Output Current Limiting:

Because the output transistors can be destroyed by excessive current flow, output current limiting circuits are often included in a power amplifier. Figure 18-27 shows the typical arrangement for a current limiting circuit.

Emitter resistors R_E2 and R_E3 are each made up of two components (R_A and R_B), as illustrated. The Output Current Limiting transistors (Q₄ and Q₅) are connected as shown, so that the voltage drop across R_B2 and R_B3 (produced by I_L) can turn Q₄ and Q₅ on. This occurs only when I_L is at the selected I_L(max) level. When Q₄ turns on, it pulls the base of Q₂ down, so that Q₂ cannot supply current in excess of I_L(max). Similarly, turning Q₅ on causes the base of Q₃ to be pulled up toward its emitter, thus again limiting the output current to I_L(max).

As already discussed for voltage regulator current limiting circuits, Q₄ and Q₅ typically turn on when V_BE ≈ 0.5 V. The component values shown in Fig. 18-27 limit the peak output current to approximately 1 A. Catch diodes D₃ and D₄ in Fig. 18-27 are usually included with Output Current Limiting, to protect the output transistors from the excessive voltage levels generated in an inductive load when the current growth is limited. The diodes prevent the load voltage from exceeding the supply voltage level.

V_BE Multiplier:

Figure 18-28 shows an alternative to diode biasing for the output stage transistors in a Class-AB amplifier. This circuit is known as a V_BE multiplier because it produces a bias voltage (V_B = V_CE6) which is a multiple of the V_BE of transistor Q₆. Referring to the circuit, note that I₁₀ is the current through the the voltage divider (R₁₀, R₁₁, and R₁₂) that biases Q₆. Because I₁₀ is much smaller than I_C6, the Q₁ collector current (1_C1) approximately equals I_C6.

It is seen that the base bias voltage for the output stage transistors can be set by suitable selection of the V_BE multiplier resistors. The bias voltage remains constant when I_C1 changes. Also, it can also be shown that the changes produced in V_B by temperature variations closely match the total V_BE temperature changes in the four output stage transistors.