There are two audio circuits in the NorCal 40A, the Automatic Gain Control, or AGC, and the Audio Amplifier. The AGC is an attenuator with JFETs that act as variable resistors. The Audio Amplifier is the LM386N-1, made by National Semiconductor. This integrated circuit appears in many different audio systems, and it costs about a dollar. The “–1” indicates a supply voltage range from 4 to 12 V. A “–4” version is available that allows a supply voltage up to 18 V. For the LM386N-1, the maximum output power is about one watt. In standby, it draws about 4 mA, which is a reasonable level for running off batteries.

Audio Amplifier

Figure 13.1 shows the schematic for the LM386N-1. It is more complicated than the previous circuits that we have looked at, and because it is an integrated circuit, most points are not accessible for measurements. There are three stages of amplification. The input is a differential amplifier, but it is made with pnp transistors rather than npn transistors. This turns things upside down. Each input has a pair of stacked pnp transistors. The stack gives the effect of a single transistor with a current gain of β2. The differential amplifier is followed by a common-emitter stage. The output is a Class-B emitter follower that is similar to the one we discussed in Section 10.6.