Radio Shack SSM-1750 User Manual download pdf (Page 18)

poses

of discussion

let us turn

to the

Fig. 4 circuit.

This basic

audio

amplifier

chain

has been

used

in a lot

of equipment

over the

years.

There

are three

stages

shown:

preamplifier,

driver

and power

amplifier.

The

preamplifier builds

up

the

voltage

level of

the

input signal.

The driver

raises the

power

level of the

amplified

signal sufficiently

to drive

the output

power

amplifier

stage.

The output

power

amplifier,

of

course,

develops

the power

to drive

the

loudspeaker.

In Fig.

4B is shown

a simple

circuit

used

in many car

and

home

radios,

though

rarely

in high

fidelity

ap-

plications.

This single

-ended

class

-A

amplifier

uses

a choke

or

autotrans-

former

for output

impedance

match-

ing.

It has several

disadvantages.

For

one

thing,

as a class

-A amplifier,

the

output

collector

current

flows

100

percent

of the

time,

even

when there

is no input

signal.

As a

result, a

lot of

heat

is

generated.

In some cases,

a

3-

to 5 -watt

fuse

resistor ( "fusistor

") is

placed

in series

with

the transistor

to

protect the

circuit

if

excess

heat

causes

QI

to

blow.

Another

disad-

vantage

is

that

fidelity

is not too

good

unless

feedback

is provided.

Though

the

fusistor

provides

a small

amount

of beneficial

degenerative

feedback,

additional

feedback

must

be provided

in most cases.

Input

(A)

(B)

03

Pc

power supply

02

R5

Q1

II

WSpeaker

T

R

Speaker

8t2

-

R7

0.1-0.51-2

fusistor

_ J

Fig.

4.

A typical

audio -amplifier

chain

contains preamplifier,

driver

amplifier

and power

amplifier

stages,

all powered

by

a common

power

supply.

This ar-

rangement

is shown

in block

diagram

form

in (A),

while

(B)

shows

the schematic

equivalent

sans

power

supply.

Two basic

kinds of

feedback cir-

cuits

are

normally used

in audio

cir-

cuits.

One

is called

the "second

collector

-to -first

emitter"

system

(Fig. 5A).

With correct

values of

components,

this

circuit

can

make a

relatively

mediocre amplifier

sound

like

a

more expensive

one.

Figure 5B

shows

the second

widely used

feed-

back system,

dubbed

the

"second

emitter-

to

first base"

system.

This

circuit

often

employs

only

one

resistor

to supply

feedback

signals.

The push

-pull

circuit

is

widely pre-

ferred

over

other

types

for both

power

handling

ability

and overall

fi-

delity.

Figure

6 shows

the

standard

transistor

push

-pull circuit

that

has

IAl

Q2

Ll

>Output

Cl

i'

.

i t

t

, r

R1

EEDBACK

Input

(B)

Output

Fig. 5.

The two basic

kinds

of feedback

circuits

normally

used

in

audio

circuits:

second

collector

to first emitter

(A)

and second

emitter

to first

base

(B).

18 /

MODERN

ELECTRONICS

/ September

1986

Say

You Saw

It In

Modern Electronics

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