miniature power amplifier
There are quite a few applicationsof an audio power amplifierin which power and hi-ficharacteristics are of secondaryimportance. If, for instance, anactive loudspeaker for a portableradio receiver is needed, compactdimensions and low currentdrain are far more importantconsiderations.
These properties are theprime design basis for the presentmini amplifier. It continuesa discrete Class-B stage. Theop amp is a Type TLC271 operatingin its high-current mode(pin 8 to ground). Any complicationsarising from the common-mode range are preventedby using the amplifier as an invertingtype. The voltage amplificationis set with feedback resistanceP1.The discrete power stageconsists of two complementarydarlingtons, each composed of aBC and a BD transistor. ResistorsR2–R5 limit the internal amplification.Capacitors C1, C2, andC6, are compensation devices.
Boucherot network R6-C3ensures amplifier stability whenthe load is very low or very high.Since the output transistorshave no emitter resistor, thevoltage is determined solely bythe knee voltage of T3 and T4.With a load of 4–8 , these voltagesare limited to 0.2–0.3 V, sothat the transistors can be drivenvirtually up to the supply voltage.This is the reason for theatypical high efficiency of theamplifier.The overall bandwidth of theamplifier is limited to not lessthan 21 kHz at the maximumamplification of ×5.