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Re: switching BJT from saturation into active mode


 

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A BJT is inherently a current-controlled and -controlling device. Ibe controls Ice, within the limits of the surrounding circuitry. I have no idea why you don't want to use current sources if you are after a device behaviour model. If there is another goal, it would be a good idea to tell us, otherwise we can lead you to a perfect but perfectly useless circuit.

To get the device in your earlier schematic into active mode, you'll have to drive the base with less current than I(e)/Hfe. This is known as "biasing a transistor" and is pretty standard electronic design. Most good biasing schemes that I know of use some form of current sources, since Vbe varies with temperature so much. Do a bit of net search for "transistor bias" and related terms; you should be able to come up with something useful.

In your original schematic (I'm not going to bother downloading another), note that V1 is 5V and V2 is -1V. The base current of Q1 will therefore be somewhere around ((5V) - (0.6V) - (-1V))/(100ohm + 1kohm) = 4.9mA. If Q1 has an Hfe of about 100 (just a WAG, understand), then Q1 will be capable of carrying? 490mA through R3 -- that WON'T happen given R3's value and the 5V from V1. To be in active region, Q1's base current has to be less than 0.01 * whatever current V1 can provide through R2 + R3.

If you _really_ want to use V2 and R1 to bias Q1, you'll have to increase V1 to a positive value near V1 - V(b,e): so somewhere around 4.0V to 4.5V. A base-emitter shunt resistor might also be helpful, but that will be left as an exercise for the student.

Donald.

On 4/4/25 08:09, john23 via groups.io wrote:

Hello,I prefer not to use current sources
A you can see In the attached photos and simulationI this I got it to be in active mode? by tweaking the resistors .
I made few equations for the BJT shown in the photo ,could you help me to develp a mathmatical way of finding the values for setting the PNP into active mode?
Thanks.

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