LM391 datasheet(8/12 Pages) NSC

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Application Hints (Continued)

A 20W 8X 30W 4X AMPLIFIER

Givens

Power Output

20W into 8X

30W into 4X

Input Sensitivity

1V Max

Input Impedance

100k

Bandwidth

20 Hz – 20 kHz g 025 dB

Equations (1) and (2) give

20W8X

VOP e 179V

IOP e 224A

30W4X

VOP e 155V

IOP e 387A

Therefore the supply required is

23V

224A reducing to

21V

387A

With 15% regulation and high line we get g29V from equa-

tion (3)

Sensitivity and equation (4) set minimum gain

AV t

020 c 8

1265

We will use a gain of 20 with resulting sensitivity of 632 mV

Letting RIN equal 100k gives the required input impedance

For low DC offsets at the output we let Rf2 e 100k Solving

for Rf1 gives

Rf2 e 100k

Rf1 e

100k

20 b 1

526k use 51k

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 There-

fore

fL e

4Hz

fh e 20k c 5 e 100 kHz

Solving for Cf

Cf t

2qRf1fL

78 mF use 10 mF

The recommended value for CC 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 IOpeak 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 complementa-

ry driver transistors are 60V devices with a minimum beta of

40 at 200 mA The driver transistors should be much faster

(higher fT) 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)

(7)

PD e 04 (30) e 12W

iJA s

150 C b 55 C

79 CW for TAMAX e 55 C

(9)

(10)

iSA s 79 b 21 b 10 e 48 CW

If both transistors are mounted on one heat sink the thermal

resistance should be halved to 24 CW

The maximum average power dissipation in each driver is

found using equation (8)

PDRIVER(MAX) e

400 mW

Using equation (9)

iJA s

155 b 55

237 CW

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