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The actual output impedance for most devices is not the same as the rated output impedance. A power amplifier may have a rated impedance of 8 ohms, but the actual output impedance will vary depending on circuit conditions. The rated output impedance is the impedance into which the amplifier can deliver its maximum amount of power without failing.
Figure 5: Small-signal circuit to determine output resistance of mirror; transistor Q 2 is replaced with its hybrid-pi model; a test current I X at the output generates a voltage V X, and the output resistance is R out = V X / I X. An idealized treatment of output resistance is given in the footnote.
In a standard two-transistor current mirror, the output impedance would be the dynamic early resistance of the output transistor, the equivalent of which in this case is . The Wilson current mirror has an output impedance that is higher by the factor β 2 {\displaystyle \scriptstyle {\frac {\beta }{2}}} , on the order of 50 times.
The Early voltage (V A) as seen in the output-characteristic plot of a BJT. The Early effect , named after its discoverer James M. Early , is the variation in the effective width of the base in a bipolar junction transistor (BJT) due to a variation in the applied base-to-collector voltage.
If the resistance is not constant, the previous equation cannot be called Ohm's law, but it can still be used as a definition of static/DC resistance. [4] Ohm's law is an empirical relation which accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current.
In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. [1]Quantitatively, the impedance of a two-terminal circuit element is the ratio of the complex representation of the sinusoidal voltage between its terminals, to the complex representation of the current flowing through it. [2]
Load regulation is the capability to maintain a constant voltage (or current) level on the output channel of a power supply despite changes in the supply's load (such as a change in resistance value connected across the supply output). [1] [2]
Because the amplifier input resistance is small, the driver delivers by current division a current v Thév / R S to the amplifier. The current gain is unity, so the same current is delivered to the output load R L, producing by Ohm's law an output voltage v out = v Thév R L / R S, that is, the first form of the voltage gain above.