A 40W8X 60W4X AMPLIFIER

Given

Power Output

40W8X

60W4X

Input Sensitivity

1V Max

Input Impedance

100k

Bandwidth

Equations (1) and (2) give

40W8X

60W4X

Therefore the supply required is

g

303V

316A reducing to

g

269V

548A

equation (3)

The minimum gain from equation (4) is

We select a gain of 20 resulting sensitivity is 900 mV

The input impedance and bandwidth are the same as the 20

watt amplifier so the components are the same

The maximum supplies dictate using 80V devices The

2N5882 2N5880 pair are 80V 160W transistors with a mini-

mum beta of 40 at 2A and 20 at 6A This corresponds to a

MJE722 driver pair are 80V transistors with a minimum beta

with 5 mA from the LM391

Output transistor heat sink requirements are found using

equations (7) (9) and (10)

(7)

24

e

(9)

(10)

For both output transistors on one heat sink the thermal

resistance should be 19 CW

Now using equation (8) we find the power dissipation in the

driver

24

20

e

12W

(8)

12

e

79 CW

(9)

Since a heat sink is required on the driver we should inves-

tigate the output stage thermal stability at the same time to

the protection circuitry we can then use equation (5) to find

the heat sink required for the drivers

The SOA characteristics of the 2N5882 2N5880 transistors

are shown in the following curve along with a desired pro-

tection line

SOA 2N5882 2N5880

TLH7146 – 10

The desired data points are

Since the break voltage is not equal to the supply we will

Circuit Used

TLH7146 – 11

Thevenin Equivalent

A

3

B

3

R

A

3

R

A

3

a

R

B

3

(

TLH7146 – 12

10