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diagram.
In
Rl, provided
impedance of
impedance of
OCUBIC.
.
COMMUNICATIONS
. SIMAltI
DIVISION
treatment is .available
in many
competent references
for those who wish to
pursue the matter further.
.
Basically, this test is performed
by
applying two non-harmonically re-
lated audio tones,of equal amplitude and within the pass band of the radio,
to the microphone
input and observing
the output across a dummy·load on an
oscilloscope.
In this manner,
the entire transmitter, from microphone jack
to antenna terminal, is tested in one simple operation.
These tones
can come from two
áudio signal generators, or better yet,
from a two tone
test set
specifically designed
for the purpose.
Several
such designs of adequate quality are included in amateur handbooks and other
readily available literature.
The tones must
be
very pure sine waves,
and
of equal amplitude,
and not harmonically related to each other as mentioned.
The oscilloscope used to view
the transmitter's output must be capable
of displaying the frequencies at least as high as those in use,
and prefer-
ably higher.
A 30 MHz bandwidthinstrument
is adequate for all HF amateur
bands, while the more common 15 MHz bandwidth instrument would serve for the
160, 80,
40
and 20 meter bands.
Since any
problems of nonlinearity,
bias,
etc., could be expected to show up on all bands, testing on one band only is
usually considered adequate.
Several wavefoxms and their common causes
are included here for refer-
ence, and others are available in current literature.
FEEDBACK AMPLIFIERS
The gain of individual transistors, within a given type number, unfort-
unately varies over
a wide range.
Often ,
this is satisfactory for the de-
signer's purpose,
but occasionally there arises a
situation where the gain
of the stage must be controlled
mor~
precisely.
In that case, it is often
advantageous to use an amplifier stage em-
ploying emitter degeneration (feedback) to
set the gain of the stage.
The accompany-
ing diagram
shows one such typical stage.
The
gain of
the stage
is
approximately
equal to
the ratio
of the collector load
to the unbypassed portion
of the emitter
resistance.
In the case of choke or tuned
circuit loads, the value used in this cal-
culation is the absolute value of the col-
lector load impedance.
Utilizing
this technique also raises
the input
and output impedances to higher
values than would otherwise be
the
case,
a condition
that is often desirable.
The
baridwidth of the circuit
can also
be
in-
creased, under the right circumstances, although, under other conditions, it
might be decreased instead.
It is also possibleto
select the value of the
capacitor
bypassing a portion of
the emitter resistance such as to provide
frequency compensation for the inherent
6
dB per octave
drop off in gain as
the frequency is increased, that is a characteristic of all bipolar transis-
tors.
Another type of feedback amplifier is shown in the second
this instanee, the gain isapproximately equal to R2 divided
by
that the load resistance
is much much greater than R2, and the
the source feeding the circuit i s much less than Rl.
The input
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