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Class-AB Amplifier Design

Overview

Your charger is to design, simulate, build, and test a simple, but complete class-B, or class-AB

amplifier. Do not use a complete IC. Rather, use discrete BJTs for the output stage. The designshould include a design for the power supply. The amplifier must meet the following

requirements.

Load resistance 8 Short circuit protection 0.6 A

Volume control 1000:1

Power 1.2 W

Input Resistance > 50 kInput Voltage 30100 mV

Distortion < 0.5%

Frequency Range 50 Hz10 kHz

Power supply 115 V mains

Table 1.Power Amplifier Specifications.

Designmeans completely considering each important issue. For example, students will use

the supplied 2N3055 power BJT. However, as part of the design process they must make sure,

and document, though calculations and then SPICE simulations, that the power ratings of the

BJT are adequate. If their design requires a heat sink, they should calculate the thermal

resistance of the heat sink, and specify a part number and cost. A useful mechanism is to

maintain a Bill of Materials (BOM). For each part, there should be vendor (i.e., Digikey,

Mouser) part number, and cost. Further, students must be able to motivate the choice of the

component.

Team

This is a team (2 members per team) effort. The instructor and T.A. will evaluate circuits and

reports on a team-by-team basis.

Design & Simulation Roadmap & Suggestions

1. Decide on a general approach, block-diagram-style, and check with the instructor.2. First, capture the core of the design in SPICE. Core means the high-gain front-end, and

class-AB output stage, but no short-circuit protection during this phase.

3. Simulate the design, and fix problems.4. Add volume control and simulate.

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5. Add short circuit protection and simulate.6. Build the circuit and test the amplifier.7. Design the power supply.8. Build and test the power supply.9. Put the whole project together10.Do not reinvent the wheelreuse some of the circuits in the textbook, but do not recycle

too much of othersideas, otherwise you will not learn as much as you could.

11.Use op-amps for the volume control and driver for the power output stage.12.Use negative feedback to reduce cross-over distortion, rather than more complex bias

circuits.

13.Pick a design that uses supplies. This requires a dual power supply, but the overalldesign is easier.

14.Use BJTs rather than FETs for the output transistors.15.The design specifications are such that you do NOT need to use Darlington transistors for

the output transistors.16.Invest in a plastic breadboard so you dont have to disassemble/reassemble the circuit.17.Do not be shy to ask the instructor for help.

Parts

The Electronic Circuits stocks the parts listed in the tables below. If students need other

components, they need to coordinate with the instructor, allowing adequate (1 week) time to

order components.

PN2222 or PN2222A Small-signal npn BJTs.

2N3906 and 2N2907 Small-signal pnp BJTs.

2N3055 Power npn BJT, complementary to MJE2955.

MJE2955Power npn BJT, complementary to 2N3055. The student version ofMicro-Cap does not have this part, however, a SPICE model is given

below, and on the class website.

1N914 or 1N4148 Small-signal/switching diodes.

2N7000 N-channel MOSFET.

Resistors E24 series + a selection of power resistors.

Capacitors

Selection on non-polarized capacitors, and electrolytic capacitors that

range 10 F4,700 F. As soon as students have a working design inSPICE, check and make sure the components are available.

Table 2. Parts that are stocked in the Electronic Circuits Lab.

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CountGain BW

(MHz)Input

Vos

(mV)

Slew Rate

(s/V)

Output Drive

(mA)

UA741CN Single 1 BJT 5 0.5 ~15

NE5532 Single 10 BJT 5 9 ~ 40

LF353 Dual 4 JFET 10 13 ~ 10

LF347 Quad 4 JFET 5 13 ~ 25

Table 3. Op-Amps that are available. These also have Micro-Cap SPICE models.

Some parts such as potentiometers. Students must specify a potentiometer (part of the volume

control) as part of their design. After the T.A. or instructor has checked the design, they may

suggest an alternate part: this will enable a bulk order of the same part for all teams, rather than a

different potentiometer for each team. The same applies to transformers, bride rectifiers and so

on.

Spice Model for MJE 2955 pnp Power BJT

*.model MJE2955 ako:NSC_5A PNP() ; case Mot 90 (s)

.MODEL MJE2955 PNP (BF=137.6 BR=5.88 CJC=870.4p CJE=390.1p FC=.5 IKF=1.642

+ IKR=3.555 IS=66.19f ISC=273.5f ISE=862.2f ITF=71.33 MJC=.6481 MJE=.4343

+ NC=1.24 NE=1.481 NK=.5695 RB=.1 RC=79.39m TF=23.21n TR=235.4n VAF=100 VJC=.75

+ VJE=.75 VTF=10 XTB=2 XTF=5.982)

Amplifier

Students must provide a working SPICE simulation file, as well as a detailed schematic and a

Bill of Materials (BOM) that list all the components, important specifications, vendor part

number, and cost. After the T.A. or instructor has checked the design, they may suggest

alternate parts: this will enable a bulk order of the same part for all teams, rather than different

parts for each team.

Power Supply

The power supply is a costly circuit that one can design and test independently from the

amplifier. Students must provide a detailed schematic and BOM that list all the components,

important specifications, vendor part number, and cost. For example, for the smoothing

capacitors one would specify the capacitance, working voltage, ripple current, vendor part

number, and the cost. After the T.A. or instructor has checked the design, they may suggest

alternate parts: this will enable a bulk order of the same part for all teams, rather than different

parts for each team.

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Professionalism

Reread the thoughts on working together that I posted at the start of the semester:

www.iihr.uiowa.edu/~hml/people/kruger/Teaching/ece_55041_2008/Policies.php

Report

Each team must submit a final report in MS-Word format. This report can be short, but must

include all the details such as neat schematic, BOM, table(s) that lists design- and actual

performance, and anything else the team feels are necessary. Neatness and presentation are

important. Along with the report, each team must submit their Micro-Cap SPICE files, ready to

run. Use the T tool in SPICE to embed important information in the SPICE files. For

example, names of team members, design parameters, date, etc.

Comments Regarding SPICE

Lay out your circuit in a conventional manner. That is, similar to how circuits appear in the

textbook: positive power supply rail generally at the top, negative supply rail at the bottom,

rather than one long ground wire, use the ground symbol, and so on. Label components

properly. For example, label the load resistance RL, the signal source VS, etc. This makes

the circuit easier to read, and helps when simulating the transient- and ac responses. Label

various nodes in a sensible manner. For example, label the output node VO, Vout, or

something similar. Use the Micro-Cap tie component to reduce clutter on schematics.

Figure 1.Usage of Micro-Cap SPICE tie component, intelligent labeling of notes, usage of the

text tool for annotation, allows for uncluttered and clear schematics.

Use the built-in functions in Micro-Cap SPICE to measure currents, power, etc., to check your

design. For example, the screen capture below shows how to determine the power dissipated by

the load with AVG(PD(RL)). Here RL is the load resistance, PD is a Micro-Cap SPICE

function for determining the power dissipated, and AVG determines the average value.

http://www.iihr.uiowa.edu/~hml/people/kruger/Teaching/ece_55041_2008/Policies.phphttp://www.iihr.uiowa.edu/~hml/people/kruger/Teaching/ece_55041_2008/Policies.phphttp://www.iihr.uiowa.edu/~hml/people/kruger/Teaching/ece_55041_2008/Policies.php

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Figure 2.Transient analysis using Micro-Cap SPICEs built-in functions and resulting output.

Survival Guide

Given the tight schedule, what should you do if you are behind with the design? Here are some

pointers.

1. Do not miss the deadlines; otherwise you lose a substantial portion of the grade.2. A design is more than just a schematic. It should include part numbers that are available

for purchase at Digikey, Mouser, or the EE Electronics Shop, and include component

cost.

3. Sometimes you have to cut your losses. If you are short on time, consider omitting theshort-circuit protection and the power supply, but keep the volume controlit is easy to

implement. You will lose points, but could still have a successful lab.

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Time Line

The grade sheet on the next page summarizes the milestones.

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Grade Sheet

Name(s): ___________________________

When What Weight Grade

Nov 17, @ 9 p.m. E-mailed the design to the instructor?,10%

Nov 19 Start fixing, modifying based on instructors

feedback.

Dec 1, @ 3 p.m. E-mailed final design so instructor can order parts?,15%

Dec 3, @ noon Instructor has parts available in Electronic CircuitsLab.

Dec 5, Dec 8 Demonstrate amplifier in Electronic Circuits Lab.T.A. will assistset up an appointment with T.A.

Does amplifier deliver 1.2 W with minimal

distortion into 8speaker with 50 mV input? 25%Does the amplifier have a 1000:1 volume control? 10%

Is there an adequate power supply? (if students

dont have a working power supply), we will use abench power supply.

20%

Does the amplifier have short circuit protection? 5%

Dec 10 Demonstrate amplifier (2n chance). Students

grade will be multiplied with 0.8.

Before end of classes Report 15%1Students that miss the deadline will forfeit these points.

2 This assumes the amplifier works, otherwise the points are forfeited.3Design is more than just a schematicit should include part numbers that are available for purchase,

along with critical specifications. For example, for smoothing capacitors: capacitance, working voltage,

and ripple current are important.