The Bipolar Junction Transistor Discussion

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.Collector The BJT is a three terminal device

Base

Emitter

3. There are two types of BJT transistors. They are the NPN type, and the PNP e c b N P

type. e

c

b

b c

e

(a) NPN (b) PNP Ic Ie Ib

4.Th e

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. Ie Ic Ib 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 Vc

Ve.

Vb

Ve

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 Vbe Vcb Vce Vbc Veb Vec

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

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.