Analog Electronics

86 Analog Electronics

but

V,

supplies

a

pulsating current, namely

a

full-wave rectified sine wave

of

peak value

21,,.

Now the average value

of

half

a

cycle

of

a sine wave

is

2/tt of

the peak

(a

handy result

to

remember),

so

the power drawn from

the

supply

V,

works

out at

(8/Tr)(Ks7/?i/)

compared with

41/s'//?,

for

the class

A stage. Thus

the

maximum efficiency

at

full

output

for a

push-pull class

Β

stage works

out at

η

=

ΤΓ/ 4

or

78%, against 50%

for the

class

A

stage.

Of course, with practical devices, more like Figure

4.1c than

b, the

achievable efficiency will

be

less,

say 70%

at

best; this gives rise

to the

rule

of

thumb

that

a

class

Β