Detector and audio circuits

Once the IF amplifier builds the signal up to a point where it can be successfully demodulated with good volume, you will want a detector circuit to recover the modulating audio. The simplest AM detector is the envelope detector shown in Fig. 5-22. The envelope detector is a single-diode rectifier at the output of the last IF transformer. The diode should be a germanium type, such as 1N34, IN60, 1N270, ECG-109, or NTE-109 devices. A volume-control potentiometer serves as the load for the diode, and a 0-GI-jjlF capacitor filters out a residual IF signal that remains in the circuit following demodulation.

D1 IN 60 1N34


CI O.ljiF

lDkO f ~ 0 ]|lF Alternate Circuit

5-22 AM envelope detector circuit.

The detector stage must be capacitor-coupied to protect the following stage because the demodulation process produces a smaJl dc offset. The capacitor strips off the dc and makes the signal once again ac. In some nases, the audio stage following the detector contains the volume control, so an alternate circuit shown in Fig. 5-22 is used at the detector

The basis for the audio stage is the easily obtained LM-386 IC audio subsystem clup (Fig, 5-23 A). This chip is easily obtained in replacement sen ^conductor lines, from RadioShack and from a number of mail-order sources. Although the low power level leaves something to be desired t the LM-386 is both better beltaved and more easily obtained than certain higher-powered audio subsystem chips.

Figure 5-23B shows the basic circuit for the LM-386 chip when it is used as an audio stage for a receiver project. Mot ice that the circuit is extremely simple in that it basically has just input, output, ground, and V+ connections. The circuit is able to

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