EGR220 Than & Gab Lab #9

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Introduction and Objectives

Although BJT (Bipolar Junction Transistor) has lost its

polarity in IC (Integrated circuit ) design, it is really

useful in discrete circuit design [1]. In this lab, we

were instructed to measure and analyze the common

emitter and common collector configurations of the

BJT amplifier to determine the voltage gain, input

resistance and output resistance for each configuration.

The primary objectives of this lab are:

1. To analyze and understand DC biasing of BJT

2. To understand common emitter and common

collector BJT amplifiers

3. To able to calculate voltage gain, input

resistance and output resistance

Equipments and Components used

In this lab, the equipments and components we used

are:- Transistor: 2N2222A (x3) Resistors: 510Ω

22KΩ, 100Ω, 10KΩ a breadboard, a waveform

generator, ±20V power supply, a multi-meter, wires

and cords.

Procedures

Procedure 1: Analyzing Common Emitter

Configuration

vO=V

o+v

o(t)

RC=510W

RS=22kW

Figure 1

vi(t)

VI

Ri

Ro

+15V

We connected the circuit as in figure1. Then, we

adjusted the VI until VO = 5V. We also measured the VI

and VBE, and found that VBE = 0.8V and VI = 3.5V.

When compared to our pre-lab calculation (in which VI

= 3.44V and VBE = 0.75V), the measurement was

pretty close. Since, the input signal 1Vpp sine wave,

the biased was enough for signal swing. Then, we

provided input sine wave 1Vpp with 1KHz, and

captured both input and output signals on

Oscilloscope.

Figure 2: Comparison of Vo and Vi for (1Vpp Sine

Wave)

From the screen image, we could see that due to dc

biasing, the output signal consists of both DC and AC

components.

From our measurement, we could see that the output

signal is out of phase with input signal, and the output

voltage (without DC) is 3.3V. From our pre-lab,

calculation, VO = -3.58V. Therefore, we could

conclude that the calculation is close enough to our

measurement.

We also measured the input resistance and output

resistance. In order to measure input resistance, we fist

measured the IB, which is 0.15mA.We calculated

rπ=VT/IB. Since Rin = rπ , Rin = 166Ω. In order to

measure output resistance, we first removed the 15V

power supply and RC, then we gave VO = 5V, and

measured IC, which was 10mA. From that we

calculated output resistance, which was 500Ω. From

our pre-lab calculation, we got rπ = 166Ω and Rout =

460Ω. We found that Rout ≈ RC.

Since VO was biased at 5V, and VCC = 15V, the

maximum swing of output signal is ±5V. From our pre-

lab calculation, input signal should be less than or

EGR220 Than & Gab Lab #9

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equal to 1.29V. Therefore, when we gave input signal

= 1.5V, we saw that the output signal was clipped off.

Figure 3: Clipped off output signal

When we gave VI = -3, the VBE will be reversed

biased, and the BJT will not work for amplification.

Therefore, when we captured the output signal, no

output signal was detected.

Figure 4: Reverse Biased Amplification

Procedure 2: Analyzing Common Collector

Configuration

Figure 5

We connected the circuit as in figure 5. As in

procedure 1, we adjusted the VI until, we got VO =

2V. We also measured VI and VBE, and got VI =

0.75V and VBE = 0.75V, and IB = 0.4 μA. Then,

we captured the screen image of input and output

signal by giving input sine wave 1Vpp with 1KHz

frequency.

Figure 6: Comparison of Vo and Vi for (1Vpp Sine

Wave)

From the screen image, we could see that the

amplitude of output signal was less than that of

input signal, which showed that the voltage gain is

less than 1. Our pre-lab calculation showed that

AV = 0.44 V/V. Our measurement showed that

VO/VI ≈ 0.5V/V.

Then, we measured the input and output

resistance. Since, Common collector is not

unilateral amplifier, we need to take RE into

account in calculation. We fist measured IB, which

is pretty small, and IC. We got IB = 0.47μA and IC

= 0.08mA. Then we calculated rπ and re, and

calculated

ro = (VA + VCE)/ IC

Rin = Rib = (β+1)(re + (ro // RE))

where VA = 92V, β =163.33 (from Lab #8) and

VCE = 8V. We got Rin = 37KΩ.

In order to measure output resistance, we removed

10V power supply and provided Vo = 5V and

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measured IE. Then, we calculated Rout. We found

that IE = 27mA; therefore, Rout = 185Ω. From our

pre-lab calculation, we got Rout = 185.85Ω.

Therefore, the calculation was close enough to the

measurement.

Since the amplitude of output signal is less than

than that of input signal, there is no limitation of

input signal except for the reason that the

transistor might be cut off. Therefore, the

maximum output voltage is 2V. Therefore, the

maximum input signal is 4.5V.

Figure 6: Cut off Output Signal (at Vin = 5Vpp).

Therefore, when we gave 5Vpp for input, the

output signal was cut off.

Discussion We found that in common emitter circuit, the input

resistance is greater than output resistance, and the

voltage gain is very high, almost 300 times. But in

common collector circuit, the output resistance is

higher than input resistance, and the voltage gain is

less than 1. However, the current gain is really high in

common collector circuit.

References [1] Sedra, Adel S., and Smith. Kenneth C. “Microelectronics

Circuits”. 5th. New York: Oxford University Press, 2004.