NPN Common Emitter Bias

It can be done, but it gets complex. "Radio Frequency Transistors -- Principals and Practical Applications" by Dye and Granberg has some examples.

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.

I've seen a circuit where the NPN is biased using a PNP transistor.

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.

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.

Not easily.

Any objection to an IC if it's small signal ?

What's the application ?

Graham

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

AND: TC of output DC bias point = -2mV/°C * DCGAIN

...Jim Thompson

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.

Well, just run the low side of the bottom resistor to a negative voltage. That's a great way to bias gaasfets, too.

John

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

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

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

Yes, but you need feedback of a different sort. My ascii art is crap so I will describe one type of circuit. Collector resistor to Vcc, emitter to gnd. Resistor from collector to base. Resistor from base to gnd. Collector volts is essentially vbe multiplied by the collector/base base/gnd resistor ratio. This sets both ac and dc gain. You can separate ac gain by slitting the collector/base resistor into two equal parts and decoupling the junction to ground.

HTH

Ian

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

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

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

Ian