Why is it that the audio circuit power supply can output normally even without negative voltage?

Audio signals generally have positive and negative signals. We are accustomed to using 1kHz sine waves to describe or test them.

Figure 1 Audio signals have positive and negative

Generally, the model in the circuit is similar to the figure below, with a positive voltage +VCC and a negative voltage -VEE power supply.

Figure 2 Positive and negative voltage power supply

The application of transistor circuit is similar to the following model. For the convenience of example, we assume that the power supply voltage is ±12V.

This circuit uses NPN+PNP transistors to form a push-pull circuit with strong driving capability. It is actually called an OCL circuit. There is a lot of information on the Internet, so you can check it out. I will not repeat it here.

Figure 3 OCL circuit model

Then, there is another circuit, which is also called OTL circuit. The power supply requirement is relatively simple. It does not require negative voltage power supply. It can output positive and negative voltages normally by only providing positive voltage VCC, as shown in the figure below. Why is this? In response to this question, this article will make a comprehensive analysis.

Figure 4 OTL circuit

Before explaining, let's first roughly sort out the functions of each component.

1. Capacitor C2 can couple the input AC audio signal and has a DC isolation function to prevent the input of DC signals.

2. R4 introduces local negative feedback. When the current and temperature of the transistor change, the voltage drop of R4 will also change, thereby suppressing the change of current and stabilizing the static point.