LM391 Audio Power Driver

Application Hints (Continued)
A 20W 8X 30W 4X AMPLIFIER
Givens
Power Output
20W into 8X
30W into 4X
1V Max
100k
20 Hz–20 kHz g 025 dB
Input Sensitivity
Input Impedance
Bandwidth
Equations (1) and (2) give
20W8X V OP e 179V I OP e 224A
30W4X V OP e 155V I OP e 387A
Therefore the supply required is
g23V 224A reducing to
g21V 387A
With 15% regulation and high line we get g29V from equation
(3)
Sensitivity and equation (4) set minimum gain
A V
t 020 c 8 e 1265
1
We will use a gain of 20 with resulting sensitivity of 632 mV
Letting R IN equal 100k gives the required input impedance
For low DC offsets at the output we let R f2 e 100k Solving
for R f1 gives
R f2 e 100k
R f1 e
100k e 526k use 51k
20 b 1
The bandwidth requirement must be stated as a pole ie
the 3 dB frequency Five times away from a pole gives 017
dB down which is better than the required 025 dB Therefore
f L e 20 5 e 4Hz
Solving for C f
1
C f
t e 78 mF use 10 mF
2qR f1 f L
The recommended value for C C is 5 pF for gains of 20 or
larger This gives a gain-bandwidth product of 64 MHz and
a resulting bandwidth of 320 kHz better than required
The breakdown voltage requirement is set by the maximum
supply we need a minimum of 58V and will use 60V We
must now select a 60V power transistor with reasonable
beta at I Opeak 387A The TIP42 TIP41 complementary pair
are 60V 60W transistors with a minimum beta of 30 at 4A
The driver transistor must supply the base drive given 5 mA
drive from the LM391 The MJE711 MJE721 complementary
driver transistors are 60V devices with a minimum beta of
40 at 200 mA The driver transistors should be much faster
(higher f T ) than the output transistors to insure that the R-C
on the output will prevent instability
To find the heat sink required for each output transistor we
use equations (7) (9) and (10)
P D e 04 (30) e 12W
(7)
150C b 55C
i JA
s e 79CW for T AMAX e 55C (9)
12
i SA
s 79 b 21 b 10 e 48CW (10)
If both transistors are mounted on one heat sink the thermal
resistance should be halved to 24CW
The maximum average power dissipation in each driver is
found using equation (8)
P DRIVER(MAX) e 12 e 400 mW
30
Using equation (9)
155 b 55
i JA
s e 237CW
04
f h e20k c 5 e 100 kHz
8