Lab Report VII
The Common Emitter Amplifier
10th December 2009 Swapnil Agrawal – Lab 7 - 210049237
Table of Contents
1. INTRODCTION 2
1. Background 2
2. Aim 2
3. Apparatus 2
2. ANALYSIS, CALCULATIONS AND OBSERVATIONS 3
1. Rise and Fall Times 3
2. A NOT Gate 5
3. SOURCES OF ERROR 9
4. CONCLUSION 9
5. REFERENCES 9
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The transistor, like the diode, utilizes the property of the PN junction in a
semiconductor. Unlike the diode, the transistor is a three-port device (with the three
ports referred to as the Base (B), Collector (C), and Emitter (E). The main functions of
transistors are as switches and amplifiers. The common emitter amplifier increases
amplitude of the voltage from an input signal. Since, there is input and output, there is
also some gain (which is the ratio of the power going out to the power coming in)
The purpose of this lab is to understand the properties of the transistor as an amplifier.
a) Agilent 33120A function/Arbitrary Waveform Generator
b) Agilent E3630A Triple Output DC Power Supply
c) Agilent 34401A Digital Multimeter
d) Breadboard, Resistors, Capacitors and 2N3904 (NPN).
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ANALYSIS, CALCULATIONS AND OBSERVATIONS
7.1 Biasing the Amplifier
The circuit of a simple DC Bias Circuit shown on the next page is constructed. All
components are checked before they are used in the circuit. V iccinput from of 12 V
from the DC Power Supply.
Resistor Resistance (Ω) Color band
R1 5.6 kΩ Green, Blue, Red, Maroon, Maroon
R2 680 Ω Gold, Brown, Blue, Dark Blue
R3 6.8 kΩ Gold, Red, Blue, Dark Blue
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R4 1 kΩ Gold, Red, Black, Brown
R5 68 Ω Gold, Orange, Blue, Dark Blue
The NPN Transistor has 2 modes. It is either on or off. It has a turn on voltage of around
0.6 V i.e. it would turn on if the voltage across is more than 0.6 V. The voltage measured
across CE i.e. CEwas found out to be 9.40 V and therefore, the transistor was not
saturated. Also, there was a potential drop across each resistor showing that the
transistor is in the “ON” state.
But if the transistor was in saturated, the circuit would switch off, and a significant
amount of current (coming from the base) would cut off. When a transistor is saturated
the collector current C is determined by the supply voltage and the external resistance
in the collector circuit, not by the transistor's current gain. As a result the ratc BI /I for a
saturated transistor is less than the current gain which is not suitable for our case.
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7.2 Amplifier Gain and Frequency Response
Now, an AC input was connected to the base as shown in the figure below V outwas taken at the
collector of the transistor.
Our chosen amplitude A = 50.00 mV
Polarized Coupling capacitors can have a very high capacitance, allowing filters made with them
to have very low corner frequencies. This "Cut-off", "Corner" or "Breakpoint" frequency is
defined as being the frequency point where the capacitive reactance and resistance are equal.
The .22μF is used to prevent any DC elements from affecting the circuit. The function of
resistors is to bias the transistor and keep it functioning in active mode and not accidentally
into saturation mode.
The gain for this circuit should be the ratio of the resistance at the collector to the resistance at
the emitter i.e.
For a 10 kHz input signal, with amplitude 50.00 mV, V outwas measured to be 0.5 V. Therefore,
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which is consistent with the gain that was expected.
The frequency was then varied from 1 Hz to 1 MHz and the following results were obtained.
Note: Circuit 1 is with the coupling capacitor and Circuit 2 is without the coupling capacitor.
Frequency (in Hz) V outCircuit 1) (in mV)VoutCircuit 2) (in mV)Gain 1 Gain 2
1 24 15 0.48 0.48
25 76 79 1.52 1.52
50 183 145 3.66 3.66
75 204 206 4.08 4.08
100 252 252 5.04 5.04
150 320 316 6.4 6.4
200 356 356 7.12 7.12
300 392 386 7.84 7.84
400 416 408 8.32 8.32