REV. E

–8–

OP297

PRECISION POSITIVE PEAK DETECTOR

ethylene to minimize dielectric absorption and leakage. The

of the OP297.

2N930

1/2

OP297

+15V

0.1 F

1k

1N4148

RESET

5

6

7

1

28

3

0.1 F

1/2

OP297

1k

1k

–15V

1k

Figure 11. Precision Positive Peak Detector

SIMPLE BRIDGE CONDITIONING AMPLIFIER

Figure 12 shows a simple bridge conditioning amplifier using the

OP297. The transfer function is

VV

R

RR

R

R

OUT

REF

F

=

+









∆

∆

The REF43 provides an accurate and stable reference voltage for

be 0.1% or better with a low temperature coefficient.

15V

3

2

1

1/2

OP297

R +

R

R

R

5

6

7

1/2

OP297

4

8

REF43

R +

R

4

Figure 12. A Simple Bridge Conditioning Amplifier

Using the OP297

NONLINEAR CIRCUITS

Due to its low input bias currents, the OP297 is an ideal log

amplifier in nonlinear circuits such as the square and square-

root circuits shown in Figures 13 and 14. Using the squaring

circuit of Figure 13 as an example, the analysis begins by writing a

voltage loop equation across transistors Q1, Q2, Q3, and Q4.

Vln

I

I

Vln

I

I

Vln

I

I

Vln

I

I

T

IN

S

T

IN

S

T

O

S

T

REF

S

1

1

2

2

3

3

4

4









+









=









+









All the transistors of the MAT04 are precisely matched and at

2 ln I

ln I

ln I

ln I

IN

O

REF

O

=+

=

×

()

I

REF

Exponentiating both sides of the equation leads to

I

I

I

O

IN

REF

2

Op amp A2 forms a current-to-voltage converter, which gives

× I

V

R

I

V

R

OUT

REF

IN

=

















2

1

2

A similar analysis made for the square-root circuit of Figure 14

leads to its transfer function

VR

VI

R

OUT

IN

REF

=

()( )

2

1

1/2

OP297

7

6

5

R2

33k

C2

100pF

1/2

OP297

–15V

1

2

3

V+

V–

R3

50k

Q3

R1

33k

Q1

Q2

Q4

13

14

12

R4

50k

8

9

10

4

8

6

5

7

2

3

1

C1

100pF

MAT04E

Figure 13. Squaring Amplifier

1/2

OP297

7

6

5

R2

33k

C2

100pF

1/2

OP297

–15V

1

2

3

V+

V–

R3

50k

R1

33k

Q1

R4

50k

7

9

10

4

8

3

1

C1

100pF

MAT04E

8

Q3

5

6

Q4

13

12

14

Q2

Figure 14. Square-Root Amplifier