illrrrrrrW

How ever , in mo st class A amplifiers, the
bias level is no t adjustable by the opera tor,
so this particular examp le won 't be met
ofte n in pra cti ce.
If th e in put signal is out of the range
int ended by the designer, other bad things
hap pen. If the input signal is too weak, the
pro bable effec t would be simply low ou t­
put , possibly contamina te d with no ise, but
most class A amplifiers are int ended to
work with signa ls which range righ t down
to th e noise level any way , so the designer
has pro bably intended his circuit to work
with input signa ls approaching zero volt s.
When grid current flows, t he input signal is
distorted before it ever reaches the act ive
device, and the distortion can't be remo ved
lat er. Even if grid cur rent does not flow.it's
possible to «saturate" th e ac tive device so
th at it no longer changes in resistance as
the input signal changes, and this too leads
to distortion .
Meanw hile, the negative-going peaks of
the in put signal add to th e o perating bias,
and ma y add to it enou gh to change the
op erating con ditions to class AB instead of
class A. This also prod uces distortion in the
a utput signal.
@
®
Fig. 8, In this exam ple the bias is prop erly set
bu t inp~t signal is excessive. SJgnal A is at
maximum level,' anq prod uces linear output X.
Signal B, at too high a level, runs outside the
linear range and produ ces distorted output Y
with bot h bottoming and flat·topping eviden t.
Too mu ch input signal, on the other
hand , pro duces all sorts of ill effec ts, and
yo u can find ho rrible examples all over any
of th e bands.
When in put signal level in a class A
amplifier is too high , either or both of two
prob lems arise (Fig. 8). T he positive-going
peaks of the input signal completely overcome
th e amplifier 's op era ting bias, which
permits grid current to flow . If the designer
has intend ed grid cur rent, and allow ed for
it, thi s is no t in itself bad - but most class
A amplifiers are not mean t to be used this
way , and so grid cur rent in 99 out of 100
of th em indi cates excessive input signa l.
74
An osc illoscope will sho w you which of
these two problems is present in an amplifier.
If the pos itive peaks are causing the
problem, the volta ge waveform of th e
ou tput signal will tend to be flatten ed off
at its negati ve peak ; if it 's th e negative peak
of the inp ut signal doin g the dirt y wo rk ,
the out put signal will be flattened at its
positive peak.
In either case, the cure is simple : reduce
the level of th e input signal. The no rma l
control for doing so is th e audio gain
contro l on the transmitt er panel, or th e
volume contro l on th e receiver. If your
transmitter has no audio gain control (a nd
73 MAGA Z INE