NORTH SOUTH UNIVERSITY
DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING
EEE111/ETE111 ANALOG ELECTRONICS I
Experiment No: 07
Name of the Experiment: The BJT Biasing Circuits.
Objective:
Study of the BJT Biasing Circuits.
Theory:
Biasing a BJT circuit means to provide appropriate direct potentials and currents, using external sources,
to establish an operating point or Q-point in the active region. Once the Q-point is established, the time
varying excursions of input signal should cause an output signal of same waveform. If the output signal is
not a faithful reproduction of the input signal, for example, if it is clipped on one side, the operating
point is unsatisfactory and should be relocated on the collector characteristics. Therefore, the main
objective of biasing a BJT circuit is to choose the proper Q-point for faithful reproduction of the input
signal. There are different types of biasing circuit. However, in the laboratory, we will study only the
fixed bias and self bias circuit. In the fixed bias circuit, shown if figure 6.1, the base current I B is
determined by the base resistance RB and it remains constant. The main drawback of this circuit is the
instability of Q-point with the variation of β of the transistor. In the laboratory, we will test the stability
using two transistors with different β. In the self bias circuit shown if figure 6.2, this problem is
overcome by using the self biasing resistor RE to the emitter terminal.
Equipments And Components :
Serial no. Component Details Specification Quantity
1. NPN Transistor C828, BD135 1 piece each
2. Resistor 470Ω, 560Ω, 220KΩ 1 piece each
3. POT 10KΩ 1 unit
4. Trainer Board 1 unit
5. DC Power Supply 1 unit
6. Digital Multimeter 1 unit
7. Chords and wire as required
NORTH SOUTH UNIVERSITY
DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING
EEE111/ETE111 ANALOG ELECTRONICS I
Experimental Setup:
RB 100 KΩ RC 470 Ω
220 KΩ
Figure 7.1: Experimental Circuit 1.
RB1 10 KΩ RC 470 Ω
10 V
RB2 10 KΩ 560 Ω
Figure 7.2: Experimental Circuit 2.
Procedure:
1. Arrange the circuit shown in figure - 6.1 by C828. Record RC and set RB to maximum value.
2. Decrease POT RB gradually so that VCE = VCC / 2.
3. Measure voltage across RC and VCE.
4. Record the Q-point (VCE, IC).
5. Replace the C828 transistor by BD135 and repeat step 3 and 4.
6. Arrange the circuit shown in figure - 6.2 by C828. Record RC and set RB to minimum value.
7. Increase POT RB2 gradually so that VCE = VCC / 2.
8. Measure voltage across RC and VCE.
9. Record the Q-point (VCE, IC).
10. Replace the C828 transistor by BD135 and repeat step 8 and 9.
NORTH SOUTH UNIVERSITY
DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING
EEE111/ETE111 ANALOG ELECTRONICS I
Data Sheet :
Table 7.1 : Data for Fixed Bias Circuit.
Transistor RC VC IC = VC / RC VCE Q-point
(Ω) (volt) (Amp) (volt)
C828
BD135
Table 7.2 : Data for Self Bias Circuit.
Transistor RC VC IC = VC / RC VCE Q-point
(Ω) (volt) (Amp) (volt)
C828
BD135
Report:
1. Which circuit shows better stability? Explain in the context of the results obtained in the
laboratory.
2. Draw the DC load line for both the circuits and show the Q-point.
3. Add the PSPICE simulation waveforms of all the experimental circuits.
