Circuit Theory/Dependent sources

 

Electronic amplifiers use two variables: current and voltage. Either can be used as input, and either as output leading to four types of amplifiers. In idealized form they are represented by each of the four types of dependent source used in linear analysis, as shown in the figure, namely:

Input	Output	Dependent source	Amplifier type
I	I	current controlled current source CCCS	current amplifier
I	V	current controlled voltage source CCVS	transresistance amplifier
V	I	voltage controlled current source VCCS	transconductance amplifier
V	V	voltage controlled voltage source VCVS	voltage amplifier

Each type of amplifier in its ideal form has an ideal input and output resistance that is the same as that of the corresponding dependent source:^[1]

Amplifier type	Dependent source	Input impedance	Output impedance
Current	CCCS	0	∞
Transresistance	CCVS	0	0
Transconductance	VCCS	∞	∞
Voltage	VCVS	∞	0

In practice the ideal impedances are only approximated. For any particular circuit, a small-signal analysis often is used to find the impedance actually achieved. A small-signal AC test current I_x is applied to the input or output node, all external sources are set to zero, and the corresponding alternating voltage V_x across the test current source determines the impedance seen at that node as R = V_x / I_x.

1. ↑ It is a curiosity to note that this table is a "Zwicky box"; in particular, it encompasses all possibilities.

This article uses material from the Wikipedia article  Metasyntactic variable, which is released under the  Creative Commons Attribution-ShareAlike 3.0 Unported License.