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OP297FP Datasheet(PDF) 10 Page - Analog Devices |
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OP297FP Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page 8/21/97 4:00 PM OP297 –10– REV. D 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. VT1 ln IIN IS1 +V T 2 ln IIN IS2 =V T 3 ln IO IS3 +V T 4 ln IREF 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 × I REF) Exponentiating both sides of the equation leads to: IO = (IIN )2 IREF Op amp A2 forms a current-to-voltage converter which gives VOUT = R2 × 10. Substituting (V IN/R1) for IIN and the above equation for IO yields: VOUT = R2 IREF V IN R1 2 A similar analysis made for the square-root circuit of Figure 35 leads to its transfer function: VOUT = R2 (V IN )(IREF ) R1 Figure 34. Squaring Amplifier VIN 33k Ω 1/2 OP-297 A1 2 3 8 1 4 V– V+ C1 100pF C2 100pF R2 33k Ω R1 IO IIN IREF VOUT 1/2 OP-297 A2 6 5 7 MAT-04E Q1 1 3 2 Q4 14 12 13 Q2 7 5 6 Q3 8 10 9 50k Ω R3 50k Ω R4 –15V – + – + |
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