Very Basic Question: Transistor Switching

nelsonys

Joined Apr 6, 2011
21
We know that BJT transistor can be used as solid-state switches by driving them in either cutoff or saturation mode.

There is some part I do not understand well about fully "ON" state in saturation mode:

My understanding is to drive a transistor in saturation, both BE and CB have to be forward biased, but looking at all the examples provided on the books, say for example 5V input signal, collector supply voltage of 15V, I do not understand how to forward bias the CB junction since Vb is smaller than Vc.

What I can think of is by using a large Rb value, much larger than the Rload connected to the collector side so that the voltage drop on the base is large than the collector in order to forward bias the CB junction.

Books usually just mention to supply maximum Ib in order to get maximum Ic, then the transistor is driven to saturation mode.

Kindly advise on this very very fundamental problems.

Thanks!

HallMark

Joined Apr 3, 2011
89
We know that BJT transistor can be used as solid-state switches by driving them in either cutoff or saturation mode.

There is some part I do not understand well about fully "ON" state in saturation mode:

My understanding is to drive a transistor in saturation, both BE and CB have to be forward biased, but looking at all the examples provided on the books, say for example 5V input signal, collector supply voltage of 15V, I do not understand how to forward bias the CB junction since Vb is smaller than Vc.

What I can think of is by using a large Rb value, much larger than the Rload connected to the collector side so that the voltage drop on the base is large than the collector in order to forward bias the CB junction.

Books usually just mention to supply maximum Ib in order to get maximum Ic, then the transistor is driven to saturation mode.

Kindly advise on this very very fundamental problems.

Thanks!
Welcome to Forum,

Here is the E-Book developed by our AAC member and they have explained very neatly everything and in very good language have a look at this chapter i am sure you will get your answer.

nelsonys

Joined Apr 6, 2011
21
Thanks HallMark!

I have read this book, but did not solve my doubts for the moment =)

nelsonys

Joined Apr 6, 2011
21

Thanks!

debjit625

Joined Apr 17, 2010
790
My understanding is to drive a transistor in saturation, both BE and CB have to be forward biased
Yes,thats true think it like this when the transistor is saturated their is voltage drop across CE about 0.2V to 0.4V and their is a max voltage drop across RL(load resistor connected between power supply and collector) wrt the voltage drop across BE 0.7 volt this cause CB to be forward biased .

You may look at 'Ebers-Moll' model google it

Good Luck