15 To 60 Watt Audio Amplifiers Using Complementary Darlington Output Transistors - Applications Engineering

Oct 12, 2026

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Monolithic power Darlington transistors simplify the design of high-fidelity audio power amplifiers by using complementary Darlington output transistors and removing extra driver transistors, driver heat sinks, and output biasing resistors used in previous approaches. The note provides circuit and performance information for 15 W to 60 W amplifier designs, including design rules for center-voltage biasing, idle current adjustment, gain behavior, compensation, and techniques such as bootstrapping or constant-current biasing to control crossover distortion and stability.

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Introduction

15-20 Watt Amplifier

The 15-60 Watt AC Coupled Circuit

Parts List and Typical Performance Tables

YRATM3M3J9MO MI2U 293LMA OK 7 TUTUO WOT0 15 TO 60 WATT AUDIO AMPLIFIERSUSING COMPLEMENTARYDARLINGTON OUTPUT TRANSISTORS Prepared byRichard G.RuehsApplications Engineering The use of monolithic power darlingtontransistors can simplify the design of high-fidelity power amplifiers.Circuit and per-formance information are provided tofacilitate the design of 15 watt to 60 wattamplifiers utilizing the power darlingtondevices. MOTOROLA Semiconductor Products Inc. INTRODUCTION Recently developed silicon monolithic darlington powertransistors permit significant simplification of audio poweramplifier circuitry.The amplifiers described in this noteuse complementary power darlington output transistors,eliminating the two driver transistors used in previousmethods.A significant cost savings should be realized since,in addition to the driver devices,the driver heat sinks andoutput biasing resistors are also eliminated.Also,theprinted circuit board for the amplifier could become verycompact. Three circuits will be discussed:a 15 to 20 watt,medi- um performance amplifier,a 15 to 60 watt high perform- ance amplifier with ac-coupled output,and a15 to 60 watt high performance amplifier with dc-coupled output 15-20 WATT AMPLIFIER The circuit of the 15-20 watt amplifier is shown in Figure 1.To ensure maximum signal swing,the dc “center”volt-age(the point between R8 and R9)must be one-half ofVcc.This is accomplished by dc feedback from this pointto the base of Q1,through R3.Resistors R3 and R2 also FIGURE 1-Four Transistor Audio Amplifier for 15 to 20 W Output # 15 TO 60 WATT AUDIO AMPLIFIERS USING COMPLEMENTARYDARLINGTON OUTPUT TRANSISTORS TABLE1 | Parts List | | | | --- | --- | --- | | RMS Power | 15W | 20W | | Load | 4Ω 8Ω | 8Ω
4Ω | | Vcc
R2
R3
R6 | 40V
32V
4.3k 5.6k
82k 120 k
3.9k
3.3k | 46V
36V
5.1 k 4.3k
100k 130 k
3.3 k 4.7 k | Typical PerformanceIdle Current(Adj.with Rv)20 mANominal Input Sensitivity forFull Rated Output1 VrmsTotal Harmonic Distortion @Rated Output,50 Hz to 20kHz0.5%Total Harmonic Distortion @1W Output,1kHz0.2%Input Impedance10kSΩ form a voltage divider which provides dc bias to the baseof Q1.Loading of this divider by the base current of Q1could cause the center voltage to vary with changes in hFE of Q1.To prevent this change,the direct current inthe R2,R3 divider is made at least ten times greater thanthe maximum base current of Q1. Transistor Q2 is used to forward bias the output darling-ton devices.Resistors R4,Ry and R1 form a resistivedivider which sets the collector to emitter voltage of Q2at approximately 2.4 V for biasing of the output.Ry ismade variable so that the Ic of Q2 can be adjusted andconsequently the dc"idle current"in the output tran-sistors can be set to minimize cross-over distortion.Twentymilliamps of idle current is sufficient to eliminate thisdistortion. The VCE voltage ofQ2tracks the VBE(on)temperaturecharacteristics of Q3 and Q4 adequately.Therefore,if Q2were mounted on the heat sink with the output transistors,the dc idle current would remain within practical limits overthe temperature range. To ensure maximum swing during peak negative signalexcursions,R6 is connected to the speaker side of the out-put coupling capacitor.This makes use of the dc chargeon the output coupling capacitor to provide drive currentto the base of Q4 thru R6(bootstrapping). Parts values and typical performance characteristics forthe 15 to 20 watt circuit are shown in Table I. THE 15-60 WATT AC COUPLED CIRCUIT The 15 to 60 watt ac-coupled circuit is shown in Figure 2.As in the previous circuit,the center voltage must beone half Vcc for maximum output swing.Resistors R1, R2 and R3 form a voltage divider which sets the dc voltageon the base of Q1 at approximately 1.5 volts above 1/2Vcc.This will maintain the center voltage at 1/2 Vccsince there is a constant 1.5 volt drop from the

FAQs

What design simplification does the monolithic power Darlington approach enable for audio power amplifiers?

It permits significant simplification by using complementary Darlington output transistors, eliminating the two driver transistors and associated driver heat sinks and output biasing resistors used previously.

How is the dc center voltage maintained for maximum output swing in the 15-20 W amplifier?

The dc “center” voltage must be one-half of Vcc, achieved through dc feedback from the R8-R9 midpoint to the base of Q1 via R3.

What methods are used to minimize crossover distortion in the described amplifier circuits?

Idle current is adjusted using a variable resistance (e.g., Ry) so sufficient idle current eliminates crossover distortion; additionally, circuit choices such as bootstrapping in the ac-coupled version or a constant current source reduce the need for bootstrap effects on frequency.

Oct 12, 2026

Upload by Sarah