Ramsey Electronics FX-440 Manual page 76

Stage B:
Receiver Audio Amplifier
The secret to the fine performance of the LM380 audio amplifier lies in
careful selection and physical positioning of the several external
components required to complete its circuit. The LM380 is a self
contained general purpose audio amplifier capable of over 2 watts. audio
output with a voltage gain of 50. Audio from the FM discriminator (U1) is
fed through C7 through the 1
OK
volume control (R7) to pin 2, the
amplifier input. The amplified output at pin 8 is available through C34 to
both the speaker jack and pin 4 of the Packet 1/0 Jack. Capacitor C41 in
series with R108 across this amplified output are recommended good
practice to prevent self-oscillation of the amplifier IC.
Pin 1 is bypassed to ground through C48 in normal operation. If pin 1 is
grounded directly, the internal bias of the LM380 is upset, and the
amplifier is silenced. We are able to put this characteristic to practical
use, using Q6 as a simple switch. When 8 volts is applied through R107
and
022
to the base of
06,
the transistor collector grounds pin
1
of U2,
thus silencing the receiver during transmit. The COR output of U1 (pin
10) also mutes the amplifier,
a
feature which we'll discuss in the context
of Stage CR.
LM380 Audio Amplifier IC
BYPASS
Vee
+INPUT
N.C.
GND
- INPUT
M.C.
GND
OUTPUT
Assm. 16
FX-440