cap. Yes, I know the OP didn't specify frequency and probably means/needs audio-band operation or some such ... I was responding to Andrew's post. When the OPs omit details, we get to make them up, don't we ? ;-) Grins, James Arthur
From: Tim Wescott on 6 Jan 2006 00:54
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. > > > 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. > > If you're building a class AB linear the average collector current can (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 |