Search results
Results from the WOW.Com Content Network
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.
This feedback means the output impedance of the circuit is increased, because the feedback involving R 2 forces use of a larger voltage to drive a given current. Output resistance is found using a small-signal model for the circuit, shown in Figure 2. Transistor Q 1 is replaced by its small-signal emitter resistance r E because it is diode ...
If the output is taken as a voltage, the amplifier is a transresistance amplifier and delivers a voltage dependent on the load impedance, for example v out = i in R L for a resistor load R L much smaller in value than the amplifier output resistance R out. That is, the voltage gain in this case (explained in more detail below) is
In this circuit (NMOS) the gate terminal of the transistor serves as the signal input, the source is the output, and the drain is common to both (input and output), hence its name. Because of its low dependence on the load resistor on the voltage gain, it can be used to drive low resistance loads, such as a speaker.
Real-world voltage sources instead have a non-zero output impedance, which is preferably very low (often much less than 1 ohm). Conversely, a current source provides a constant current, as long as the impedance of the load is sufficiently lower than the current source's parallel impedance (which is preferably very high and ideally infinite).
However, the FET device's output resistance typically is not high enough for a reasonable transconductance amplifier (ideally infinite), nor low enough for a decent voltage amplifier (ideally zero). As seen below in the formula, the voltage gain depends on the load resistance, so it cannot be applied to drive low-resistance devices, such as a ...
A small output impedance means that the series combination of the original voltage source and the voltage follower presents a Thévenin voltage source with a lower Thévenin resistance at its output node; that is, the combination of voltage source with voltage follower makes a more ideal voltage source than the original one.
The measure of that independence is the output impedance of the circuit, the ratio of a change in output voltage to the change in current it causes. Fig. 3 shows a small signal model of a Wilson current mirror drawn with a test voltage source, v test {\displaystyle \scriptstyle v_{\text{test}}} , attached to the output.