biasing

Discussion in 'Electronic Basics' started by novice, Jan 5, 2006.

1. noviceGuest

seasons greetings, I'm new to the electronics subject, basically i'm
a computer science student.i'm unable to understand the biasing in
transistors and why it is needed? also what is a quiescent point what
is the significance of it? hoping for the positive response. thanks a

2. PeteSGuest

This subject is covered thoroughly in texts (for printed books try Art
of Electronics, online there is an excellent discussion at
http://www.americanmicrosemi.com/tutorials/bipolartransistor.htm )

There are many online resources that can help you with this. Basic
amplifier theory, fundamental transistor operation and 'small signal'
are all subjects you should look at, amongst others.

Transistor biasing refers to setting the 'quiescent' state (i.e. the
state of the device with no input signal) of the transistor
appropriately. For a linear amplifier, (such as a simple common emitter
type) the biasing would usually be set such that the collector voltage
(Vc) is approximately half the supply voltage.
That does not mean we would always set it that way - it depends on what
we want the transistor to 'do'.

The 'quiescent point' is usually taken to mean the voltage at the
output terminal (which is normally the emitter or the collector,
depending on application) as set by the biasing of the device.
Note that the above definition is rather loose - the quiescent point
may be a current or a voltage and refer to any terminal (or all three)
- it depends on what information needs to be conveyed.

We bias transistors so that when we apply a signal to the input, the
output presents some useful signal in response. This may or may not be
an exact copy (larger or smaller) of the input.
When talking of this, we also get into 'conduction angles' . For a
sinusoid input, the conduction angle refers to the part of the input
(angle) for which the transistor actually conducts. This can be
anything from a few degrees (class C) to 360 degrees (conducts for the
entire input cycle, class A).

This is such a huge subject that I suggest googling for 'transistor
biasing' plus the other subjects above and reading up a little. You may
also want to get a basic book on amplifiers.

Good luck!

Cheers

PeteS

3. John PopelishGuest

Think of a transistor as an electrical analog of a water valve (except
that it alters the flow of charge, called current, instead of the flow
of water). Lets say that the changes in flow from water from a valve
represent a signal that has been amplified. When there is no signal,
there has to be a middle amount of flow, so that changes in either
direction can occur from that zero signal condition. Quiescent just
means with zero signal. Biasing is getting the no signal flow where
it needs to be, so that when a signal arrives, the flow can change, as
needed from that average operating point.