OP275

REV. A

–10–

importance. Like the transformer based design, either output

can be shorted to ground for unbalanced line driver applications

without changing the circuit gain of 1. Other circuit gains can be

set according to the equation in the diagram. This allows the

design to be easily set to noninverting, inverting, or differential

operation.

A 3-Pole, 40 kHz Low-Pass Filter

The closely matched and uniform ac characteristics of the

OP275 make it ideal for use in GIC (Generalized Impedance

Converter) and FDNR (Frequency-Dependent Negative Resis-

tor) filter applications. The circuit in Figure 18 illustrates a lin-

ear-phase, 3-pole, 40 kHz low-pass filter using an OP275 as an

inductance simulator (gyrator). The circuit uses one OP275 (A2

and A3) for the FDNR and one OP275 (A1 and A4) as an input

buffer and bias current source for A3. Amplifier A4 is config-

ured in a gain of 2 to set the pass band magnitude response to

0 dB. The benefits of this filter topology over classical ap-

proaches are that the op amp used in the FDNR is not in the

signal path and that the filter’s performance is relatively insensi-

tive to component variations. Also, the configuration is such that

large signal levels can be handled without overloading any of the

the filter’s internal nodes. As shown in Figure 19, the OP275’s

symmetric slew rate and low distortion produce a clean, well-

behaved transient response.

3

2

1

A1

R1

95.3k

Ω

R2

787

Ω

C1

2200pF

C2

2200pF

R3

1.82k

Ω

C3

2200pF

R4

1.87k

Ω

R5

1.82k

Ω

A2

1

2

3

5

6

7

A3

R6

4.12k

Ω

C4

2200pF

R7

100k

Ω

5

6

7

A4

R8

1k

Ω

R9

1k

Ω

A1, A4 = 1/2 OP275

A2, A3 = 1/2 OP275

Figure 18. A 3-Pole, 40 kHz Low-Pass Filter

10Vp-p

10kHz

SCALE: VERTICAL–2V/ DIV

HORIZONTAL–10µs/ DIV

10

0%

100

90

Figure 19. Low-Pass Filter Transient Response

of the circuit and dc offset errors. If the parallel combination of

Ω, then an additional resistor, R

should be used in series with the noninverting input. The value

maintain the low distortion performance of the OP275.

Driving Capacitive Loads

The OP275 was designed to drive both resistive loads to 600

Ω

and capacitive loads of over 1000 pF and maintain stability.

While there is a degradation in bandwidth when driving capaci-

tive loads, the designer need not worry about device stability.

The graph in Figure 16 shows the 0 dB bandwidth of the OP275

with capacitive loads from 10 pF to 1000 pF.

10

9

8

7

6

5

4

3

2

1

0

0

200

400

600

800

1000

C

High Speed, Low Noise Differential Line Driver

The circuit of Figure 17 is a unique line driver widely used in

industrial applications. With

±18 V supplies, the line driver can

deliver a differential signal of 30 V p-p into a 2.5 k

Ω load. The

high slew rate and wide bandwidth of the OP275 combine to

yield a full power bandwidth of 130 kHz while the low noise

front end produces a referred-to-input noise voltage spectral

density of 10 nV/

√Hz.

1

2

3

A2

1

3

2

A1

5

6

7

A3

V

O1

R3

2k

R9

50

R11

1k

P1

10k

R12

1k

R10

50

R8

2k

R2

2k

R5

2k

R4

2k

R1

2k

R7

2k

A1 = 1/2 OP275

A2, A3 = 1/2 OP275

GAIN =

SET R2, R4, R5 = R1 AND R6, R7, R8 = R3

R3

R1

R6

2k

Figure 17. High Speed, Low Noise Differential Line Driver

The design is a transformerless, balanced transmission system

where output common-mode rejection of noise is of paramount