FORM 6 PHYSICS: ELECTRONICS PART 2

Common collector configuration

Under this configuration the transistor has current gain but no voltage gain

=Amplification factor

            =

             =

For common emitter

                           V_{o =} I_C R₂

                             V_{i =} I_BR_B

From, I_{E =} I_B + I_C

Common emitter characteristic curve

The circuit above is for investigating the variation of current with voltage in the input and output circuits.

OUTPUT CHARACTERISTICS
I_C-V_CE with I_B constant .

The results are plotted below.
The knee of the curves shown corresponds to a low P.d(0.2) the output for higher P.d the output I_C varies linearly with V_CE for a given value of base current I_B.

The linear part of the characteristic is the one used in the audio frequency (a.f) amplifiercircuits so that the output is undistorted.

INPUT CHARACTERISTICS

I_B-V_BE with V_CE constant

The results are as follows:-
The input characteristics is non-linear

TRANSFER CHARACTERISTICS
I_C-I_B  with V_CE constant

The results are as shown below:-
The output current I_C varies linearly with the input current I_B. The current transfer ratio or current gain is given by

In the figure  below

 

Questions

1.   An npn transistor has a current gain (Beta) value of 200. Calculate the base current  required to switch a resister of 4µA.

2. An npn transistor has a dc base bias voltage  of and an input base resistor  of 100kΩ.What will be the of base current into the transistor

(The transistor is a silicon type)

Data

From Kirchhoff law

For silicon

 =10v

  = 100kΩ

 =0.6v (wasted voltage)

Solution

––=0

–=

Example

Given the circuit below, determine

The transistor has =150

The transistor of silicon type

Solution

Second Kirchhoff’s law in the input circuit

× = 0

5V

A

A

           

=0.6v

10

= 3.4V

=5+ ( )

=5+ (10000×4.4×10^-4)  (6.6)

=2.8v