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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.