Week of October 22nd - ESE123 Capacitor non-idealities...

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Figure : Schematic symbol for the op amp e : Schematic symbol showing power supply connections ESE123 Class Notes for week of October 22nd Fall 2007 Capacitor non-idealities In our previous lecture we noted that the capacitor is a device that can store energy in an electric field. We use the passive sign convention to assign the voltage and current reference directions as we did with the resistor. During the time when the instantaneous power (the product of the current and voltage) is positive, the capacitor is absorbing power or charging . When the instantaneous power is negative the capacitor is delivering power to the circuit or discharging . This cycle may repeat billions of time per second. Over a long period of time, the average power of an ideal capacitor is zero because the total energy absorbed is equal to the total energy delivered. Real capacitors are not ideal however, and there is some loss in the energy conversion process. Real capacitors dissipate some power just like resistors do. If you are specifying a capacitor where the charging currents will be high, you will need to consult the manufacturer’s specifications to determine if the capacitor can handle the anticipated power levels. Special capacitors are manufactured for these applications. Similarly, a charged capacitor should maintain its charge indefinitely if it is removed from its circuit, but instead you will find that all capacitors slowly loose their charge, this phenomenon is known as leakage . You may need to specify low leakage capacitors for some applications. Capacitors can be used in timing circuits. In these applications, it is important that the capacitance is precisely known and does not vary with temperature. Real capacitors have tolerances that can be quite large, and they also have temperature coefficients that describe how the capacitance changes with temperature. If you are designing a circuit that uses capacitors for timing you will need to consult the manufacturer’s data sheet for the relevant specifications. In any application, it is necessary to observe the maximum voltage specification for the capacitor. Higher voltages correspond to larger devices (for a given capacitance). The lesson here is that while resistors behave pretty much like their ideal model (V=IR), capacitors can vary quite a bit from the ideal, and it is necessary to understand the various compromises when choosing a part. For instance, electrolytic capacitors have a very high capacitance for their volume (and they are cheap), but they can only tolerate the application of voltage in one direction; reversed polarity may result in catastrophic failure. Because of the various compromises inherent in capacitor construction,
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