Question

To set up a common base transistor circuit and to study its input and output characteristics and transfer characteristics to calculate the current gain

Answer 1

we can calculate it by using following formula:

AC current gain= [delta(IC)/delta(IE)]

DC current gain= IC/IE

Answer 2

Operation:

Bipolar Junction Transistor (BJT) is a three terminal (emitter, base, collector) semiconductor device. There are two types of BJTs, namely NPN and PNP. It consists of two PN junctions, namely emitter junction and collector junction.

The basic circuit diagram for studying input characteristics is shown in the circuit diagram. The input is applied between emitter and base, the output is taken between collector and base. Here base of the transistor is common to both input and output and hence the name is Common Base Configuration.

Input characteristics are obtained between the input current and input voltage at constant output voltage. It is plotted between VEE and IE at constant VCB in CB configuration.

Output characteristics are obtained between the output voltage and output current at constant input current. It is plotted between VCB and IC at constant IE in CB configuration.

Procedure:

Input Characteristics:

• Connect the circuit as shown in the circuit diagram.

• Keep output voltage VCB = 0V by varying VCC.

• Varying VEE gradually, note down emitter current IE and emitter-base voltage(VEE).

• Step size is not fixed because of nonlinear curve. Initially vary VEE in steps of 0.1 V. Once the current starts increasing vary VEE in steps of 1V up to 12V.

• Repeat above procedure (step 3) for VCB = 4V.

Output Characteristics:

• Connect the circuit as shown in the circuit diagram.

• Keep emitter current IE = 5mA by varying VEE.

• Varying VCC gradually in steps of 1V up to 12V and note down collector current IC and collector-base voltage(VCB).

• Repeat above procedure (step 3) for IE = 10mA.

• Repeat above procedure (step 3) for IE = 10mA.

Plotting Graph

• Plot the input characteristics for different values of VCB by taking VEE on X-axis and IE on Y-axis taking VCB as constant parameter.

• Plot the output characteristics by taking VCB on X-axis and taking IC on Y-axis taking IE as a constant parameter.

Calculations from Graph:

The h-parameters are to be calculated from the following formulae:

• Input Characteristics: To obtain input resistance, find VEE and IE for a constant VCB on one of the input characteristics.

• Input impedance  = hib = Ri = VEE  / IE (VCB  = constant)

• Reverse voltage gain = hrb = VEB / VCB   (IE = constant)

• Output Characteristics: To obtain output resistance, find IC  and VCB  at a constant IE.

• Output admitance = hob = 1/Ro = IC / VCB  (IE = constant)

• Forward current gain = hfb = IC / IE (VCB = constant)