Download 2. - AIUB Solution

Title: Bipolar Junction Transistor (BJT): Study of Single Stage Transistor
Amplifier
Abstract:
The aim of the ac analysis is to determine the voltage amplification (AV), current
amplification (Ai), input impedance (Zi), output impedance (Zo), and the phase relation
between the input voltage (Vi) and the output voltage (Vo). After performing the dc
analysis, we will now calculate the small signal parameters depending on the model being
used, draw the small signal equivalent circuit and then perform the ac analysis.
The main objectives of this experiment are to1) Trace the circuit diagram of a single stage transistor Amplifier;
2) Measure Beta (β) of the transistor with multimeter.
3) Measure the Q – Point.
4) Measure the maximum signal that can be amplified with the amplifier without any
distortion.
5) Measure the voltage gain of the amplifier at 1KHz.
6) Measure the voltage gain of the amplifier at different values of load resistance.
Theoretical Background:
The analysis is done assuming that the signal frequency is sufficiently high. Subsequently
it can be assumed that all the coupling capacitors (C1 and C2) and the bypass capacitor
(CE) act as perfect short circuits. Such a frequency is said to be in the mid band of the
amplifier.
The hybrid-π model and T-model can be used for the ac analysis. Those models are valid
only for small signals. The general forms of these models are shown in the following
figures 1 and 2.
Where, Transconductance
Common emitter input resistance
Common base input resistance
Since in this experiment we will mainly concentrate on single stage amplifier where most
widely used single stage transistor amplifier in common emitter configuration is shown in the
figure below.
Pre-lab homework:
Apparatus:
Precaution:
Transistors are sensitive to be damaged by electrical overloads, heat, humidity, and
radiation. Damage of this nature often occurs by applying the incorrect polarity voltage to
the collector circuit or excessive voltage to the input circuit. One of the most frequent
causes of damage to a transistor is the electrostatic discharge from the human body
when the device is handled.
Experimental Procedure:
1. The value of β was measured by using a multimeter for each transistor..
2. Calculated DC operating point of the transistor circuit.
3. Implement the circuit as shown in the figure.
4. Measured the operating point with the help of table: 1 and compare with your
calculated value.
5. Feed ac Signal of 1 kHz at the input and observed the input and output on the CRO.
6. Increased the input signal till the output wave shape starts getting distorted. Measured
this input signal. This is the maximum input signal that the amplifier can amplify without
any distortion.
7. Now feed an ac signal that is less than the maximum signal handling capacity of the
amplifier. Fix the input signal frequency at 1 KHz, Draw the input and output voltage wave
shape and calculate gain.
8. Connected different load resistors and find the voltage gain of the amplifier for each.
Observations and data table:
1. Q – Point of the amplifier.
Vcc
Vc
10V
1.87V
10V
-8.84v
Vcc-Vc
8.13V
Ic=(Vcc-Vc)/Rc
Vce
IB
0.25mA
0.12V
0.18 Ma
12mA
-12.68V
0.16Ma
2. Maximum signal that can be handled by the amplifier without introducing distortion =
50V and Operating frequency = 50KHz
3. Voltage Gain of the amplifier:
Load resistor
10K
100K
Input voltage
300 (pp) mV
300 (pp) mV
Simulation and Measurement:
Output voltage
3.2(pp) V
3.4(pp) V
Gain
10.67
11.33
Calculation:
IB=
Vcc−VBE
RB+(β+1)RE
=
(10−0.7)
32.3K+(75+1)0.32K
=0.16Ma
IC= β IB = 75*0.16 Ma=12mA
VCE =VCC – IC(RC+RE)=10 – 12mA(1.57K+.32K)= -12.68V
VC=VCC-ICRC =10V –(12mA*1.57K)= -8.84
Gain = VO/Vi = 3.2V/300mV = 10.67
Question and Answers :
1. Why do we need capacitors and resistors in this circuit?
Capacitors are used to cutoff the dc signal in the transistor amplifier. Resistors are
used to restrict the flow of the current.
2. Describe the experiment as a whole?
Here the voltage gain of different load resistor was measured. The frequency was
fixed at a maximum level where the waveform wouldn’t distort.
Discussion:
1. The dc power source needs to be switched off while changing the transistor.
2. The trainer board needs to be checked if working properly.
3. The circuit was connected properly and was also checked before taking the
readings.
References:
1. American International University–Bangladesh (AIUB) Electronic Devices Lab
Manual.
2. A.S. Sedra, K.C. Smith, “Microelectronic Circuits,” Oxford University Press (1998).
3. J. Keown, ORCAD PSpice and Circuit Analysis, Prentice Hall Press (2001)
4. P. Horowitz, W. Hill, “The Art of Electronics,” Cambridge University Press (1989).