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OP297 Datasheet, PDF (10/16 Pages) Analog Devices – Dual Low Bias Current Precision Operational Amplifier
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8/21/97 4:00 PM
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 34 and 35. Using the squaring
circuit of Figure 34 as an example, the analysis begins by writing
a voltage loop equation across transistors Q1, Q2, Q3 and Q4.
V
T1

ln
I IN
IS1


+VT2

ln
I IN
IS2


=VT3

ln
IO
IS3


+VT4
ln


I REF
IS4


All the transistors of the MAT04 are precisely matched and at
the same temperature, so the IS and VT terms cancel, giving:
2 ln IIN = ln IO + ln IREF = ln (IO × IREF)
Exponentiating both sides of the equation leads to:
IO
= (IIN )2
I REF
Op amp A2 forms a current-to-voltage converter which gives
VOUT = R2 × 10. Substituting (VIN/R1) for IIN and the above
equation for IO yields:
V OUT
=


R2
I REF


V IN  2
 R1 
A similar analysis made for the square-root circuit of Figure 35
leads to its transfer function:
V OUT
= R2
(V IN )(IREF )
R1
R1 IIN
VIN
33kΩ
1
2 Q1
3
C1
100pF
V+
7
6 Q2
5
2– 8
1/2
OP-297
1
3 +A1 4
V–
C2
100pF
R2
33kΩ
6–
1/2
OP-297
7
IO
5 +A2
VOUT
MAT-04E
8
Q3 9
10
IREF
14
Q4
13
12
R3
50kΩ
R4
50kΩ
–15V
Figure 34. Squaring Amplifier
–10–
REV. D
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