Dependent sources can be classified as follows:

• Voltage-controlled voltage source: The source delivers the voltage as per the voltage of the dependent element. ${\displaystyle V={f_{a}}({v_{x}})}$ 
• Voltage-controlled current source: The source delivers the current as per the voltage of the dependent element. ${\displaystyle I={f_{b}}({v_{x}})}$ 
• Current-controlled current source: The source delivers the current as per the current of the dependent element. ${\displaystyle I={f_{c}}({i_{x}})}$ 
• Current-controlled voltage source: The source delivers the voltage as per the current of the dependent element. ${\displaystyle V={f_{d}}({i_{x}})}$ 

Dependent sources are not necessarily linear. For example, MOSFET switches can be modeled as a voltage-controlled current source when ${\displaystyle V_{\rm {DS}}>V_{\rm {GS}}-V_{T}}$  and ${\displaystyle V_{\rm {GS}}>V_{T}}$ .

However, the relationship between the current flowing through it and ${\displaystyle V_{\rm {DS}}}$  is approximately:

${\displaystyle I_{\rm {D}}={\frac {\mu _{n}C_{\rm {ox}}}{2}}{\frac {W}{L}}(V_{\rm {GS}}-V_{\rm {th}})^{2}\left(1+\lambda (V_{\rm {DS}}-V_{\rm {DSsat}})\right).}$ ^[2][3]

In this case, the current is not linear to ${\displaystyle V_{\rm {DS}}}$ , but rather approximately proportional to the square of ${\displaystyle V_{\rm {DS}}-V_{T}}$ .

As for the case of linear dependent sources, the proportionality constant between dependent and independent variables is dimensionless if they are both currents (or both voltages). A voltage controlled by a current has a proportionality factor expressed in units of resistance (ohms), and this constant is sometimes called "transresistance". A current controlled by a voltage has units of conductance (siemens), and is called "transconductance". Transconductance is a commonly used specification for measuring the performance of field effect transistors and vacuum tubes.^[1]