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Actual passive two-terminal components can be represented by some network of lumped and distributed ideal inductors, capacitors, and resistors, in the sense that the real component behaves as the network does. Some of the components of the equivalent circuit can vary with conditions, e.g., frequency and temperature.
The equivalent resistance R th is the resistance that the circuit between terminals A and B would have if all ideal voltage sources in the circuit were replaced by a short circuit and all ideal current sources were replaced by an open circuit (i.e., the sources are set to provide zero voltages and currents).
At each stage, resistors for the "rung" and "leg" are chosen so that the rung value matches the leg value plus the equivalent resistance of the previous rungs. The rung and leg resistors can be formed by pairing other resistors in series or parallel in order to increase the number of available combinations. This process can be automated.
Many circuits can be analyzed as a combination of series and parallel circuits, along with other configurations. In a series circuit, the current that flows through each of the components is the same, and the voltage across the circuit is the sum of the individual voltage drops across each component. [ 1 ]
The Norton equivalent circuit is used to represent any network of linear sources and impedances at a given frequency. Norton's theorem and its dual, Thévenin's theorem , are widely used for circuit analysis simplification and to study circuit's initial-condition and steady-state response.
The theorems are useful in 'circuit analysis' especially for analyzing circuits with feedback [1] and certain transistor amplifiers at high frequencies. [ 2 ] There is a close relationship between Miller theorem and Miller effect: the theorem may be considered as a generalization of the effect and the effect may be thought as of a special case ...
A voltage drop occurs across each resistor in the network causing each successive "rung" of the ladder (each node of the circuit) to have a higher voltage than the previous one. Since the ladder is a series circuit, the current is the same throughout, and is given by the total voltage divided by the total series resistance (V/R eq).
Figure 4. These circuits are equivalent: (A) A resistor at nonzero temperature with internal thermal noise; (B) Its Thévenin equivalent circuit: a noiseless resistor in series with a noise voltage source; (C) Its Norton equivalent circuit: a noiseless resistance in parallel with a noise current source.