Download 4. single stage bipolar junction transistor (bjt) amplifiers

EEE 307-Analog Electronics Laboratory
ABTU-EEE
4. SINGLE STAGE BIPOLAR JUNCTION TRANSISTOR (BJT)
AMPLIFIERS
I. Objectives and Contents
The goal of this experiment is to become familiar with BJT as an amplifier and to evaluate the basic
configurations of BJT amplifiers experimentally. The experiment will consist two parts. In the first part,
student will analyze a single stage BJT amplifier, however in the second part they are asked to design an
amplifier experimentally.
II. Equipment and Components:

Power Supply

Digital Multimeter

Oscilloscope

Signal Generator

Agilent Vee
III. Prerequisites:
Familiarity to the basic BJT operation is required.
I. Theoretical Background
a. In this experiment students will investigate a simple BJT amplifier shown in the figure.1.
Figure 1 –BJT Amplifier
The npn transistor is connected in a common emitter configuration. The input and output
voltage signals are coupled to and from the amplifier with the use of coupling capacitors. The
base resistors RB1 and RB 2 bias the transistor to a Q-point. The collector resistor RC converts
the output current to an output voltage signal. The amplifier output drives a load with resistance RL . The
simple biasing scheme used in this circuit leaves the Q-point sensitive to changes in the value of the
transistor  . The small-signal AC equivalent circuit for this amplifier is shown in the figure.
Figure 2 – AC Equivalent
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The value of the small-signal input resistance is determined by the DC base current; r  nVT / I BQ where
n is the emission coefficient. The small-signal transconductance is given by g m  I CQ / nVT .
b. In the following, some information is presented about design and construct of a single-stage bipolar
junction transistor (BJT) amplifier. The elaborated circuit topology is shown in the figure 3. The npn
transistor is connected in a common emitter configuration. The input and output voltage signals are
coupled to and from the amplifier with the use of coupling capacitors C1 and C2 . The biasing network
consisting of resistors R1 and R2 biases the transistor to a DC Q-point. The emitter resistor
RE  RE1  RE 2 provides bias stability against variations in  . The output of the amplifier is connected
to a load that has a resistance of RL . The total emitter resistance RE is provided by two separate resistors
in series. The second emitter resistor RE 2 is bypassed with a capacitor CE in order to increase the voltage
gain. However, a small amount of emitter resistance ( RE1 ) is left in the AC circuit, the purpose of which
is explained later.
Figure 3 – Single stage BJT amplifier
The small-signal AC equivalent circuit for this amplifier is shown in the figure.
Figure 4 – Small-signal AC Equivalent circuit
It is assumed that rb  0, r  , r0   . It is also assumed that all capacitors can be considered to be short
circuit at the frequency of operation. The value of the small-signal input resistance is determined by the
DC base current r  nVT / I BQ . The transistor small-signal transconductance is given by g m  I CQ / nVT .
It is important to note that the presence of the bypass capacitor CE and the output coupling capacitor C2
result in the AC load line to be different than the DC load line. In designing the biasing network, it is
important to set the Q-point at the center of the AC load line for maximum symmetrical output voltage
swing. It is advisable to leave some margin of safety when considering the maximum peak-to-peak
undistorted (unclipped) output voltage swing while setting the Q-point.
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The unbypassed emitter resistor RE1 serves three functions:
1. Adjust the voltage gain to a desired value: When RE1  0 , the voltage gain of the amplifier is as
high as it can be for the circuit at hand. Setting RE1 to a nonzero value provides a mechanism to
decrease the gain to a desired value.
2. Provide stability against variations in rπ due to variations in the emission coefficient n: The
emission coefficient of a transistor may show a variation from transistor to transistor much like the
transistor  . This variation influences the value of rπ, which in turn affects the voltage gain and
the input resistance of the amplifier. If RE1 is relatively large so that r (   1) RE1 , the voltage
gain and the input resistance are not influenced by changes in the value of n.
3. Provide linear (undistorted) operation for larger input signals: When RE1  0 , the input voltage is
limited to values that are much smaller than VT for linear (undistorted) operation. This is so
because the small-signal approximation exp(vbe / VT ) 1  vbe / VT fail unless vbe VT . However, if
RE1 is relatively large so that r
(   1) RE1 , the base current is mainly determined by the linear
resistance (   1) RE1 , rather than the nonlinear i-v curve approximated by the resistance rπ.
II. PRELIMINARY WORK
1. Each of the students is tested before starting experiment by research assistant. This test may be
written or oral examination. Theoretical knowledge of the experiment that should be gotten from
experimental sheet and other sources (lecture notes, books etc.). The theoretical information about
experiment is not limited to study only experimental sheet, students have to research other
sources to get enough knowledge.
2. Students should know the purpose of the experiment. They should know how the experiment can
be done and which measuring elements can be used. They should also get measuring elements catalog
information.
3. Read the datasheet of BC238 transistor series. What is the typical base to collector amplification
factor of BC238 BJT? What does the gain-bandwidth product mean?
4. Please answer the following questions before attending the Lab.

What is an amplifier?

Which parameters affect the performance of an amplifier?

What are the main features of single-transistor amplifier stages?

What are the important frequency values of gain-frequency curve of an amplifier? How are
they obtained?
5. In the laboratory you will construct the amplifier circuit using the following values (for the first
experiment Figure1. and Figure.2):
Table.1
VCC
15 V
RB1
470 kΩ
RB2
1.8 kΩ
RC
470 Ω
RL
100 kΩ
C
10 µF
The transistor that you will use is BC238B. This transistor has 200    320 . In the
preliminary work section, you are asked to base your calculations on three different values of  ,
namely 1  250, 2  200, and 3  320 . Other transistor parameters are VCE ( SAT )  0.2V and
VBE (ON )  0.6V .
Assume
that
the
emission
coefficient n  1 ,
even
transistors usually exhibit a higher value. For each  value (250, 200, 320):
6. Analyze the DC circuit to determine the Q-point. Find IBQ,
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though
ICQ,
BC238B
and
VCEQ.
EEE 307-Analog Electronics Laboratory
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7. Draw the load line and the transistor iC–vCE characteristics, and indicate the Q-point.
(Separate graph for each β value.)
8. Calculate the peak-to-peak maximum undistorted voltage swing at the output.
9. Draw the AC equivalent model assuming that the capacitors are short circuit at the operating
frequency. Calculate r , g m , and the the voltage gain Av = vout/vin.
10. Simulate the amplifier circuit using LTSPICE. First do a Bias Point analysis to determine
DC voltages and currents. Next, do a Time Domain analysis using an input sine wave
of 2 mV peak-to-peak at a frequency of 1 kHz, and determine the voltage gain. Repeat
with input amplitudes of 40 mV, 140 mV, and 500 mV; note the changes on the output
waveform. Comment on your results.
EXPERIMENT-I (Figure.1 and Figure.2)
In this experiment you are going to use the silicon npn transistor BC238B. Please check the datasheet of
BC238 before attending the laboratory. For this transistor, VBE (ON ) 0.6V and VCE ( SAT ) 0.2V .
Before constructing the circuit, verify the values of the resistors. Make sure that all resistors are within
2%-%5 of their marked values. This will assure that your current measurements are accurate. Construct
the amplifier circuit using the values indicated in the preliminary work section.
1. Before connecting the signal generator, measure IBQ, ICQ, and VCEQ, and compare these with
your calculations. Draw the load line on a graph and indicate the Q-point.
2. Using your measurements from the previous part, determine the β of the transistor that you are
using.
3. Now you have measured the particular β (for n=1.4) values for the specific transistor that you
are using. Calculate the voltage gain of the amplifier using these values.
4. Set the input voltage signal to a sinusoid with 1 kHz frequency and 100 mV peak-to-peak
amplitude. Measure the voltage gain of the amplifier and compare with your calculations from
the previous part. In that step, you should also use Agilent Vee pro. In order to apply a signal
with different frequency (For instance you may apply between 100 Hz- 10 Khz). Observe the
input and output voltage waveforms on the oscilloscope.
5. Gradually increase the input signal amplitude and determine the onset of distortion at the output.
6. Further increase the input signal amplitude and determine the onset of clipping. Measure the
peak-to-peak maximum undistorted output voltage swing. (Here “undistorted” really means
“unclipped.”) Comment on how this value is related to the location of the Q-point on the load
line.
7. Print the output waveform for three different values of peak-to-peak input amplitude:
40 mV, 140 mV, and 500 mV. On each plot, indicate any distortion or clipping that you may
see.
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