LM389 National Semiconductor, LM389 Datasheet - Page 5

• Manufacturer Part Number: LM389
• Description: LM389 Low Voltage Audio Power Amplifier with NPN Transistor Array
• Manufacturer: National Semiconductor

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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 1 35 k
capacitor is put from pin 4 to 12 bypassing the 1 35 k
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 150
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 k
tor) For 6 dB effective bass boost R j 15 k
value for good stable operation is R
open If pins 4 and 12 are bypassed then R as low as 2 k
can be used This restriction is because the amplifier is only
compensated for closed-loop gains greater than 9V V
Input Biasing
The schematic shows that both inputs are biased to ground
with a 50 k
tors is about 250 nA so the inputs are at about 12 5 mV
when left open If the dc source resistance driving the
LM389 is higher than 250 k
additional offset (about 2 5 mV at the input 50 mV at the
output) If the dc source resistance is less than 10 k
shorting the unused input to ground will keep the offset low
(about 2 5 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
1 35 k
resistor between pins 4 and 12) it is necessary to
resistor The base current of the input transis-
resistor sets the gain at 20 (26 dB) If a
it will contribute very little
e
internal resistor If
10 k
the lowest
if pin 4 is
FIGURE 1 AM Radio
resis-
then
5
bypass the unused input preventing degradation of gain
and possible instabilities This is done with a 0 1 F 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 0 1 F 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 7 1V
can be used as a zener diode at currents from 1
5 mA These transistors make good LED driver devices
V
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
SAT
is only 150 mV when sinking 10 mA
TL H 7847– 6
A to