LM391 National Semiconductor Corporation, LM391 Datasheet - Page 7

• Manufacturer Part Number: LM391
• Description: Audio Power Driver (discontinued)
• Manufacturer: National Semiconductor Corporation

Application Hints
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 Put a 1 M
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 k
than 1 M
correctly The slight difference in resistors results in about
15 mV of offset The 40W 8
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
outputs the input is V
Typical Applications
Output Transistors Selection Guide
20W
30W
40W
60W
Output
Power
to insure that the shutdown circuitry will operate
8
4
8
4
IN
1 and V
(Continued)
amplifier schematic shows
MJE711
MJE171
D43C8
MJE712
MJE172
D43C11
(Continued)
IN
PNP
2 is disconnected
Driver Transistor
resistor from
Bridge Circuit Diagram
rather
Table A
MJE721
MJE181
D42C8
MJE722
MJE182
D42C11
NPN
7
OSCILLATIONS
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
and or ground loops A 10
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 grounds bypass 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
bias) The potentiometer R
give about 25 mA of current in the output stage This current
is usually monitored at the supply or by measuring the volt-
age across R
E
GROUNDING
TIP42A
2N6490
2N5882
PNP
Output Transistor
B
from pins 6– 7 is adjusted to
F 50V electrolytic on each
TIP41A
2N6487
2N5880
NPN
TL H 7146– 7