CROSS OVER DISTORTION

A normally used technique for the purpose of eliminating crossover distortion is by way of biasing the transistors so that they are on the point of conducting. With BJTs, this would mean the biasing them so that vBE is equal to 0.7 V when there is no signal. The very easy way to do all this is to making use of the 0.7 V voltage level drop across a forward-biased diode. In the circuit diagram shown below, current flows through the chain that contains R1, D1, D2 and R2.

The voltage drops of 0.7 V happens across the base-emitter junctions of both of these transistors. These transistors will raise the base voltage of Q1 to 0.7 V which is above the value of vIN. When no signal is present, vIN is “0” so the base of Q1 is at 10.7 Volt. It conducts as soon as vIN increases above the level zero. Similarly, the value of base of Q2 is at 20.7 V and Q2 is in ready state to conduct as soon as vIN falls below 0. In few words, when vIN begins to increase or decrease, one of the transistors is in ready state to conduct. There will be no or very little crossover distortion. The modification we have made to the Class B amplifier is termed as Class AB amplifier.

The use of diodes have an additional benfit. Just Consider Q1 and its associated diode D1. The diode is a forward biased pn junction, this is just like the base-emitter junction of Q1. As the level of temperature changes, the forward voltage drop (vBE) across the base-emitter junction also changes. But the drop across the diode changes too, and compensates for changes in the transistor. Whatever the temperature level is, the voltage drops of D1 and Q1 are equal and Q1 remain biased state, and it is ready to begin conducting with any increase of vIN. The same scenario applies to Q2 and D2. This is the way by which the diodes enhance the temperature stability of the amplifier.