Adding current-limiting circuitry to a power amplifier's or a linear voltage regulator's emitter-follower output stage protects both the output transistor and the downstream circuitry from excessive-current damage. Figure 1 shows the classic current-limiter circuit: Transistor Q2 senses the output-current-induced voltage drop across ballast resistor R2 and diverts base current from Darlington-connected transistors Q1 and Q3. Transistor Q2's base-emitter voltage, VBE, sets the circuit's current-limit threshold. Unfortunately, a small-signal transistor's VBE exhibits a temperature coefficient of –2 mV/°C, which causes a substantial variation in the current-limiting threshold over the circuit's operating-temperature range.
|Figure 1.||A small-signal transistor, Q2, provides an output-current
limit for this emitter-follower power amplifier.
You can improve the circuit's performance by replacing Q2 with IC1, an adjustable shunt regulator (Figure 2). With an input threshold voltage of 0.6 V, the MAX8515 allows use of a lower value for current-sense resistor R2 and thus helps minimize R2's power and thermal losses. Alternative commonly available shunt regulators present input voltages of 1.25 to 2.5 V. In addition, a separate power-supply input connection allows the MAX8515 to maintain accuracy when its internal output transistor approaches saturation.
|Figure 2.||Substituting a shunt regulator, IC1, for Q2 in Figure 1
improves the output-current-limit accuracy.
Figure 3 compares current-limit accuracy for the circuits of Figure 1 and Figure 2 over an operating-temperature range of –40 to +85 °C. Neglecting the temperature coefficient of sense resistor R2, the shunt-regulator version maintains its output current to an accuracy of better than 2%, and the small-signal-transistor version exhibits a 25% current variation over the operating-temperature range.
|Figure 3.||Output-current-versus-temperature plots for the circuits of figures 1 and 2
show improved accuracy for the shunt-regulated circuit (bottom trace)
over the discrete-transistor version (top trace).