This tutorial will concentrate on the “front end” of the modern audio amplifier design. In Addition to this, this tutorial establishes some of the basics relating to integrated circuit operational amplifiers.
Figure below is a kind of semi-block diagram showing the input stage of most high-power audio amplifiers. Q1, D1, D2, and P1 form a circuit called a constant-current source. For this discussion purposes, suppose that the D1 drops the same value of voltage as the base-emitter junction of Q does. That means that the voltage drop across D2 shall also be the voltage drop across P1. If the value of voltage drop across D2/P1 is 0.7 volt, and P1 is adjusted to be 700 ohms, the value of emitter current flow will be 1 milliamp. Because the collector current of Q1 will almost equal to the emitter current, the level of collector current is also “held” at 1 milliamp. It is important to Note here that the collector current is regulated; it does not depends on the collector load or the amplitude of the source voltage. The variables that are controlling the collector current are only D2’s forward threshold voltage and the setting of P1. Therefore, we can appropriately name it as a constant-current source. Transistors Q2 and Q3 form a differential amplifier.
