Application Hints

GENERALIZED AUDIO POWER AMP DESIGN

Givens

Power Output

Load Impedance

Input Sensitivity

Input Impedance

Bandwidth

The power output and load impedance determine the power

supply requirements Output signal swing and current are

found from

(1)

(2)

ed voltage usually about 15% higher Supply voltage will

also rise 10% during high line conditions

max supplies

g

(3)

The input sensitivity and output power specs determine the

required gain

e

(4)

Normally the gain is set between 20 and 200 for a 25 watt

8 ohm amplifier this results in a sensitivity of 710 mV and 71

mV respectively The higher the gain the higher the THD

as can be seen from the characteristics curves Higher gain

also results in more hum and noise at the output

can cause board layout problems and DC offsets at the out-

The output transistors and drivers must have a breakdown

voltage greater than the voltage determined by equation (3)

The current gain of the drive and output device must be high

The power transistors must be able to dissipate approxi-

mately 40% of the maximum output power the drivers must

dissipate this amount divided by the current gain of the out-

puts See the output transistor selection guide Table A

To prevent thermal runaway of the AB bias current the fol-

lowing equation must be valid

(5)

where

tion to ambient in CW

equation (3)

K is the temperature coefficient of the driver base-emitter

voltage typically 2 mV C

is rearranged to be

(6)

The maximum average power dissipation in each output

transistor is

(7)

The power dissipation in the driver transistor is

(8)

Heat sink requirements are found using the following formu-

las

(9)

(10)

where

most mountings

5