Lab 5

Electronics 1
The purpose of Lab 5 is to build and evaluate transistor biasing circuits. Three types of circuits are
evaluated; base biased, voltage divider and emitter biased. Operating Q point is determined using
measured values of voltages and currents.

The first circuit to be examined is a base-biased circuit which consists of a resistor (RB) connected
between the collector supply voltage and the base as shown in Figure 1.
VCC
15V

RB RC
330kΩ 1kΩ

Figure 1

Measurements are taken using two different transistors, T1 & T2. The results, as evidenced by the data
in Figure 2, show that base-biased circuits are highly dependent on β.
Transistor VB VC VE VCE IC IB βDC
T1 .707V 6.91V 0 6.9V 8.2mA 43.6µA 188
T2 .707V 6.28V 0 6.3V 8.8mA 43.7µA 201

Figure 2

Load line and Q-points are in Figure 3.

15 Ma

8.8 Ma Q1
8.2 Ma Q2

6.3 V 6.9 V 15 V

Figure 3

2
By inserting an emitter resistor, as shown in Figure 4, the circuit becomes an emitter-feedback bias
circuit.

VCC
15V

RB RC
330kΩ 1kΩ

RE
2.2kΩ

Figure 4

As the data in Figure 4 indicates, this circuit is less susceptible to variations in β.

Transistor VB VC VE VCE IC IB βDC
T1 8.7V 11.4V 8.2V 3.2V 3.7mA 18.7µA 198
T2 7.9V 11.7V 7.4V 4.3V 3.4mA 21.3µA 160

Figure 4

Load line and Q-points are in Figure 5.

4.7 Ma

3.7 Ma Q1
3.4 Ma Q2

3.2 V 4.3 V 24 V

Figure 5
The next circuit is a voltage-divider circuit as shown in Figure 6.

3
 15V

R1 RC
4.7kΩ 1kΩ

R2 RE
2.2kΩ 1kΩ

Figure 6

Voltage measurements are taken and calculations are made for I C and βDC for each transistor as shown in
Figure 7.
Transistor VB VC VE VCE IC IB βDC
T1 4.8V 11V 4V 7V 4.1mA 25µA 164
T2 4.8V 11V 4.1V 7V 4.1mA 22µA 186

Figure 7
See Figure 8 for load lines and Q-points. In this circuit β has minimal effect on current and voltage.

7.5 Ma

Q1
4.1 Ma
Q2

7V 15 V

Figure 8

The final circuit, shown in Figure 9, is an emitter-bias circuit.

4
 15V

RC
1kΩ

RE
2.2kΩ 1kΩ

-VEE=-9V

Figure 9

Measurements are shown in Figure 10.

Transistor VB VC VE VCE IC IB βDC
T1 -.09V 6.6V -.8V 7.5V 8.4mA 44µA 189
T2 -.1V 6.9V -.8V 7.7V 8.1mA 50µA 166

Figure 10

For better understanding of this circuit, an equivalent circuit is shown in Figure 11.

VCC
15V

RC
1kΩ
15 V

2.2kΩ RE
9V
1kΩ

9V
-VEE=-9V

Figure 11

Load lines and Q-points are shown in Figure 12.

12 Ma

5
 8.4 Ma Q1
8.1 Ma Q2

7.5 V 7.7 V 24 V

Figure 12

Removing the base resistor and connecting base directly to ground results in the following readings.

VC = 6.8V
VE = -.7V

There results illustrate that RB has minimal effect on voltage and current.

Transistor biasing is important because it establishes a DC operating point, a Q-point, which puts the
transistor in the amplification zone. An amplifier biased with incorrect voltages might operate in cutoff
or saturation resulting in output voltages that are clipped on the positive or negative half-cycles.
Additionally, proper biasing allows for a signal to be amplified without distortion which, in many cases, is
desirable.