advertisement The LT1166: Power Output Stage Automatic Bias System Control IC – Design Note 126 Dale Eagar Class AB amplifiers are popular because of their “near Class A” performance and their ability to operate on considerably less quiescent power than Class A. Class AB amplifiers are easy to construct, rugged and reliable. However, there is an aspect of these amplifiers that can cause perplexity, consternation and finally hair loss––their bias scheme. The problem is that the very parameter that makes Class AB so good, namely, low quiescent current, is poorly controlled. The LT ®1166 offers control over the quiescent current directly, removing the necessity of temperature tracking, matching transistors or trim pots. In addition, it removes all excess crossover distortion caused by improperly set quiescent current, and significantly reduces the distortion caused by the effects of nonlinear transconductance in the output transistors. Functional Description The LT1166 (Figure 1), combined with external transistors, implements a unity-gain buffer. The circuit controls the Class AB output stage by incorporating two control loops, the current-control loop and the voltage-control loop. The current-control loop (Figure 2) operates independently of the voltage loop while keeping the product of V1 and V2 constant. The voltage loop maintains the output voltage at the input voltage level by driving both gates up or down. The two loops, although mutually independent, act in harmony to provide a component insensitive, temperature insensitive, simple Class AB bias network. Parallel Operation Parallel operation is an effective way to get more output power by connecting multiple power drivers. All that is required is a small ballast resistor to ensure current sharing between the drivers and an inductor to isolate the drivers at high frequencies. In Figure 3 one power slice can deliver ±6A at 100VPK or 300WRMS into 16Ω. Adding another slice boosts the power output to 600WRMS into 8Ω and adding two or more drivers theoretically raises the power output to 1200WRMS into 4Ω. Due to IR losses across the sense resistors, the FET RON resistance at 10A and some sagging 03/96/126 V+ >4mA Q1 GATE ISENSE R1 INPUT IN LT1166 V1 OUTPUT OUT R2 V2 ISENSE GATE Q2 >4mA V– DN126 F01 Figure 1. Basic LT1166 Circuit Configuration V+ V1* V3** V2* *WHEN THE OUPUT CURRENT IS ZERO V1 = V2 = 20mV **V3 IS ADJUSTED SO THAT (V1)(V2) = 0.0004 DN126 F02 V– Figure 2. LT1166 Current-Control Loop of the power supply, the circuit of Figure 3 actually delivers 350WRMS into 8Ω. Performance photos are shown in Figures 4 and 5. Frequency compensation is provided by the 2k input resistor, 180µH inductor and the 1nF compensation capacitors. The common node in the auxiliary power supplies is connected to the amplifier output to generate the floating ±15V supplies. , LTC and LT are registered trademarks of Linear Technology Corporation. POWER SLICE 15V + R1 100Ω 10µF 100V FB 2N3906 R15 390Ω 1nF R9* 9.1k R2 100Ω VTOP 8 SENSE + R10* 1k LT1004-2.5 12.5V 3 C4 0.1µF R14 1k ILIM + 7 + – RIN 2k 2 6 LT1360 2 180µH LT1166 VOUT VIN 4 R7* 10k –12.5V 3 6 SENSE – VBOTTOM 5 R8* 1k C1 1µF R3 0.22Ω C2 R6 1µF 1k R4 0.22Ω R17 0.22Ω L1** 0.4µH R11 100Ω 4 R16 390Ω R5 1k 7 – ILIM VIN IRF230 1 IRF9240 1nF FB LT1004-2.5 2N3904 + R13 200Ω 10µF –100V R12 100Ω –15V C3 3300pF POWER SLICE ~ + 110V AC DIODE BRIDGE ~ – 7815 15V + C7 1000µF 35V + C5 220µF 25V + C8 1000µF 35V + C6 220µF 25V –15V 7915 1Ω L3*** 1.5µH 10A FAST-BLOW AUXILIARY SUPPLIES *0.1% RESISTORS **4 TURNS T37-52 (MICROMETALS) ***6 TURNS T80-52 (MICROMETALS) VOUT DN126 F03 Figure 3. 350W Power Amplifier PO = 350W, R L = 8Ω Figure 4. 0.3% THD at 10kHz DN126 F04 CL = 1µF DN126 F05 Figure 5. 2kHz Square Wave For literature on our amplifiers, call 1-800-4-LINEAR. For applications help, call (408) 432-1900, Ext. 456 Linear Technology Corporation LT/GP 0396 155K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 LINEAR TECHNOLOGY CORPORATION 1996