6.2.5 General Feedback Elements

Chapter 6.2.5 General Feedback Elements

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 6. OPERATIONAL AMPLIFIERS114and only drops 18 db/octave.For complex poles we must use either integrators or differentiators. Consider figure 6.14.CRRIVinVoutFigure 6.14: Active filter with complex poles.The closed-loop gain isG =−ZRCRI=−R/jωCRI (R +1/jωC)=−RRI (1 + jωRC).(6.27)By exchanging the input resistor for a capacitor we can change between a low-pass andhigh-pass filter.6.2.5General Feedback ElementsThe feedback elements in an operation amplifier design can be more complicated than asimple resistor and capacitor. An interesting feedback element is the analog multiplier asdefined in figure 6.15.Vy(t)Vx(t)Vz(t)=Vx(t)Vy(t)MULTFigure 6.15: Five-terminal network that performs the multiplication operation on two voltagesignals.The multiplier circuit itself can be thought of as another op-amp with a feedback resistorwhose value is determined by a second input voltage. Multiplication circuits with the abilityto handle input voltages of either sign (four-quadrant multipliers) are available as integratedcircuits and have a number of direct uses as multipliers. But when used in a feedback looparound an operational amplifier, other useful functional forms result.The circuit of figure 6.16 gives an output that is the ratio of two signals, whereas thecircuit of figure 6.17 yields the analog square-root of the input voltages.