REV. D
OP497
–9–
2
3
1
7
8
5
6
4
–5V
+5V
REF43
6
2
4
+5V
R
2.5 V
R +
R
RF
VOUT
VREF
VOUT = VREF (
)
R
R +
R
RF
R
1/4
OP497
1/4
OP497
Figure 9. A Simple Bridge Conditioning Amplifier Using
the OP497
NONLINEAR CIRCUITS
Due to its low input bias currents, the OP497 is an ideal log
amplifier in nonlinear circuits such as the square and square
root circuits shown in Figures 10 and 11. Using the squaring
circuit of Figure 10 as an example, the analysis begins by writing a
voltage-loop equation across transistors Q1, Q2, Q3, and Q4.
VIn
I
VIn
I
VIn I
I
VIn
I
T
IN
S
T
IN
S
T
O
S
T
REF
S
1
2
3
4
+
=
+
All the transistors of the MAT04 are precisely matched and at
the same temperature, so the IS and VT terms cancel, giving:
2InI
InI
In I
I
IN
O
REF
O
REF
=+
=
×
()
Exponentiating both sides of thick equation leads to:
I
O
IN
2
REF
= ()
Op amp A2 forms a current-to-voltage converter which gives
VOUT = R2
× IO. Substituting (VIN/R1) for IIN and the above
equation for IO, yields:
V
R2
I
V
R1
OUT
REF
IN
2
=
1
2
3
6
7
5
C1
100pF
V+
2
3
8
1
4
V–
VIN
IIN
C2
100pF
6
5
7
VOUT
I O
9
8
10
Q1
Q3
Q2
14
12
Q4
13
–15V
IREF
MAT-04E
R1
133k
R2
33k
R3
50k
R4
50k
1/4
OP497
1/4
OP497
Figure 10. Squaring Amplifier
A similar analysis made for the square-root circuit of Figure 11
leads to its transfer function:
VR2
VI
R1
OUT
IN
REF
=
()( )
In these circuits, IREF is a function of the negative power sup-
ply. To maintain accuracy, the negative supply should be well
regulated. For applications where very high accuracy is required,
a voltage reference may be used to set IREF. An important con-
sideration for the squaring circuit is that a sufficiently large
input voltage can force the output beyond the operating range
of the output op amp. Resistor R4 can be changed to scale IREF,
or Rl and R2 can be varied to keep the output voltage within
the usable range.
1
2
3
6
7
5
C1
100pF
V+
2
3
8
1
4
V–
6
5
7
VOUT
IO
9
8
10
Q1
Q3
Q2
14
12
Q4
13
–15V
IREF
MAT-04E
IIN
VIN
R1
33k
R2
33k
R5
2k
R3
50k
R4
50k
C2
100pF
1/4
OP497
1/4
OP497
Figure 11. Square-Root Amplifier
Unadjusted accuracy of the square-root circuit is better than
0.1% over an input voltage range of 100 mV to 10 V. For a
similar input voltage range, the accuracy of the squaring circuit
is better than 0.5%.