Passive devices are the main building blocks of electronic circuits and without them these circuits would either not work at all or become unstable. So what are “Passive Devices”? Well passive devices are the main elements used in electronics such as resistors, inductors, capacitors and transformers required to build electronic circuits. Passive devices are those that do not require electrical strength to function unlike “active devices” such as transistors, operational amplifiers and integrated circuits that require to be powered in some way to make them work.
Passive devices do not have gain or directionality and as such they will always have a gain less than one and consequently can not generate, oscillate or amplify a signal. Passive devices can be connected together in a circuit, either in a series or similar combination to control complicate circuits or signals, produce a phase shift to the signal or to provide some form of feedback but they can not multiply a signal by more than one because they have no strength gain. In fact passive devices consume strength making them like attenuators unlike active elements that generate or provide strength to a circuit.
The part values of passive devices such as resistance in Ohms or capacitance in Farads are always positive (i.e. >0) in value and never negative although some elements may have a negative coefficient. Also, passive devices are bi-directional elements that is they can be connected either way around within a circuit with the polarity of the voltage being that current flows from the positive to negative terminals unless the have a specific polarity marking such as electrolytic capacitors.
In both electrical circuit theory and circuit examination passive devices are generally called electrical elements so let us take a fleeting look at three of the most shared basic passive elements namely, Resistance, Capacitance and Inductance.
The resistor is a basic passive part that opposes the flow of electrical current by it. The amount of opposition to the flow of current is called the resistance of the resistor and is denoted by the symbol “R”. Resistance is a measure of how easily or how difficult electrons can flow by a particular path in an electrical circuit and is expressed as a value in units called Ohms. One ohm is the value of resistance that arises when a current of one ampere flows by a resistor that has one volt across its terminals. Then the resistance of a resistor can be defined in terms of the voltage drop across the resistor and the current flowing by the resistor as related by Ohm’s law: I = V/R
Where: R is the resistance, V is the voltage across the resistor, and I is the current flowing by the resistor. This relationship between the voltage and current (v-i relationship) in a resistor is linear and the strength absorbed by a resistor is represented by: P = VI. An ideal resistor will dissipate electrical energy without storing it as an electrical charge or as magnetic energy.
Inductance which has the symbol “L” and is measured in Henries (H), is the component used for the storage of energy in the form of an electromagnetic field. Electromagnetic energy is stored within the turns of a wire as long as a time varying current i(t) keeps flowing by the inductor. Self-inductance, L is the character of an inductor which opposes any changes in the current, I as defined by the continued of proportionality with the voltage generated in the wire being proportional to the rate of change of current flowing by it with respect to time.
An inductor is another passive device that can store or deliver energy but cannot generate it. An ideal inductor is lossless, meaning that it can store energy indefinitely as no energy is lost as heat. Inductors present a low impedance path to DC current and a high impedance path to AC current. The impedance of an inductor called inductive reactance varies with frequency and in an ideal inductor the current of the AC sine wave lags the voltage by 90 degrees.
Then we can define inductance L as the measure of an inductor’s “resistance” to the change of current with the larger the value of L, the lower the rate of change of current. Like resistance, inductance is always a positive value.
Our final passive device is the capacitor. Unlike the inductor which stores its energy magnetically, a capacitor stores its energy electrostatically as a charge across its plates. A capacitor is made up of two or more conducting plates which are separated by a dielectric material. Capacitance, C is the character of a capacitor which opposes any changes in the voltage across it as defined by the continued of proportionality as the current flowing by it is proportional to the rate of change of voltage across it with respect to time.
The capacitance of a similar plate capacitor is the ratio of the amount of charge, Q stored to the voltage, V across its plates and is measured in Farads, symbol C, ie C=Q/V. Capacitors present a low impedance path to AC signals but will block DC. The impedance of a capacitor called capacitive reactance varies with frequency and in an ideal capacitor the voltage of the AC sine wave lags the current by 90o. Like resistance, capacitance is always a positive value.