Dynamic Measurements - Elenco Electronics AM/FM-108TK Manual

Adjust your VOM
to
read
9
volts and connect
it
as
shown in Figure
4.
Make sure that
the
battery, or a
9
volt power supply (if available), is properly
connected
and turn the power ON.
The voltage at TP1 should
be
between 3 to 6
volts.
lf you get this reading, go on to
the
next
test.
lf
your circuit fails this test, turn the power
TRANSISTOR BIAS TEST
Move
the positive lead of your VOM to the base of Q11.
Make sure that the power
is
ON. The voltage should
be
between
.5
and .8V higher than the voltage
at
TP1.
All
silicon transistors biased
for
conduction
will
have
approximately .7V from the base to the emitter.
Now
move
the positive lead
of
your VOM to the base of
Q12.
The voltage at this point should be between .5 and
.8V
OFF and check that all of the transistors are
correctly
inserted in the correct locations. The
E
on the transistor
indicates the emitter lead and should always be in the
hole with the E next to
it.
Check that all resistor values
are the correct value and not interchanged.
lower than the voltage at
TP1.
This is because Q12
is
a
PNP type transistor. Turn the power OFF. lf
your
circuit fails this test, check
the Q11 and Q12
are
properly inserted in the circuit board. All
static tests
must pass before proceeding to the Dynamic Tests or
the next section.
lf you do not
have a variable power supply,
skip to the next test.
DYNAMIC MEASUREMENTS
DC GAIN
The DC gain of the audio amplifier is set
by
the current
in transistor
Q10.
Looking at the circuit and assuming
the output bias is
112
of
V+ or 4.5 volts, the base of
Q11
will be .7V higher or 5.2 volts. This is because there
is
a negligible voltage drop across R4B. This means there
is a 3.8 voltage drop across R44. The current through
R44 can now be calculated as 3.8/R44 or
3.8/3.3k
which equals 1.15 milliamps. Since D5 and R42
are
used for biasing transistors Q11 and Q12, the current
through Q10 can be assumed
to be
1.15
milliamps.
The DC gain of Q10 can be calculated as the collector
resistor, R44, divided by the emitter resistor plus the
Effective Emitter Resistance. The
effective
emitter
resistance is actually the dynamic resistance of silicon
and can be calculated by the approximate equation:
Rj = 26
/
l(in milliamps)
therefore,
Rj
=
26
I
1.15 = 22.6
ohms.
Now the DC gain
can be calculated
as:
R44
/
(R46
+
Rj)
or
3300
/
@7
+
22.6) which
equals
47.4.
Figure
5
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