Simple BJT Bias fails?

Hi All,

I'm unsure if this is a problem with CircuitLab or a problem with my understanding.

Previously, with breadboards I've biased a transistors base at half the supply voltage. I've then applied an audio input and checked the base voltage and output voltage on a scope.

But, in circuitlab, create a potential divider with two resistors and the mid point reads correctly. As soon as I add a transistor the base voltage drops to somewhere in the region of 1V. I've tried many different combinations of components and this always seems to happen.

I've thrown a circuit together to demonstrate this. On this occasion I've not calculated values, just plucked resistors values out of the air. Even so, I'd expect the base to sit at 6V (half the supplies 12V):-

Any ideas?



by thespirit3
June 10, 2015


"I'm unsure if this is a problem with CircuitLab or a problem with my understanding."

Sorry but it's your understanding.

CL is showing pretty much exactly what a real circuit would do.

A forward biased BE junction of a bipolar transistor looks like a forward biased diode. Forward bias a silicon diode - the semiconductor material from which a 2N2222 is made and the simulation model therefore represents - and push about 550uA through it and you will get a voltage of between about 0.55V and 0.7V.

Your sim pushes 537uA into the base to ground and you get a Vbe of 627mV.

No way will you see a base voltage in this circuit of 6V. (Actually you can in simulation but in a real circuit the smoke from the transistor will obscure the readings on the DVM and scope.)

I suspect that the circuits you are thinking of that have base voltages of almost what the potential divider would give if open circuit, have a resistor between the emitter and ground.

With your 20k + 20 base bias ladder, if you reduce the collector resistor from 2k to 510R and insert a 1k resistor from emitter to ground then you'll get a base voltage of about 5.5V. Note that the Vbe is still around 609mV.


by signality
June 10, 2015

Thanks for taking the time to respond. The frustrating thing is, 10 years ago I was reasonably proficient at all these things. Now, I feel like I'm learning from scratch and making all the same newbie mistakes that I did the first time around.

Your comment regarding the forward bias of the BE junction is ringing all sorts of bells from my distant past. Of course, you're 100% right. I now remember calculating the emitter resistor to bias things properly.

I'll get there, eventually. Thanks again!


by thespirit3
June 11, 2015

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