Lecture 12
•Review:
•Source transformations
•Maximum power transfer
•Derivation of maximum power transfer
•Thévenin theorem examples
•Operational Amplifiers
•Related educational modules:
–Sections 1.7.5, 1.8.0, 1.8.1
Using Source Transformations in Circuit Analysis
• Any voltage source in series with a resistance can be modeled as a
current source in parallel with the same resistance and vice-versa
Maximum Power Transfer
• The load receives the maximum amount of power if
RL = RTH
• Why?
Maximum Power Transfer – Derivation
• Load voltage:
VL  VOC
RL
RL  RTH
• Delivered power:
V
V 
RL

PL 

RL
RL  RL  RTH
2
L
2
OC
PL
RL



2
Maximizing power
• Set derivative of power to zero:
PL

0
RL
RL
• Chain rule:
 2

RL
0
VOC
2
RL  RTH  

 ( RL  RTH )2  2 RL ( RTH  RL ) 
V 
0
4
( RL  RTH )


2
OC
• Set numerator to zero:
RL  RTH
Maximum Power Delivered
• Delivered power:
2
OC
V
PL 
RL

RL

 RL  RTH
• Letting RL = RTH:
2
VOC
PL 
4 RTH



2
Example 1: Maximum power transfer
(a) Determine the load resistance, R, which absorbs the
maximum power from the circuit.
(b) What is the maximum power delivered to the load?
Example 1(a): Load Design
Example 1(b): Power delivered
Example 2
• Determine the Norton equivalent of the circuit of example 1
Operational Amplifiers
• So far, with the exception of our ideal power
sources, all the circuit elements we have examined
have been passive
– Total energy delivered by the circuit to the element is
non-negative
• We now introduce another class of active devices
– Operational Amplifiers (op-amps)
– Note: These require an external power supply!
Operational Amplifiers – overview
• We will analyze op-amps as a “device” or “black
box”, without worrying about their internal circuitry
– This may make it appear as if KVL, KCL do not apply to the
operational amplifier
– Our analysis is based on “rules” for the overall op-amp
operation, and not performing a detailed analysis of the
internal circuitry
• We want to use op-amps to perform operations, not
design and build the op-amps themselves
uA741 op-amp schematic
• Source: RFIC Technologies web site
Ideal Operational Amplifiers
• Typical circuit schematic symbol:
ip +
vp - vn = vin
in -
• Three-terminal device (2 inputs, 1 output)
• Operation characterized by:
– Voltage difference between input terminals (vin)
– Currents into the input terminals (ip and in)
Ideal Operational Amplifier “Rules”
• More complete circuit
symbol
• (Power supplies shown)
• Assumptions:
• ip = 0, in = 0
• vin = 0
• V - < vout < V +
Notes on op-amp operation
1. Output current is generally not known (it is
provided by external power supplies)
2. KCL at input nodes is generally a good starting
point in op-amp circuit analysis
3. vin is multiplied by a large number to get vout
4. vout is limited by the external power supplies
Op-amp circuit – example 1
• Find Vout
Op-amp circuit – example 2
• Find Vout