ADA4075-2

Data Sheet

Rev. C | Page 18 of 24

BALANCED LINE DRIVER

The circuit of Figure 67 shows a balanced line driver designed

for audio use. Such drivers are intended to mimic an output

transformer in operation, whereby the common-mode voltage

can be impressed by the load. Furthermore, either output can be

shorted to ground in single-ended applications without affecting

the overall operation.

Circuits of this type use positive and negative feedback to obtain a

high common-mode output impedance, and they are somewhat

notorious for component sensitivity and susceptibility to latch-up.

This circuit uses several techniques to avoid spurious behavior.

First, the 4-op-amp arrangement ensures that the input impedance

is load independent (the input impedance can become negative

with some configurations). Note that the output op amps are

packaged with the input op amps to maximize drive capability.

Second, the positive feedback is ac-coupled by C2 and C3, which

eliminates the need for offset trim. Because the circuit is ac-coupled

at the input, these capacitors do not have significant dc voltage

across them, thus tantalum types of capacitors can be used.

Finally, even with these precautions, it is vital that the positive

feedback be accurately controlled. This is partly achieved by

using 1% resistors. In addition, the following setup procedure

ensures that the positive feedback does not become excessive:

1. Set R11 to its midposition (or short the ends together,

whichever is easier) and temporarily short the negative

output to ground.

2. Apply a 10 V p-p sine wave at approximately 1 kHz to the

input and adjust R7 to provide 930 mV p-p at TEST (see

Figure 67).

3. Remove the short from the negative output (and across

R11, if used) and adjust R11 until the output waveforms

are symmetric.

The overall gain of the driver is equal to 2, which provides an

extra 6 dB of headroom in balanced differential mode. The

output noise is about −109 dBV in a 20 kHz bandwidth.

C3

10µF

R2

4.7k

Ω

OUT+

OUT–

1/2

ADA4075-2

R5

4.7k

Ω

R6

4.7k

Ω

R7

250Ω

R8

100

Ω

R9

4.7k

Ω

R13

100

Ω

R4

4.7kΩ

A2

1/2

ADA4075-2

A1

IN

C5

50pF

C1

10µF

C2

10µF

TEST

SYMMETRY

TRIM

FEEDBACK

TRIM

1/2

ADA4075-2

R12

4.7k

Ω

R11

250Ω

R17

4.7k

Ω

R16

100

Ω

R10

4.7k

Ω

R15

4.7k

Ω

A4

C6

50pF

1/2

ADA4075-2

A3

C4

50pF

R14

100

Ω

R1

10k

Ω

NOTES

1. ALL RESISTORS SHOULD HAVE 1% TOLERANCE.

2. A1/A2 IN SAME PACKAGE; A3/A4 IN SAME PACKAGE.

R3

4.7k

Ω

R18

4.7k

Ω

Figure 67. Balanced Line Driver