OP497
+5V
2
2.5 V V
REF
REF43 6
R
4
R
R
F
R
2
1/4
R+
R
3
C2
100pF
6
R2
33k
1/4
OP497
1
V
OUT
1
2
Q1
3
6
7
Q2
5
8
R1
133k
V+
I
IN
2
1/4
3
8
1
C1
100pF
I
O
5
OP497
7
V
OUT
+5V
6
1/4
5
8
V
OUT
= V
REF
(
7
R
R
)
F
R+ R
R
MAT-04E
Q3
10
9
14
I
REF
Q4
12
13
OP497
4
–5V
V
IN
OP497
4
V–
Figure 9. A Simple Bridge Conditioning Amplifier Using
the OP497
NONLINEAR CIRCUITS
R3
50k
R4
50k
–15V
Figure 10. Squaring Amplifier
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 Q
1
, Q
2
, Q
3
, and Q
4
.
A similar analysis made for the square-root circuit of Figure 11
leads to its transfer function:
V
OUT
=
R2
(
V
IN
)(
I
REF
)
R1
I
I
I
I
V
T
1
In
IN
+
V
T
2
In
IN
=
V
T
3
In
I
O
+
V
T
4
In
REF
I
S
1
I
S
2
I
S
4
I
S
3
All the transistors of the MAT04 are precisely matched and at
the same temperature, so the I
S
and V
T
terms cancel, giving:
2InI
IN
=
InI
O
+
InI
REF
=
In
(
I
O
×
I
REF
)
Exponentiating both sides of thick equation leads to:
In these circuits, I
REF
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 I
REF
. 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 I
REF
,
or Rl and R2 can be varied to keep the output voltage within
the usable range.
R2
33k
I
O
(
I
IN
)
2
=
I
REF
I
O
I
IN
C1
100pF
V+
V
IN
R1
33k
2
3
8
1/4
6
Q1 1
2
3
6
5
C2
100pF
1/4
Op amp A
2
forms a current-to-voltage converter which gives
V
OUT
= R2
×
I
O
. Substituting (V
IN
/R1) for I
IN
and the above
equation for I
O
, yields:
OP497
7
V
OUT
MAT-04E
14
13
8
9
Q4
12
I
REF
V
OUT
R2
V
IN
=
I
REF
R1
2
7
Q3
Q2
5 10
OP497
4
V–
1
R5
2k
R3
50k
R4
50k
–15V
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%.
REV. D
–9–
AD [ ANALOG DEVICES ]
