Lecture 12 Maximum Signal Transfer ECE 205 Prof

Lecture 12 Maximum Signal Transfer ECE 205 Prof. Ali Keyhani

Maximum Signal Transfer • In a given interface maximizing the voltage, current or the power from the source to load is important depending on the application • The goal is to adjust the load to have maximum power, voltage or current • For simplicity source and load circuits are assumed to be resistive • The source circuit is shown by its Thevenin equivalent circuit and the load is represented by equivalent resistance RL

• Applying voltage division results in interface voltage: • If then the interface voltage will be maximum • Ideally when or when load is open circuit the voltage is maximum. Therefore:

• The current delivered to the interface: • For a fixed source the current is maximized when RL is made small: RL 0 • Ideally when load resistance is zero (short circuit) the interface current is maximized:

Maximum Power Transfer • The power delivered at the interface: • Maximum voltage (RL ∞): p 0 • Maximum current (RL 0): p 0 Note: conditions for maximum voltage or current will not lead to maximum power

Maximum Power Transfer • The equation is differentiated with respect to RL to find the maximum power:

• Maximum power transfer occurs when load resistance equals to Thevenin resistance of the source

Normalized plots of current , voltage and power versus RL/RT

Example 1 For a circuit with v. T=5 V and RT=50Ω, and a load resistance of RL=10Ω: A) Find the maximum voltage, current and power levels available from the source B) Determine the actual signal levels delivered to the load

Solution: A)

B) Actual power delivered:

Interface Circuit Design • Sometimes it is necessary to insert an interface circuit between source and load to achieve desired signal levels • The interface circuits are called two -port network – Port connected to the source: input – Port connected to the load: output

Example 2 • Find the load resistance to have the interface signals :

Example 2 Solution: Interface voltage constraint: Interface current constraint:

Example 3 • Design the interface circuit so that the source delivers v 2=2 v to the load and the input resistance seen by the input port is RIN=300 Ω