Common collector configuration(confusion)

[paulmdrdo]

I don't understand why in this configuration the collector-base junction is said to be reverse-biased. It seems to me that Vbb is forward biasing it. because the base is at higher + potential than the collector which is at ground. can you enlighten me about it. Thanks a lot!

 

[LvW]

Yes - you are right. The B-C junction is forward biased. However the unit cannot work as an amplifier because the polarity of Vce is false.

 

[FvM]

Yes - you are right. The B-C junction is forward biased. However the unit cannot work as an amplifier because the polarity of Vce is false.

I fear the drawing with bottomside collector terminal is confusing things a lot.

In fact Vee has correct polarity for regular transistor operation, but Vbb polariry is wrong.

 

[LvW]

I fear the drawing with bottomside collector terminal is confusing things a lot.

Yes - you are right. I have assumed a grounded emitter automatically. And therefore, false polarity of Vbb.

 

[Audioguru]

The entire circuit is wrong.
The base-emitter is not biased correctly and the emitter has no load.
Why is it drawn upside-down?

 

[paulmdrdo]

That image is from a book authored by Robert Bolyestad. And I also saw it being used in a video lecture from nptel that exact image. How is that wrong? That is a common-collector configuration. Please help.

 

[FvM]

Strictly speaking, the circui isn't necessarily wrong. If Vbb is small (e.g. < 500 mV), the transistor may still work in active region without saturation. But the circuit seems at least useless because no output load is connected.

Why are you stuck to this strange circuit? Wouldn't it be better to read a profound textbook or internet resource about transistor circuits?

And it's about to helpless to expect an explanation from Edaboard users why the circuit has been setup as is.

 

[paulmdrdo]

what do you mean by Vbb <500mv? It would still be in saturation even if you make Vbb <500mv. The c-b junction is still forward bias with Vbb < 500mv, say 300mv. can you explain? please bear with me. THANKS!

 

[Audioguru]

Your transistor is completely wrong. I fixed it:

 

[FvM]

The c-b junction is still forward bias with Vbb < 500mv, say 300mv.

I believe the discussion has been superseeded by Audioguru's corrected schematic.

I know that operation with forward biased base-collector junction is formally considered as satuarated state. But with e.g. 300 mV forward bias, the transistor still behaves "unsaturated", involving high current gain (at least at moderate currents without considerable voltage drop at intrinsic resistance). This is behaviour e.g. utilized in schottky clamp circuits which limits the forward bias to a few 100 mV.