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switching BJT from saturation into active mode


 

Hello,In the circuit below I have Vc>Vb.
I need some how to make Vb larger then Vc.
I there a method using R1 R2 and R3 which could swith the Vbc junction state?
Given the circuit how can I make the Vcb juntion reversed biased?
Thanks.
Ltspice file is attached.
/g/electronics101/files/john233/03_04_25.zip


 

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V2 is turning the Q1 on very hard, such that it is in saturation, and Vc (product of Ic and R3) is quite large. If you don't want Vc to be so large, you have to reduce the base drive (V2/R1) of Q1 or divert some of that current away from the B-E junction.

What is this supposed to do?

Donald.

On 4/3/25 08:19, john23 via groups.io wrote:

Hello,In the circuit below I have Vc>Vb.
I need some how to make Vb larger then Vc.
I there a method using R1 R2 and R3 which could swith the Vbc junction state?
Given the circuit how can I make the Vcb juntion reversed biased?
Thanks.
Ltspice file is attached.


 

Hello Donald ,Veb supposed to be forward biased so emitter? needs to be less then base.
I tried to increase R1 drastickly but it didnt work.
could you help with mathematical way in my circuit to set it into active mode?

The goal is to test small signal responce in saturation and in active mode to see the difference.
Thanks.


 

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In that case, you have to reduce the base drive. Separate bias and signals on the base would probably help.

I would change the series V2 and V3 into one current source - the DC offset is the bias and the Amplitude provides your signal. You can .STEP the DC offset to set different bias conditions.

Donald.

On 4/3/25 14:16, john23 via groups.io wrote:

Hello Donald ,Veb supposed to be forward biased so emitter? needs to be less then base.
I tried to increase R1 drastickly but it didnt work.
could you help with mathematical way in my circuit to set it into active mode?

The goal is to test small signal responce in saturation and in active mode to see the difference.
Thanks.


 

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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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.