SSM2275 데이터 시트보기 (PDF) - Analog Devices

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SSM2275

Rail-to-Rail Output Audio Amplifiers

Analog Devices 

SSM2275/SSM2475

Capacitive Loading

The output of the SSM2275/SSM2475 can tolerate a degree of

capacitive loading. However, under certain conditions, a heavy

capacitive load could create excess phase shift at the output and

put the device into oscillation. The degree of capacitive loading

is dependent on the gain of the amplifier. At unity gain, the am-

plifier could become unstable at loads greater than 600 pF. At

gain greater than unity, the amplifier can handle a higher degree

of capacitive load without oscillating. Figure 35 shows how to

configure the device to prevent oscillations from occurring.

CFB

CFB

RFB

RFB

RI

RI

VIN

VOUT

VOUT

VIN

RB

50k⍀

SSM2275

CL

RB

50k⍀

SSM2275

CL

INVERTING GAIN AMPLIFIER

NONINVERTING GAIN AMPLIFIER

Figure 35. Configurations for Driving Heavy Capacitive

Loads

RB should be at least 50 kΩ. To minimize offset voltage, the

parallel combination of RFB and RI should be equal to RB. Set-

ting a minimum CF of 15 pF bandlimits the amplifier enough to

eliminate any oscillation problems from any sized capacitive

load. The low-pass frequency is determined by:

f−3dB

=

2π

1

RFBCF

(6)

With RFB = 50 kΩ and CF = 15 pF, this results in an amplifier

with a 210 kHz bandwidth that can be used with any capacitive

load. If the amplifier is being used in a noninverting unity gain

configuration and RI is omitted, CFB should be at least 100 pF.

If the offset voltage can be tolerated at the output, RFB can be

replaced by a short and CFB can be removed entirely. With the

typical input bias current of 200 nA and RB = 50 kΩ, the in-

crease in offset voltage would be 10 mV. This configuration will

stabilize the amplifier under all capacitive loads.

Single Supply Differential Line Driver

Figure 36 shows a single supply differential line driver circuit

that can drive a 600 Ω load with less than 0.001% distortion.

The design mimics the performance of a fully balanced trans-

former based solution. However, this design occupies much less

board space while maintaining low distortion and can operate

down to dc. Like the transformer based design, either output

can be shorted to ground for unbalanced line driver applications

without changing the circuit gain of 1.

R13 and R14 set up the common-mode output voltage equal to

half of the supply voltage. C1 is used to couple the input signal

and can be omitted if the input’s dc voltage is equal to half of

the supply voltage. The minimum input impedance of the cir-

cuit as seen from VIN is:

( ) ( ) RIN = R1+ R5 || R3 + R7 || R11

(7)

For the values given in Figure 36, RIN = 5 kΩ. With C1 omitted

the circuit will provide a balanced output down to dc, otherwise

the –3 dB corner for the input frequency is set by:

f−3dB

=

2π

1

RIN CL

(8)

The circuit can also be configured to provide additional gain if

desired. The gain of the circuit is:

AV

= VOUT

V IN

=

2(R2)

R1

(9)

where VOUT = VO1 – VO2, R1 = R3 = R5 = R7 and,

R2 = R4 = R6 = R8

Figure 37 shows the THD+N versus frequency response of the

circuit while driving a 600 Ω load at 1 V rms.

C3

33pF

R1

10k⍀

R2

10k⍀

C1*

10␮F

+12V

R5

10k⍀

R9

50⍀

V01

VIN

R11

10k⍀

R12

10k⍀

SSM2475-A

+12V

+5V

R13

100k⍀

C4

33pF

R6

10k⍀

C2

10␮F

R14

100k⍀

SSM2475-C

R7

10k⍀

R8

10k⍀

C3

10␮F

+12V

R3

10k⍀

R10

50⍀

V02

C1* IS OPTIONAL

SSM2475-B

R4

10k⍀

C4

10␮F

Figure 36. A Low Noise, Single Supply Differential

Line Driver

0.1

VSY = 12V

RL = 600⍀

0.01

0.001

0.0001

20

100

1k

10k 20k

FREQUENCY – Hz

Figure 37. THD+N vs. Frequency of Differential Line Driver

–12–

REV. A

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