Discussion in 'Electronic Basics' started by Animesh Maurya, Aug 30, 2003.

1. ### Animesh MauryaGuest

Sorry the subject line was "Analogy about Transistor" rather that
"sci.electronics, sci.electronics.basics, sci.physics" which was some
how mispasted.

Animesh Maurya

2. ### Roger JohanssonGuest

I have a simple explanation and model of how transistors (bipolar)
work.

Current units from the collector input can only pass the emitter if
they are in company of a current unit coming into the transistor
through the base.

But each current unit going through the base-emitter path can take
with it a number of current units coming from the collector.
That number is the amplification factor.

To be more concrete let's say that the current unit is milliamperes
(mA) and the amplification factor is 400.

Each mA we send through the base-emitter path then allows 400 mA from
the collector to accompany it through the emitter.

To control how many mA we want to send through the base-emitter we can
use a base resistor in series with the base.

The voltage at the other end of that resistor controls the base
current, and controls how much collector current can pass.

3. ### William J. BeatyGuest

Your analogy seems to be missing the entire transistor action. In
the above design, tiny changes in the base circuit will not produce
large changes in the collector circuit. This fluid analogy simply
acts like two resistors and two power supplies.

The core concept in transistor or vacuum tube amplification is
"controllable valve action," where a tiny amount of energy is
used to open/close some sort of fluid valve. Or in other words,
small energy expended per second is used to control a large amount
of energy per second being produced by some sort of power supply.
Perforated plates essentially behave as resistors, not as
controllable valves.

In transistors, the base/emitter junction acts as a genuine
"electricity valve" which determines the value of current
in emitter and collector leads. To understand transistors, don't
use the common-base configuration. Instead imagine an "ideal"
transistor where the base width is zero and the base current is
zero, yet where Vbe still determines Ic (still like a diode, but
a diode where the Vf forward voltage is applied between two terminals,
while the main current path is through two entirely DIFFERENT
terminals.)

PS
Any analogy which tries to explain why base current can control
the collector current is doomed to failure. Why? Because in real-
world transistors, base current doesn't control collector current!

Here's someone's simple but excellent analogy:

http://www.satcure-focus.com/tutor/page4.htm

See? In the above analogy, base current doesn't control
collector current. Instead, the pressure applied to the
base will open the "collector valve" as well as causing
fluid leakage through the base, determining both the Ib and
the Ic. The central concept is "Vbe controls Ic."
Understand that, and you understand transistors. Oh, yeah,
by the way, Vbe also controls Ib, so Ib just happens to be
proportional to Ic; a useful phenomenon for simplifying design
calcs, but for actually UNDERSTANDING transistors, the whole
is no mechanism in bipolar transistors whereby Ib can directly
determine Ic.

Here's my own oversimplified explanation (w/lots of ASCII art!) :

http://amasci.com/amateur/transis.html

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty Research Engineer
UW Chem Dept, Bagley Hall RM74
Box 351700, Seattle, WA 98195-1700
ph206-543-6195 http//staff.washington.edu/wbeaty/

4. ### Animesh MauryaGuest

Yes I agree that the model needs some more addition before it can
satisfactorily explain anything. Perhaps it was just an outline. Any
further help will be appreciated.

Animesh Maurya

5. ### Animesh MauryaGuest

Your explanation is very nice, the same I was looking for. It cleared my concept.

Thanks