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TL/H/7380

LM380

Power

Audio

Amplifier

AN-69

National Semiconductor
Application Note 69
December 1972

LM380 Power Audio
Amplifier

INTRODUCTION

The LM380 is a power audio amplifier intended for consum-
er applications. It features an internally fixed gain of 50
(34 dB) and an output which automatically centers itself at
one-half of the supply voltage. A unique input stage allows
inputs to be ground referenced or AC coupled as required.
The output stage of the LM380 is protected with both short
circuit current limiting and thermal shutdown circuitry. All of
these internally provided features result in a minimum exter-
nal parts count integrated circuit for audio applications.

This paper describes the circuit operation of the LM380, its
power handling capability, methods of volume and tone con-
trol, distortion, and various application circuits such as a
bridge amplifier, a power supply splitter, and a high input
impedance audio amplifier.

CIRCUIT DESCRIPTION

Figure 1 shows a simplified circuit schematic of the LM380.

The input stage is a PNP emitter-follower driving a PNP dif-
ferential pair with a slave current-source load. The PNP

input is chosen to reference the input to ground, thus en-
abling the input transducer to be directly coupled.

The output is biased to half the supply voltage by resistor
ratio R

1

/R

2

. Negative DC feedback, through resistor R

2

,

balances the differential stage with the output at half supply,
since R

1

e

2 R

2

(

Figure 1 ).

The second stage is a common emitter voltage gain amplifi-
er with a current-source load. Internal compensation is pro-
vided by the pole-splitting capacitor C

Ê

. Pole-splitting com-

pensation is used to preserve wide power bandwidth
(100 kHz at 2W, 8

X

). The output is a quasi-complementary

pair emitter-follower.

The amplifier gain is internally fixed to 34 dB or 50. This is
accomplished by the internal feedback network R

2

– R

3

. The

gain is twice that of the ratio R

2

/R

3

due to the slave current-

source which provides the full differential gain of the input
stage.

TABLE I. Electrical Characteristics (Note 1)

Parameter

Conditions

Min

Typ

Max

Units

Power Output (rms)

8

X

loads, 3% T.H.D. (Notes 3,4)

2.5

Wrms

Gain

40

50

60

V/V

Output Voltage Swing

8

X

load

14

V

p-p

Input Resistance

150k

X

Total Harmonic Distortion

P

o

e

1W, (Notes 4 & 5)

0.2

%

Power Supply Rejection

C

bypass

e

5

m

F,

²

e

120 Hz

38

dB

(Note 2)

Supply Voltage Range

8

22

V

Bandwidth

P

o

e

2W, R

L

e

8

X

100k

Hz

Quiescent Output Voltage

8

9

10

V

Quiescent Supply Current

7

25

mA

Short Circuit Current

1.3

A

Note 1:

V

S

e

18V; T

A

e

25

§

C unless otherwise specified.

Note 2:

Rejection ratio referred to output.

Note 3:

With device Pins 3, 4, 5, 10, 11, 12 soldered into a

(/16

×

epoxy glass board with 2 ounce copper foil with a minimum surface of six square inches.

Note 4:

If oscillation exists under some load conditions, add a 2.7

X

resistor and 0.1

m

F series network from Pin 8 to ground.

Note 5:

C

bypass

e

0.47

m

F on Pin 1.

Note 6:

Pins 3, 4, 5, 10, 11, 12 at 50

§

C derates 25

§

C/W above 50

§

C case.

C

1995 National Semiconductor Corporation

RRD-B30M115/Printed in U. S. A.

Содержание LM380

Страница 1: ... half supply since R1 e 2 R2 Figure 1 The second stage is a common emitter voltage gain amplifi er with a current source load Internal compensation is pro vided by the pole splitting capacitor CÊ Pole splitting com pensation is used to preserve wide power bandwidth 100 kHz at 2W 8X The output is a quasi complementary pair emitter follower The amplifier gain is internally fixed to 34 dB or 50 This ...

Страница 2: ...kage will support 3 watts dissipation at 50 C am bient or 3 7 watts at 25 C ambient Figure 2 shows the maximum package dissipation versus ambient temperature for various amounts of heat sinking TL H 7380 2 FIGURE 2 Device Dissipation vs Ambient Temperature Figures 3a b and c show device dissipation versus output power for various supply voltages and loads TL H 7380 3 FIGURE 3a Device Dissipation v...

Страница 3: ...ting input is used the inverting input is left floating When the inverting input is used and the non in verting input is left floating the amplifier may be found to be sensitive to board layout since stray coupling to the floating input is positive feedback This can be avoided by employ ing one of three alternatives 1 AC grounding the unused input with a small capacitor This is preferred when usin...

Страница 4: ...ume and Tone Control This common mode volume control can be combined with a common mode tone control as seen in Figure 10 This circuit has a distinct advantage over the circuit of Fig ure 7 when transducers of high source impedance are used in that the full input impedance of the amplifier is realized It also has an advantage with transducers of low source im pedance since the signal attenuation o...

Страница 5: ...ereby increasing the power capability by a factor of four over the single amplifier However in most cases the package dissipation will be the first parameter limiting power delivered to the load When this is the case the power capability of the bridge will be only twice that of TL H 7380 17 FIGURE 15A 8X Load the single amplifier Figures 15A and B show output power versus device package dissipatio...

Страница 6: ...r T1 A turns ratio of 25 and a device gain of 50 allows a maxi mum loop gain of 1250 RV provides a common mode volume control Switching S1 to the listen position reverses the role of the master and remote speakers LOW COST DUAL SUPPLY The circuit shown in Figure 19 demonstrates a minimum parts count method of symmetrically splitting a supply volt age Unlike the normal R C and power zener diode tec...

Страница 7: ... value of R2 is determined by the product of the gate reverse leakage IGSS and R2 This voltage should be 10 to 100 times smaller than VP The output impedance of the FET source follower is Ro e 1 gm 11 so that the determining resistance for the interstage RC time constant is the input resistance of the LM380 BOOSTED GAIN USING POSITIVE FEEDBACK For applications requiring gains higher than the inter...

Страница 8: ...ions for use provided in the labeling can effectiveness be reasonably expected to result in a significant injury to the user National Semiconductor National Semiconductor National Semiconductor National Semiconductor Corporation Europe Hong Kong Ltd Japan Ltd 1111 West Bardin Road Fax a49 0 180 530 85 86 13th Floor Straight Block Tel 81 043 299 2309 Arlington TX 76017 Email cnjwge tevm2 nsc com Oc...

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