electronics lab 9
TRANSCRIPT
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Electronics Lab
Experiment # 9
Exp: To setup & investigate performance of a BJT amplifier experimentallyand study its Various Characteristics
Apparatus:
1. Common Emitter Amplifier (BC407)
2. Jumper Wires
3. Breadboard
4. Function generator/ variable AC power supply (6VAC)
5. DC power supply(12V)6. Capacitor 10F x 2
7. Resistors
i. 8-10 K x 1
ii. 27 -30 K x1
iii. 300 -500 K x1
8. Oscilloscope
Oscilloscope probes x 3 (1 crocodile clipper end probe, 2 passive probes)
Objectives:
To understand basic common emitter amplifier theoretically.
To investigate performance of a BJT amplifier experimentally.
To be familiar with testing skills of oscilloscope.
Pre-Lab:
In the common emitter amplifier, the emitter is common to both the input and output circuit. Figure 1
illustrates an NPN transistor connected in the common emitter configuration.
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Figure 1 Common Emitter NPN Amplifier
The current relationship of the transistor remains: IE = IB + IC , which is the basic equation applied to all
junction transistors. However, note that in the common emitter amplifier, the base current IB is the input
current, and the collector current IC is the output current.
The forward current gain of the transistor in the common emitter configuration is the ratio of the output
current, or collector current, to the input current, which is the base current. The ratio is referred to as the
beta of the transistor. Beta is expressed by the formula:
Typical betas range between 50 and 200. The true significance of beta lies in the fact that any variations
in base current will result in a much greater change in collector current.
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Figure 2 Amplifying circuit
Voltage gain of the amplifier is dependent upon two factors. The first is the beta of the transistor. For agiven amplifier circuit, the greater the beta, the greater the voltage gain. Secondly, the value of the
collector load resistor is an important factor. The higher the resistance, the greater the voltage change is
across it for a given current change. However, there is a limit to the value of the resistor that can be used.
If the resistance is too high, the voltage drop across it becomes excessive, and distortion results. This
condition is referred to as saturation and occurs when the voltage drop across the collector resistor
becomes close to the value of applied voltage VCC .
Important Facts:
The common emitter amplifier is the most often used transistor amplifier circuit in the electronics
industry.
The current gain of the transistor in the common emitter configuration is the ratio of the collector
current to the base current.
In the common emitter amplifier, the input signal is applied between the base and the emitter, and
the output appears between the collector and the emitter.
For linear operation, fixed DC bias potentials must be applied to the transistor.
The amount of collector voltage and the collector current with no signal input control the
operating point of the amplifier.
A load line is drawn on a set of collector characteristic curves and can be used to determine
instantaneous voltages and currents in a transistor amplifier operation.
Forward bias on the emitter-base junction can be provided by connecting the base of thetransmitter to the power supply voltage (VCC ) through a high value of resistance.
The voltage gain of the common emitter amplifier can be very high.
The common emitter amplifier input resistance is approximately equal to beta times R IB , the
resistance of the forward biased base-emitter junction.
The output resistance of the common emitter amplifier is approximately equal to the collector
load resistance RL.
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The power gain of the common emitter amplifier is the best of three transistor amplifier
configurations and is approximately equal to the voltage gain times the current gain.
The voltage gain of the amplifier depends somewhat on the size of the collector load resistor.
Within limits, the larger the resistor, the higher the voltage gain.
Transistor as an Amplifier:
Procedure:
1. Set up the circuit as follows:
Figure 3 Amplifying circuit
2. Set up the function generator to produce an output with a 6V p-p AC signal and supply it as
controlling voltage to input of circuit. At the same time also attach the Limiting voltage Vcc at thecollector.
3. Set both passive probes one at input ad other at output of the circuit and calibrate the oscilloscope
accordingly.
4. Once all connections are established, power up the circuit.
5. Now slowly operate by changing the base voltage and keep on noting the output voltage.6. Fill in the measurements and plot a voltage measurement graph of Output to input. Also calculate the
amplification ratio of the circuit through readings taken.
Results:
7. Draw the output waveform observed from oscilloscope when input voltage a the base was 3.3 V p-p
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8.
Figure 43 Positive clamper measurements
9. Fill the following with measured results
S.
No.
Base Voltage Applied
V
Collector voltage Observed
V
Ratio
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