Variable Capacitance Multiplier Design with Op Amp

Implementation and analysis of a variable capacitance multiplier circuit are presented in this video. This circuit is useful in applications where a large adjustable capacitance is needed. This Analog circuit is implemented with two operational amplifiers and one potentiometer. In an alternative implementation we can use a digital potentiometer or RDAC. How does capacitor multiplier work is intuitively discussed in this video. This circuit is utilizing a combination of positive feedback and more dominant negative feedback. The positive feedback is enabling the larger than one scaling of the value of the internal 100 nano Farad capacitor in this circuit. The first Op Amp stage is wired as a buffer (unity-gain amplifier) and the second OpAmp stage is effectively an inverting amplifier in the frequency range of interest for this circuit. Both Op Amps are assumed to be stable and operating in linear region (not saturated) enforced by the dominant negative feedback that also enforces virtual short between positive and negative input terminals of op amp (virtual short). To find input impedance of this programmable capacitor multiplier circuit, a test voltage is applied at the input of the circuit and then the resulting input current is found. It is then shown that the input impedance is purely simulating an scaled capacitor whose value is adjustable by the potentiometer in the circuit. For more circuit design videos please see the analog circuit and signal processing video playlist    • Electrical Engineering, Analog, Digit...  .