Independent and dependent sources

INDEPENDENT AND DEPENDENT SOURCES:

The ideal sources discussed above are independent sources. They are also termed as uncontrolled sources. Their voltages and currents are not affected by other parts of the circuit. Their voltages and currents are fixed.

Dependent sources are also known as controlled sources. In case of dependent sources, voltage and current are not fixed. In this case, voltage or current is dependent on the voltage or current existing at some other location in the circuit. Such sources are represented as shown below by a diamond shape. Voltage source is marked with polarities and the current source is shown by an arrow.

Controlled sources are used in many models like transistors and vacuum tubes. There are four types of controlled sources as mentioned below. This classification depends on whether control variable is voltage or current and the source controlled is a voltage source or current source. The four types are

(i) Voltage-controlled voltage source (VCVS). Eg. Operational amplifier

(ii) Voltage controlled current source (VCCS). Eg. Field effect transistor

(iii) Current-controlled voltage source (CCVS).

(iv) Current-controlled current source (CCCS). Eg. Common base transistor

The above controlled sources are shown in the following figures.

The factors μ, g_m, α and r_m are scalar constants. Hence these controlled sources are linear, time - variant active elements. For these controlled sources, either input voltage or input current is zero and they are capable of delivering unlimited output power. So they exhibit infinite power gain.

Controlled source is unilateral. For example, observe the figure of VCVS. V₁ controls the output V₂.But I₂ has no influence on the input.