CTU: EE 375 – Electronics 1: Lab 3: BJT Amplifier

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Colorado Technical University

EE 375 – Electronics 1

Lab 3: BJT Amplifier

March 2010

L. Schwappach and C. Fresch

ABSTRACT: This lab report was completed as a course requirement to obtain full course credit in EE375, Electronics 1 at

Colorado Technical University. This lab report examines the Bipolar Junction Transistors Amplifiers. This device is used in many

applications such as amplifying and switching. This lab will focus on the gain, input/output resistance and the frequency bandwidth

using a common emitter circuit. In particular how to achieve a gain of 7 with the selected load resistance of 10kohm and how to obtain

the Re, Rc,R1,and R2.

If you have any questions or concerns in regards to this laboratory assignment, this laboratory report, the process used in

designing the indicated circuitry, or the final conclusions and recommendations derived, please send an email to

LSchwappach@yahoo.com or cfresch24@comcast.net. All computer drawn figures and pictures used in this report are of original and

authentic content. The authors authorize the use of any and all content included in this report for academic use.

I. INTRODUCTION

HE Bipolar-Junction-Transistors-Amplifiers are

active devices used in many applications such as

switching and amplifying and etc. The DC biasing determines

the operating point of the device and its performance

characteristics. The BJT transistor structure contains three

regions, the emitter, base and collector. The objective of this

lab is to design and gain a understanding of how an amplifier

can be built by using a BJT.

II. OBJECTIVES

The objective of this lab is to design and gain an

understanding of the physical structure, operation, and

characteristics of the bipolar junction transistors (BJT). In

particular how to determine the gain, input resistance, output

resistance, and the frequency bandwidth of the amplifier by

simulation, hand calculations, actual measurement. The last

objective is to recognize the discrepancies between the three

and why they may differ from each-other.

III. DIODE THEORY

A bipolar (junction) transistor (BJT) is a three-

terminal electronic device constructed of doped semiconductor

material and may be used in amplifying or switching

applications. Bipolar transistors are so named because their

operation involves both electrons and holes. Charge flow in a

BJT is due to bidirectional diffusion of charge carriers across a

junction between two regions of different charge

concentrations. This mode of operation is contrasted with

unipolar transistors, such as field-effect transistors, in which

only one carrier type is involved in charge flow due to drift.

By design, most of the BJT collector current is due to the flow

of charges injected from a high-concentration emitter into the

base where they are minority carriers that diffuse toward the

collector, and so BJTs are classified as minority-carrier

devices.

The proportion of electrons able to cross the base and

reach the collector is a measure of the BJT efficiency. The

heavy doping of the emitter region and light doping of the

base region cause many more electrons to be injected from the

emitter into the base than holes to be injected from the base

into the emitter. The common-emitter current gain is

represented by β; it is approximately the ratio of the DC

collector current to the DC base current in forward-active

region. It is typically greater than 100 for small-signal

transistors but can be smaller in transistors designed for high-

power applications. Another important parameter is the

common-base current gain, α. The common-base current gain

is approximately the gain of current from emitter to collector

in the forward-active region. This ratio usually has a value

close to unity; between 0.98 and 0.998. Alpha and beta are

more precisely related by the following identities (NPN

transistor):

IV. DESIGN APPROACHES/TRADE-OFFS

The performance of this lab will depend on how well

the circuit is developed. If the circuit is developed correctly

the results should be similar to the simulation results that were

obtained by PSpice and the hand calculations (analysis). The

performance of the lab also depends on how well the

equipment is calibrated and accurate the components tolerance

is.

T

CTU: EE 375 – Electronics 1: Lab 3: BJT Amplifier

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This is not a very cost effective lab except for the

development and time it took to construct the lab components.

But to save money for a lab project, whether it’s the testing or

developing phase of a new design, depending on what the

schematic is, a circuit can be reduced, if done correctly.

V. HAND CALCULATIONS

The hand calculations used for this lab (See figure 1)

can be found below.

Equations:

-Rc||Rl/Re = Av

Ri = R1||R2||Rib

Rib = Rpi + (1+Beta)Re

Rpi = Beta(Vt)/Icq

AV = 7, choose Re = 1kohms

7 = Rc||RL/Re

7 = Rc||10k/Rc + 10k

7Rc + 70k = 10RcK

Rc = 3Rc = 70 Rc = 23.3Kohms

Choose R2 = 15kohms

10 – Icq(34) -1 = (Icq –0.1)(7.6744)

9 – 34Icq = 7.67(Icq – 0.1)

9 – 34Icq = 7.67Icq – 0.767

9 = 41.674Icq – 0.767

9.767 = 41.674Icq

Icq = 0.234mA

Icq + Icq/Beta = Ie

Ie =0.234+( 0.234/200) Ie = 0.23517

Ie (Vbe) = Vb Vb = (0.23517)(0.65)

Vb = 0.885

15k(10)/R1 + 15K = 0.885

150k = 0.885R1 + 13.275

136.725 = 0.885R1

R1 = 154.49kohms

Ri = R1||R2||Rib

Rib = Rpi + (1 + Beta)Re

Rib = 22.1 + (201)(1k) Rib = 223.1

Ri = 154.49||15||223.1

Ri = 12.88kohms

VI. CIRCUIT SCHEMATICS

The circuit schematics below were built in PSpice

and allowed our team to analyze the circuit digitally before

performing the physical build.

Figure 1: BJT LT-Spice diagram showing Common

Emitter Circuit for EE-375 Lab #3

VII. COMPONENT LIST

The following is a list of components that were used in

constructing the BJT amplifier from the giving specs in lab

#3. Component values were selected by the professor.

A digital multimeter for measuring circuit

voltages, resistor resistances, and capacitor

capacitance.

A oscilloscope for viewing the input and output

waveforms of the circuit.

A power supply capable of producing Vcc = 10V

A Pulse Generator capable of delivering input

voltage of (100mV) and a signal at about 2Khz.

A 2N3904 Transistor, V(BE) = .65V, Vt = 0.026V

Beta (B) = 200

5 resistors RL = 10kohms, Rc = 23.2kohms,

(raised to 33kohms), Re = 1kohms, R1 =

154.49kohms (raised to 160kohms) and R2 =

15kohms.

Two capacitors C1 and C2 = 0.1uF

Bread board with wires.

NOTE: Resistors can normally provide around +/-

5%-25% difference between actual and designed

values while Capacitors generally provide around

CTU: EE 375 – Electronics 1: Lab 3: BJT Amplifier

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20%-50% difference between actual and designed

values. You can add resisters in series as (R1+R2)

to closer approximate required resistance values

and you can add Capacitors in parallel as (C1+C2)

to closely approximate required capacitance.

VIII. PSPICE SIMULATION RESULTS

The P-Spice simulation results below confirmed our

circuit schematics and allowed our team to confirm the circuit

digitally before performing the physical build.

Figure 2: BJT Amplifier P-Spice Simulation Results

Voutput and Vinput

Key: Green line = Vout, Purple line = Vin

Scale: Vout ( Y – Axis) Range: -800mV to 800mV (400mV

increments)

Vin (X – Axis) Range: 1.000s to 1.0045s (0.005s

increments)

PSpice results show that when Vin ranges from -100mV to

100mV. It is showing that Vin is 200mV peak-peak.

Vout = Pspice results show that Vout ranges from 667.466V

to -694.957V. Vout = 1.362mV. Which produces a gain of

6.81.

Figure 3: BJT Amplifier Circuit Schematic P-Spice

Simulation Results from Figure 1.

Figure 3 is a graph of a Bode line. This is showing the FL

(Low Frequency Range and High Frequency Range)

Figure 4: BJT Circuit Schematic LT-Spice Simulation

DC Bias Results

Figure 5: BJT Amplifier Circuit Schematic Actual

Experimental Results from Figure 1.

The graph above is the actual experimental results. It shows

that the Vpp voltage/200mv is appx the gain (Av). Rc and R1

were raised to increased to achieve a close gain of 7.

CTU: EE 375 – Electronics 1: Lab 3: BJT Amplifier

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IX. EXPERIMENTAL DATA

The above diagram is the experimental data.

The change in the Vpp voltage alters between 1.35V and 1.38

depending on what resistor values were used. The gain is

approximately 7, according to this result Vpp/.2mV =

1.35/0.2.

X. ANALYSIS/DATA COMPARISON

The analysis/PSpice/Experimental data results were

all accurate, but the results differed between the three. The

reasons that the results were different is because the

experimental results have equipment calibrations, component

tolerances, and actual measures values from the components.

The PSpice results is a close estimate of what the results

showed actually be. For example figure 1 shows a graph of

what the output showed look like. And the actual results

verifies that to be true. The analysis is a good estimate of what

the PSpice results should be. If the PSpice results match the

analysis results, then it’s time to work on the actual lab. All

three were not in total, agreement however, the results were

close to each other, and can be proved by applying the PSpice

Results from Figure 1 to the experimental results.

XI. CONCLUSIONS

The DC Bias values that were calculated by hand are similar

to that of the P-Spice results. The results from the different

methods proves that the P-Spice and hand calculation are

correct.

The measured characteristics closely matched those

in the specifications. A 2N3004 Transistor was used , a

requirement of a gain of 7 was closely achieved because we

obtained a gain of 6.89 from the actual experiment results. The

input resistance that had to be at least 12K was achieved

because the input resistance that we resulted in was

12.9Kohms. The signal was undistorted which is a

requirement for the lab. There was no clipping, or saturation.

Look at figure 5.

This BJT Amplifier lab may be improved to achieve

a closer gain of 7 by altering Rc. Depending on the other

resistor values, whether or not to increase or decrease the

resistance value.

XII. ATTACHMENTS

All figures above follow.

REFERENCES

[1] D. A. Neamen, “Microelectronics: circuit analysis and design - 3rd ed.”

McGraw-Hill, New York, NY, 2007. pp. 1-107.