Application Hints

Gain Control

To make the LM389 a more versatile amplifier two pins (4

and 12) are provided for gain control With pins 4 and 12

open the 135 kX resistor sets the gain at 20 (26 dB) If a

capacitor is put from pin 4 to 12 bypassing the 135 kX

resistor the gain will go up to 200 (46 dB) If a resistor is

placed in series with the capacitor the gain can be set to

any value from 20 to 200 A low frequency pole in the gain

response is caused by the capacitor working against the

external resistor in series with the 150X internal resistor If

the capacitor is eliminated and a resistor connects pin 4 to

12 then the output dc level may shift due to the additional

dc gain Gain control can also be done by capacitively cou-

pling a resistor (or FET) from pin 12 to ground

Additional external components can be placed in parallel

with the internal feedback resistors to tailor the gain and

frequency response for individual applications For example

we can compensate poor speaker bass response by fre-

quency shaping the feedback path This is done with a se-

ries RC from pin 1 to 12 (paralleling the internal 15 kX resis-

tor) For 6 dB effective bass boost R j 15 kX the lowest

open If pins 4 and 12 are bypassed then R as low as 2 kX

can be used This restriction is because the amplifier is only

compensated for closed-loop gains greater than 9VV

Input Biasing

The schematic shows that both inputs are biased to ground

witha50kX resistor The base current of the input transis-

tors is about 250 nA so the inputs are at about 125 mV

when left open If the dc source resistance driving the

LM389 is higher than 250 kX it will contribute very little

additional offset (about 25 mV at the input 50 mV at the

output) If the dc source resistance is less than 10 kX then

shorting the unused input to ground will keep the offset low

(about 25 mV at the input 50 mV at the output) For dc

source resistances between these values we can eliminate

excess offset by putting a resistor from the unused input to

ground equal in value to the dc source resistance Of

course all offset problems are eliminated if the input is ca-

pacitively coupled

When using the LM389 with higher gains (bypassing the

135 kX resistor between pins 4 and 12) it is necessary to

bypass the unused input preventing degradation of gain

and possible instabilities This is done with a 01 mF capaci-

tor or a short to ground depending on the dc source resist-

ance of the driven input

Supplies and Grounds

The LM389 has excellent supply rejection and does not re-

quire a well regulated supply However to eliminate possi-

ble high frequency stability problems the supply should be

decoupled to ground with a 01 mF capacitor The high cur-

rent ground of the output transistor pin 18 is brought out

separately from small signal ground pin 17 If the two

ground leads are returned separately to supply then the par-

asitic resistance in the power ground lead will not cause

stability problems The parasitic resistance in the signal

ground can cause stability problems and it should be mini-

mized Care should also be taken to insure that the power

dissipation does not exceed the maximum dissipation of the

package for a given temperature There are two ways to

mute the LM389 amplifier Shorting pin 3 to the supply volt-

age or shorting pin 12 to ground will turn the amplifier off

without affecting the input signal

Transistors

The three transistors on the LM389 are general purpose

devices that can be used the same as other small signal

transistors As long as the currents and voltages are kept

within the absolute maximum limitations and the collectors

are never at a negative potential with respect to pin 17

there is no limit on the way they can be used

For example the emitter-base breakdown voltage of 71V

can be used as a zener diode at currents from 1 mAto

5 mA These transistors make good LED driver devices

In the linear region these transistors have been used in AM

and FM radios tape recorders phonographs and many oth-

er applications Using the characteristic curves on noise

voltage and noise current the level of the collector current

can be set to optimize noise performance for a given source

impedance Some of the circuits that have been built are

shown in

Figures 1 – 7 This is by no means a complete list

of applications since that is limited only by the designers

imagination

TLH7847 – 6

FIGURE 1 AM Radio

5