Distortion vs Output Power

Output Power 4X Load

Device Dissipation vs

Output Power 8X Load

Device Dissipation vs

TLH7848 – 6

Application Hints

Gain Control

To make the LM390 a more versatile amplifier two pins (2

and 6) are provided for gain control With pins 2 and 6 open

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

tor is put from pin 2 to 6 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 se-

ries with the 150X internal resistor If the capacitor is elimi-

nated and a resistor connects pin 2 to 6 then the output dc

level may shift due to the additional dc gain Gain control

can also be done by capacitively coupling a resistor (or

FET) from pin 6 to ground as in

Figure 7

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 6 to 13 (paralleling the internal 15 kX resis-

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

open If pins 2 and 6 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 9 VV

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

LM390 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 LM390 with higher gains (bypassing the

135 kX resistor between pins 2 and 6) 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 on the driven input

Bootstrapping

The base of the output transistor of the LM390 is brought

out to pin 9 for Bootstrapping The output stage of the am-

plifier during positive swing is shown in

Figure 3 with its

external circuitry

output transistor The maximum output current divided by

beta is the value required for the current in R1 and R2

e

632 mA

0632

To keep the current in R2 constant during positive swing

R1 and R2 above the supply maintaining a constant voltage

due to the output coupling capacitor and the load This

gives

j

25

4 as in

Figure 4

For reduced component count the load can replace R1 The

is both the coupling and the bootstrapping capacitor (see

Figure 2 )

3