DN126 - The LT1166: Power Output Stage Automatic Bias System Control IC

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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
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