Radio Shack SSM-1750 User Manual download pdf (Page 16)

I

/Ill!irmasfous

September

1986

Solid

-State

Audio

Circuits

By Joseph J.

Carr

Audio circuits

are among

the

most interesting

of linear

circuits

with which to ex-

periment.

They are excellent

as

weekend and one

-evening

projects

for

electronics

hobbyists and

experi-

menters.

Part of the

reason

for

their

popularity

is that they

are so useful

to so

many

people; another

is that

they

are generally

well-

behaved

and,

thus, can easily

be built

with low

-cost

components.

In this

first

installment of our

two -

part article,

we

discuss

the basic

cir-

cuits

used

in modern audio-

amplify-

ing

equipment

and explore

both

discrete

transistor and

integrated

-

circuit

(IC) audio

amplifier

circuits.

The information

contained

here

will

lay the groundwork

for the

more

detailed

discussions

of circuits,

built

around

actual

components,

that

will

appear

in next month's conclusion.

Transistor

Biasing

One

of the

most important

factors

in

understanding

solid

-state audio cir-

cuits

is the

methods used to

achieve

proper biasing

of transistors.

Figure

1 shows

several of

the most common

methods.

These circuits,

or

varia-

tions

of them,

are used

in most dis-

crete

and

IC audio circuits.

Figure IA shows

the

simplest

-

and

most practical-

biasing

scheme.

Fixed base -current

bias

is

established

by current

flow

from the emitter

-

base junction

of the

transistor

through

RI to

he supply

voltage.

The amount

of bias

is dependent

on

the

value of

RI and

the supply

vol-

tage.

The primary

disadvantage

of

this bias

arrangement

is that

it pro-

vides

no

means of

automatically

limiting collector

current.

Hence,

Part

1

R1

V+

Rl

v+

R2

(A)

(B)

Cl

V,

(c)

Rl

R3

V,

(D)

Cl

Fig. 1. Several

of

the

most

common methods

for biasing

transistors.

fixed base -current

biasing

yields cir-

cuits that

can be

unstable.

Another simple

form of

self -bias,

shown

in Fig.

1B, is called collector

feedback.

Because

RI is connected

to the transistor

side of

load resistor

R2, any change

in collector

current

will

cause

a proportional

but

oppo-

site change

in transistor

bias.

For ex-

ample,

if collector current

increases

because

of a temperature

increase,

the

voltage at the collector

decreases

(becomes

less

positive).

In turn, this

reduces the current

through the

emit-

ter -base junction

and

Rl circuit.

Al-

though

this bias system

does provide

a degree

of stabilization,

it

also

in-

troduces

deneration

caused

by

feed-

back

of any ac

signal

voltage devel-

oped

across

the load

resistor.

Collector-

feedback

with

ac

bypas-

sing

is the same as

in Fig.

1B, except

that an

electrolytic

capacitor

has

been added

to

filter out (bypass)

variations.

Combination

fixed and

self

-bias

(Fig. 1D) provides

both

good stabili-

zation and

minimum

degeneration.

Fixed emitter

-base bias

is developed

by

the

RI /R3

voltage divider.

Usual-

16 / MODERN

ELECTRONICS

/ September

1986

Fig. 2.

An

amplifier

biased for dual

-

polarity

power supply operation

(A)

and a

typical dual - polarity power

supply

(B).

ly,

the

value

of

R3 is substantially

less than that

of Rl.

Resistor

R4

stabilizes

the transistor.

For exam-

ple,

if

emitter

-to- collector

current

increases because

of an

increase

of

temperature,

the

voltage drop across

R3 also

increases.

This

places

a

more -positive

voltage on the emitter,

Say

You Saw It In

Modern Electronics

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