6.3.2 Input Impedance

Chapter 6.3.2 Input Impedance

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

  • CHAPTER 6. OPERATIONAL AMPLIFIERS125H(jω)ωcω−12 db/octaveFigure 6.30: The overall transfer function when the amplifier drives a capacitive load.6.3.2Input ImpedanceWhen calculating the output impedance we still assumed an infinite input impedance. Inthis section we will calculate the finite input impedance assuming a zero output impedance.We consider a model that assumes an internal resistor RT connecting the inverting andnon-inverting input terminals of the op-amp as shown in figure 6.31Consider an inverting amplifier and remove the input resistor so that the input impedancecan be calculated directly at the amplifier’s input terminals.V1ITI1I2RFVoutRT+-Figure 6.31: Model for calculating the input impedance of the inverting amplifier.The input impedance is defined byZin =V1I1(6.72)and the current at the summing junction is