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Audio Amplifier, Mute Switch & Audio Limiter
The amplifier stage has a gain of 75x (37 db) as set by the 75K feedback resistor and the 1K input
resistor. The 1.5v bias at the + input to the amplifier is also used to bias the 400 Hz HPF above. This
stage includes the volume control to set the listening level of the receiver.
Some receivers use an RF gain pot at the front end of the receiver to control the received audio level.
However, this approach leaves the audio chain at maximum gain all the time, creating audio hiss that
cannot be avoided, even with the RF gain pot turned all the way down. The location of this volume
control, roughly in middle of the gain chain with only 37 db of gain following it, allows the design to
minimize the background audio chain hiss when the volume is turned down. This insures that for all but
exceptionally quite band conditions, the user will be listening to signals on the band and not unnecessary
receiver noise.
The output of the audio amplifier feeds the mute switch, which is a single gate CMOS analog switch.
The output of the gate is biased to 1.5v, the same as the input side, to minimize thump on the transition
to receive (mute gate off to mute gate on) transition. The 0.1 uf cap connecting the mute output to the
next stage also helps further attenuate any remanding low frequency thump. The mute gate has an R/C
delay built in to delay the release of the mute for a few msec after the transmitter to minimize thumps
from earlier in the receiver audio chain.
This receiver is designed to drive headphones. For normal headphones, 20 to 40 mV pk-pk is a very
comfortable listening level. This circuit has a diode clipper to limit the audio to a maximum of 400 mV
pk-pk. With the volume at maximum, the diode clipper starts conducting at 280 mV pk-pk at an input
RF signal of –95 dbm. The SCAF filter that follows this stage tends to smooth out high frequency
Norcal NC2030 v5
12-18-05
Page 116 of 128
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