Ramsey Electronics FZ-146 Specifications download pdf (Page 76)

FX 146 • 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 10K 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 I/O 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 D22 to the

base of Q6, the transistor collector grounds pin 1 of U2, thus silencing the

receiver during transmit. The COR output of U1 (pin 16) also mutes the

amplifier, a feature which we'll discuss in the context of Stage CR.

LM380 Audio Amplifier IC

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Ramsey Electronics FZ-146 Specifications Page 76