BJT: Emitter-Base & Base-Collector Fwd Biased w/ 0.7V: Ic, Vce?

[dexterdev]

I have a doubt regarding BJT. what happens if both emitter-base & base-collector junctions are forward biased with 0.7V ? will BJT be in saturation region? What will be the voltage across the collector and emitter and Ic ?

-Devanand T

 

[yungman]

Do you mean you actually shorted out the E and C and drive the B? If that is the case, you just have a diode! If you are talking about grounding the E and drive the B hard, you'll never get the 0.7V across the BC junction.

 

[vk6kro]

I tried this in a simulation with a 2N2222 transistor..

I put two independent + 0.7 volt supplies on the base and collector, with the emitter grounded and the negative terminals of the supplies also grounded.

The base current was 26.9 uA and the collector current was 5.29 mA. This gave a current gain of 196.

This was very similar at 10 volts on the collector.
Base current 26.8. The collector current was 5.76 mA. Current gain 214.

So, I guess the transistor is operating normally at 0.7 volts supply.

 

[jim hardy]

i suggest get a handful of cheap transistors and experiment.

0.7 volts of forward bias on C-B may or may not be okay
but beyond 100 milliamps you can destroy the junction of a small signal transistor or melt its tiny internal wires.

 

[vk6kro]

Rereading the post, I think you mean this:

http://dl.dropbox.com/u/4222062/grounded%20collector.PNG

I'm not sure the simulator I used has a good enough model to cope with this weird arrangement, however it had a shot at it and the above diagram shows the suggested currents.

The 1 ohm resistors are just for measurement purposes and should not have any effect on the result.

I'm now curious to see if this really happens. Pretty bizarre.

 

[jim hardy]

Thanks Vk6 - now i see what you meant.

In old days we tested transistors with a multimeter and multimeters were all analog then.
As you well know, Simpson 260 on RX1 scale puts out enough test current to destroy a small transistpr's C-B junction so you ALWAYS check transistors on RX100 scale.

That's point i was trying to make. As usual i was on wrong page !

old jim

 

[dexterdev]

I am talking about a situation as below :

 

[yungman]

I am talking about a situation as below :

View attachment 43724

That's what I thought, you have two separate diode in parallel like I posted! The BJT is two N region separate by the P( base). So they are different diode all together. In IC, the collector is the lightly doped tub. P base is implanted in the middle of the tub with much higher concentration of P impurity. Then the middle of the base is again doped heavily by N impurity to form the emitter. It is literally like two diodes in parallel. And is just a diode in forward bias!

 

[dexterdev]

My colleagues were telling no current will flow. But a forward biased diode must conduct current isn't it ?

 

[yungman]

You bet current will flow in your drawing. Just take a transistor and try it. You put 0.7V, that will turn on the transistor. But it is not safe to connect like it your drawing as you might pump too much current through, you should use a resistor in series to limit the current.

 

[dexterdev]

Thanks for your reply. I will try that.