TRANSIENT INTERMODULATION DISTORTION

There has been a lot of interest in recent years about tran-

sient intermodulation distortion Matti Otala of University of

Oulu Oulu Finland has published several papers on the

subject The results of these investigations show that the

open loop pole of the power amplifier should be above 20

kHz

To do this with the LM391 is easy Puta1MX resistor from

pin 3 to the output and the open loop gain is reduced to

about 46 dB Now the open loop pole is at 30 kHz The

current in this resistor causes an offset in the input stage

that can be cancelled with a resistor from pin 4 to ground

The resistor from pin 4 to ground should be 910 kX rather

than 1 MX to insure that the shutdown circuitry will operate

correctly The slight difference in resistors results in about

15 mV of offset The 40W 8X amplifier schematic shows

the hookup of these two resistors

BRIDGE AMPLIFIER

A switch can be added to convert a stereo amplifer to a

single bridge amplifer The diagram below shows where the

switch and one resistor are added When operating in the

bridge mode the output load is connected between the two

2 is disconnected

OSCILLATIONS

GROUNDING

Most power amplifiers work the first time they are turned on

They also tend to oscillate and have excess THD Most os-

cillation problems are due to inadequate supply bypassing

andor ground loops A 10 mF 50V electrolytic on each

power supply will stop supply-related oscillations However

if the signal ground is used for these bypass caps the THD

is usually excessive The signal ground must return to the

power supply alone as must the output load ground All

other groundsbypass output R-C protection etc can tie

together and then return to supply This ground is called

high frequency ground On the 40W amplifier schematic all

the grounds are labeled

Capacitive loads can cause instabilities so they are isolated

from the amplifier with an inductor and resistor in the output

lead

AB BIAS CURRENT

To reduce distortion in the output stage all the transistors

are biased ON slightly This results in class AB operation

and reduces the crossover (notch) distortion of the class B

stage to a low level (see performance curve THD vs AB

give about 25 mA of current in the output stage This current

is usually monitored at the supply or by measuring the volt-

Bridge Circuit Diagram

TLH7146 – 7

Output Transistors Selection Guide

Table A

Power

Driver Transistor

Output Transistor

Output

PNP

NPN

PNP

NPN

20W

8X

MJE711

MJE721

TIP42A

TIP41A

30W

4X

MJE171

MJE181

2N6490

2N6487

D43C8

D42C8

40W

8X

MJE712

MJE722

2N5882

2N5880

60W

4X

MJE172

MJE182

D43C11

D42C11

7