BJT Multistage Amplifiers
Formal Lab Report for ECE 333
Electronics II
Submitted by
Justin Mooney
11057418
Department of Electrical and Computer Engineering
The University of Alabama
April 22, 2010
ABSTRACT
A bipolar junction transistor is a semiconductor device used to amplify or switch
electronic signals. The device is a solid piece of semiconductor material with three connection
terminal. The purpose of this laboratory assignment is to design, build, and study the operation
of a multistage amplifier. The group accomplished this by using a differential amplifier being
driven by a single npn BJT. There are multiple stages because a single stage may not be
sufficient for most applications. Two big advantages of multistage over single stage are that it
has a higher gain, and has flexibility for higher input and output impedances.
TABLE OF CONTENTS
INTRODUCTION .......................................................................................................................... 1
LAB DESIGN ................................................................................................................................. 2
LAB PROCEDURE ........................................................................................................................ 4
CONCLUSION ............................................................................................................................... 8
1
INTRODUCTION
A bipolar junction transistor (BJT) is a three terminal electronic device that is constructed
of semiconductor material and is used for switching or amplifying applications. There are two
types of BJTs, which are PNP and NPN. Both are made of the same material but their physical
characteristics are different. The different regions of the BJT are connected to its separate
terminal E, B, and C. The NPN BJTs consist of a layer of P-doped semiconductors wedged
between two N-doped layers. The PNP BJTs are the opposite of NPN being that the N-doped
semiconductor is wedged between two N-doped semiconductors.
There are four regions of operation for a BJT. These are: forward-active mode, reverseactive mode, saturation, and cutoff. The highest current gain is usually available in forwardactive mode. Reverse-active mode is hardly used. Saturation operates as a closed electrical
switch. Cutoff usually corresponds to an open electrical switch.
In this lab the group is to design a multistage amplifier. This is the use of an active load
consisting of a two-transistor pnp BJT current source. It usually provides a high output
resistance at a larger level of DC current than a passive resistor can provide. Also, a single npn
BJT was used to drive this differential amplifier. The collector current entering the npn BJT will
be the current source driving the differential amplifier.
2
LAB DESIGN
The group first measured and the recorded the β from 2 2N2222 transistors, 2N3906
transistors, and 2 2N3904 transistors. For the first stage, input stage, a common-emitter like
transistor circuit was designed which played the role of the current source using one 2n2222
BJT. A common-emitter configuration is a simple BJT with a resistor on the collector. With the
base voltage being adjusted by a voltage divider with resistors of R1 = 5.6 kΩ and R2 = 1 kΩ.
The emitter of this BJT is connected to a resistor R3 = 1 kΩ. A diagram of this circuit is located
in Figure 1 below. The current, IC, was found by placing a 1 kΩ resistor in between the collector
and ground. The voltage was then measured across the resistor. Using simple ohm’s law, the
current was found.
0
Ic=1mA
R1
5.6k
1B
Q3
2N2222
R2
1k
R3
1k
V-
Figure 1 – Common-Emitter Transistor Circuit
Next, the group designed a differential amplifier using 2 2n3904 BJTs (Q1 and Q2) driven
by the current source designed right before this. This was also connected to an active load
formed by two 2N3906 transistors (Q4 and Q5). Connected to the emitters of Q4 and Q5 was V+.
A clear diagram of this first stage is below in Figure 2.
3
V+
2N3906
Q4
2N3906
Q5
Vout
Q1
V1
Q2
2N2222
V1
V2
2N2222
0
R4
5.6k
1B
Ic=1mA
Q6
2N2222
R5
1k
R6
1k
V+
Figure 2 – First Stage of Amplifier Circuit
The second stage is the output stage. This stage consists of another 2N3906 pnp BJT
transistor (Q6) in a common-emitter configuration. The base of Q6 is the input node and was
connected to V+ through a resistor R5 = 5 kΩ as well as to ground through a resistor R6 = 12 kΩ.
The emitter of Q6 was connected to V+ through a resistor R7 = 100 Ω and its collector was
connected to V- through a resistor R8 = 5 kΩ. The output of this stage is the collector of Q6. A
clear diagram of the entire amplifier circuit is below in Figure 3.
4
V+
2N3906
Q4
2N3906
Q6
Vout
V1
Q1
Q2
2N2222
R7
100
R5
1.2k
2N3906
Q5
R6
12k
V2
V1
2N2222
Vout3
0
0
R4
5.6k
1B
Ic=1mA
Q3
R8
5.6k
2N2222
R5
1k
R6
1k
V-
Figure 3 – Multistage Amplifier Circuit
LAB PROCEDURE
First, the group constructed the current source designed in the lab design procedure. The
digital multi meter was used to measure the output current IO to ensure it was as close to 1 mA as
possible. The values of the resistances had to be changed to achieve this. A value of 0.909 mA
was achieved. The values of resistances used were R1 = 5.6 kΩ and R2 = 1 kΩ.
The differential amplifier designed in the lab design procedure was then added to the
output node of the current source. All of this was done on the breadboard. The group made sure
V+ = +10 V and V- = -10 V.
With both inputs being grounded, the group measured and recorded the voltage at the
one-sided output of this stage. The voltage was at the output of the first stage with no load
attached to it. This voltage was VO = 9.4 V.
5
Next, the group completed the entire multistage amplifier circuit by adding the output
stage circuit, making sure V+ and V- remain the same.
With both inputs grounded, the voltage at the output of the second stage was 5.2 V. This
voltage had to be within 1 V of ground so the group added a potentiometer to achieve this
voltage. It was connected between the emitters of Q1 and Q2 with its center lead connected to the
collector of Q3.
The voltage at the output of the first stage as well as the voltage at the input of the second
stage was 0.906 V. The voltage was the same as it was in step 3 of the lab procedure; where the
voltage was measured.
The group then grounded the input node V2 and applied a sine wave to the input node V1
from the waveform generator. The amplitude was adjusted on the input to produce a nondistorted sine wave. To verify the input was correct, it was connected to the oscilloscope. The
amplitude was not too high since the amplifier had significant gain. The frequency was adjusted
so the maximum gain was obtained. The input and output signal waveform is below in Figure 5.
The input signal is yellow and the output signal is blue.
6
Figure 5 – Input and Output Signal Waveform
The amplitude of the input was 20m and the amplitude of the output was 5. Their ratio,
the differential-mode voltage gain, was 98.
Next, the input signal amplitude was increased to where the output waveform was clipped
at the positive and negative peaks. The output voltage level where the clipping occurred was 500
mV. The clipping occurred with the signal at 52.7 Hz. Clipping occurs because the amplifier is
pushed to create a signal with more power than it can supply, the signal that would occur beyond
the maximum power supplied is cut off. The waveform where clipping occurred is located
below in Figure 6; the output is in blue. The error message is an error saving the waveform.
7
Figure 6 – Clipping Output in Blue
The input signal frequency was increased until the output of the signal dropped to 70% of
its maximum value; this is the -3dB differential-mode bandwidth. This happened at frequency
334 kHz. With both input nodes, V1 and V2, connected to the sine wave generator, the inputs
amplitude was adjusted to produce a non-distorted sine wave at the output. The frequency was
adjusted to that the maximum gain was obtained. The common-mode voltage gain was 105. The
resulting input and output signal waveforms are below in Figure 7. The input is in yellow and
the output is in blue. There is a lot of noise in the input. To find the CMRR (common mode
rejection ratio) for the amplifier use the equation: 20 log ||.
8
Figure 7 – Sine Wave Input and Output
CONCLUSION
A multistage amplifier is more useful than a single stage amplifier in that it has a higher
gain and has more flexibility for higher input and output impedances. A single npn BJT was
used to drive this differential amplifier. The collector current entering the npn BJT will be the
current source driving the differential amplifier. Clipping occurs when the amplifier tries to
output a higher power than what the amplifier can achieve. It was also tough finding a good
frequency to achieve a non noisy input.