P

N

P

b

c

e

N

P

N

b

e

c

1. The Bipolar Junction Transistor (BJT) is an extremely common electronic device

to all forms of electronic circuits. It can be used for a number of useful

applications such as an amplifier, a switch, a buffer, an oscillator, a nonlinear

circuit – so forth.

2. The BJT is made by P and N type semiconductor material, which should be

familiar from the study of diodes. The BJT is a three terminal device

3. There are two types of BJT transistors. They are the NPN type, and the PNP

type.

(a) NPN (b) PNP

Emitter

Base

Collector

e

c

b

c

b

e

Ib

Ie

Ic

Ib

Ic

Ie

4. The arrows show the direction of DC current flow for both the NPN and PNP

cases. In both cases the base current (Ib) is a very small current in the order of

microamps while the collector current (Ic) and emitter current (Ie) are larger and

in the order of milliamps. Note that for the NPN transistor, the base current flows

into the transistor but for the PNP transistor, the base current flows out the

transistor. Also note Ic and Ie always flow in the same direction and in the

direction of the (black) arrow, the same arrow that tells us whether the transistor

is PNP or NPN.

5. Now for the voltages:

The voltage at the base is normally written as Vb.

The voltage at the collector is normally written as Vc.

The voltage at the emitter is normally written Ve.

Vc

Vb

Vec

Veb

Vbc

Vce

Vcb

Vbe

6. For the part voltage between collector and emitter, emitter and base and base

and collector we use either:

Vce or Vce for collector and emitter

Veb or Vbe for emitter and base

Vbc or Vcb for base and collector

It is written such like example below;

Lets;

Vc = 6V (The voltage at the collector is 6 volts)

Ve = 2V (The voltage at the emitter is 2 volts)

Then Vce is 4V because the voltage at the collector is 4V higher than the voltage

at the emitter. Also, Vec = -4V because the voltage at the emitter (measuring

point) is 4V lower than the voltage at the collector (reference point). And so on for

Veb or Vbe and Vbc or Vcb. This is the convention used for measuring voltages

between terminals of the NPN and PNP transistors. The reason for this is that in

these examples the first subscript letter is usually of higher voltage than the

second, hence all variables listed below will have positive values.

Ve

7. The analysis or design of a transistor amplifier requires a knowledge of both the

dc and the ac response of the system. Too often it is assumed that the transistor

is a magical device that can raise the level of the applied ac input without the

assistance of an external energy source. In actuality,

the improved output ac power level is the result of a transfer of energy from the

applied dc supplies.

8. The analysis or design of any electronic amplifier therefore has two components:

the dc portion and the ac portion. Fortunately, the superposition theorem is

applicable and the investigation of the dc conditions can be totally separated

from the ac response. However, one must keep in mind that during the design or

synthesis stage the choice of parameters for the required dc levels will affect the

ac response, and vice versa.

9. The dc level of operation of a transistor is controlled by a number of factors,

including the range of possible operating points on the device characteristics.

The range for the bipolar junction transistor (BJT) amplifier should be specified.

Once the desired dc current and voltage levels have been defined, a network

must be constructed that will establish the desired operating point.