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University of Bahçeşehir
Engineering Faculty
Electrical-Electronics Department
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EXPERIMENT 7
Report
BJT Amplifiers
Objective
To determine the quiescent operating conditions of the voltage divider bias BJT configurations.
Tools and Equipments Required
DMM (Digital Multi Meter)
DC Power Supply
Function Generator
Oscilloscope
680 Ω
x1
1.8 kΩ
x1
33 kΩ
x1
6.8 kΩ
2N3904
1 µF
10 µF
22 µF
x1
x1
x1
x1
x1
Theory and Descriptions
Bipolar transistors operate in three modes: cut-off, saturation, and linear. In each of these modes, the
physical characteristics of the transistor and the external circuit connected to it uniquely specify the
operating point of the transistor. In the cut-off mode, there is only a small amount of reverse current
from emitter to collector, making the transistor akin to an open switch. In the saturation mode, there is
a maximum current flow from collector to emitter. The amount of that current is limited primarily by
the external network connected to the transistor; its operation is analogous to that of a closed switch.
Both of these operating modes are used in digital circuits.
For amplification with a minimum of distortion the linear region of the transistor characteristics is
employed. A DC voltage is applied to the transistor, forward-biasing the base emitter junction and
reverse-biasing the base-collector junction, typically establishing a quiescent point near or at the center
of the linear region.
In this experiment, we will investigate two biasing networks: the fixed-bias and the voltage-divider
bias configuration. The former has the serious drawback that the location of the q-point is very
sensitive to the forward current transfer ratio () of the transistor and temperature. Because there can
be wide variations in beta and the temperature of the device, it can be difficult to predict the exact
location of the Q-point on the load line of a fixed bias configuration.
The voltage divider bias network employs a feedback arrangement that makes the base-emitter and
collector emitter voltages primarily dependent on the external circuit elements and not the beta of the
transistor. Thus, even though the beta of individual transistors may vary considerably, the location of
1
the Q-point on the load line will remain essentially fixed. The phrase “beta-independent biasing” is
often used for such an arrangement.
PROCEDURE
PART 1. Voltage-Divider Configuration.
a. Construct the network of Figure 6.2. using the 2N3904 transistor. Write down
the measured value of each resistor .
VCC
20V
R1
RC
33k
1.8k
VC
Q1
VB
2N3904
VE
R2
RE
6.8k
680
Figure 6.2.
b. Using the beta(200) for the 2N3904 transistor, calculate the theorical levels of
VB,VE, IE, IC, VC,VCE and IB for the network of Figure 6.2. Insert the results in
Table 6.3.
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2N3904
VB
VE
VC
VCE
IE
IC
IB(µA)
Calculated [Part 3(b)]
Measured [Part 3(c)]
Table 6.3.
c. Energize the network of Figure 6.2. and measure VB, VE, VC, and VCE . Record
their values in Table 6.3. In addition, measure the voltages VR1 and VR2 . Try to
measure the quantities to the hundredths or thousandths place. Calculate the currents
IE and IC and the currents I1 and I2 (using I1=VR1/R1 and I2=VR2/R2) from the voltage
readings and measured resistor values. Using the Kirchhoff’s current Law. Insert the
calculated current levels for IE,IC,and IB in Table 6.3.
How do calculated and measured values of Table 6.3. compare?
PART 2. BJT Amplifier Circuit
a) Calculate values of DC bias voltage and current for the circuit of fig.2.1 for β = 200.
Figure 2.1
PART 3. Low-Frequency Response Measurements
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a) Construct the network of fig.2.1. Record the actual resistor values in the space
provided in fig.2.1, if desired. Adjust VCC = 20 V. Apply an input AC signal,
Vsig = 10 mV, at a peak frequency of f = 10 kHz. Observe the output voltage using an
oscilloscope. If VO shows any distortion, reduce Vsig until output is undistorted, with
your lab. assistant.
b) Calculate the magnitude of amplifier gain using
AV  
RC / / RL
eq.2.6
re
AV =
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