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Activity 4
Bipolar Junction Transistors (BJTs) Characteristics
Objectives:
1. To observe BJTs characteristics
2. To measure the voltage output Vrb, Vrc, Vce, Vre
3. To compute the current Ic, Ib, and Ie, and rE
4. To measure Vi, Vo, Av and the frequencies
5. To compare the input and output phases of the circuit
Introduction:
Bipolar junction transistors (BJT) are very versatile in certain applications. It can be used
as an amplifier, a switch, or an oscillator. It has two types, the NPN and PNP transistors. It also
has three terminals labeled as the base, the collector and the emitter. In construction of BJTs, it
should consist of a p-n junction that is reversed biased and the other forward biased. In operating
a BJT, its conditions should be set depending on the stability needed to be satisfied. This is
achieved by biasing.
Fig 1.1 Biasing an NPN transistor Fig 1.2 Symbol for NPN transistor
The arrow in the graphic symbol defines the direction of emitter current through the
device.
Biasing is applying a DC voltage to a transistor in order to turn it on so that it can amplify
AC signals. There are many ways to bias a transistor. Example of the biases are Voltage Divider
output at the Collector and Emitter, and Common-Base Configuration Bias.
Materials:
Bread Board NPN transistor
10V DC Power Supply Connectors
Resistors Multitester
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Function Generator Oscilloscope
Procedure:
1. Construct the circuits below.
Figure 1. Voltage Divider Output at the Collector
Figure 2. Voltage Divider Output at the Emitter
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Figure 3. Common-Base Configuration
2. Measure the voltages across RB1, RB2, RC, RE and the Collector-Emitter Junction and
put the values on the table below.
3. Compute for the current across the collector (IC), emitter (IE) and base (IB), and rE.
4. Connect the circuit to the function generator and the oscilloscope.
5. Measure Vi, Vo, and the frequencies at mid, high and low. Put the measured values on
the table.
Results:
Voltage Divider output at the Collector
DC Response:
IB 95 A
IE 0.423 mA
IC 0.518 mA
VCE 6.52 V
VC 2.75 V
VE 0.57 V
VB1 8.66 V
VB2 1.22 V
AC Response:
Mid Frequency:
rE 61.47ohms
AV -43.125
Vi 128 mV
Vo 5.52 V
fmid 9.92 kHz
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High Frequency:
Vi 126 mV
Vo 2.76 V
fhigh 349.60 kHz
Low Frequency:
Vi 1.44 V
Vo 3.20 V
flow 1.19 Hz
Voltage Divider output at the Emitter
DC Response:
IB 109 A
IE 0.545 mA
IC 0.436 mA
VCE 6.40 V
VC 2.91 V
VE 0.60 V
VB1 8.70 V
VB2 1.24 V
AC Response:
Mid Frequency:
rE 47.71 ohms
AV 0.857
Vi 616 mV
Vo 528 mV
fmid 8.71 kHz
High Frequency:
Vi 480 mV
Vo 264 mV
fhigh 5.32 MHz
Low Frequency:
Vi 296 mV
Vo 248 mV
flow 3.61 Hz
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Common Base Configuration
DC Response:
IB 54.20 A
IE 4.39 mA
IC 4.336 mA
VCE 6.53 V
VC 6.09 V
VE -0.69 V
VRE 4.23 V
VRC 4.19 V
AC Response:
Mid Frequency:
rE 5.92 ohms
AV 85.5
Vi 40 mV
Vo 3.42 V
fmid 91.24 kHz
High Frequency:
Vi 36.40 mV
Vo 1.72 V
fhigh 2.09 MHz
Low Frequency:
Vi 106 mV
Vo 1.76 V
flow 3.04 kHz
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Graphs:
Voltage Divider Output at the Collector
Mid Frequency
Low Frequency
High Frequency
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Voltage Divider Output at the Emitter
Mid Frequency
Low Frequency
High Frequency
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Common-Base Configuration
Mid Frequency
Low Frequency
High Frequency
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Conclusion:
In this activity, the most familiar configurations of the small-signal BJT amplifiers have
been introduced. Each of the configurations has its unique characteristics with its particular AC
and Frequency Analysis.
Voltage Divider output at the Collector
With AC analysis for this configuration, the output voltage is very much greater than the
input voltage. Based on our experiment, the input is only at 128 millivolts while it outputs 5.52
volts. It has a negative voltage gain Av implying that the input and the output are out of phase
from each other.
Voltage Divider output at the Emitter
It is also called the common-collector configuration wherein the collector is shared
between the input and output AC signals and the output follows the input. Typically it has a high
input impedance, low output impedance, and a positive voltage gain slightly less than 1 which is
always non-inverting , implying that it has an opposite phase behavior with the voltage divider
output at the collector. Its input and output are in phase with each other.
Common-Base Configuration
The common base amplifier uses a bypass capacitor or a direct connection from base to
ground to hold the base at ground for the signal only. In the experiment, it is observable that it has
high voltage gain while its current gain is low value (about 1). It also has a low input resistance
but a high output resistance. It also has an in phase input-output behavior since it has a positive
voltage gain.
Frequency Analysis
The interesting and significant part of the analysis of all the three configurations is at their
HIGH frequency response. The Voltage Divider output at the Collector should consider its miller
capacitance so its fhigh is only at 349.60 kHz while the Voltage Divider output at the Emitter and
the Common Base Configuration is significantly both in megahertz, respectively at 5.32 MHz and
2.09 MHz. This is because they both have no miller capacitances to consider.