Engineering 43
OpAmp Tutorial
a.k.a.: Draw in the
Diamond
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer
BMayer@ChabotCollege.edu
Engineering-43: Engineering Circuit Analysis
1
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-43_OpAmp_Tutoria.ppt
Picture: Sp11 MTE1 OpAmp Problem
2.7 kΩ
560 Ω
+
3 mA
8.2 kΩ
1.2 kΩ
1.8 kΩ
5 mA
Io
Vo
−
Sp11 MTE1 OpAmp Errors
• Calc of v+ as +/− 41V
– Assumes voltage across
5 mA current source is
ZERO!!!
– V+ Actually 6V
• Failure to Recognize
that OpAmp can Source
or Drain current
• Assuming that the
Voltage across the 3 mA
source is ZERO!!!
• NO Calc, at all, of v+
– Just Assumed v− = 0
• Failure to use Virtual
Short  v+ = v−
• Using GND Node for KCL
– i.e., assumed that IOA = 0
withOUT considering the
– Shoud be IOA = 10.83 mA
OpAmp GND
– Relates to IOA = 0 error
Suggestions to Improve
• SHOW on OpAmp Symbol Dependent Voltage
Diamond
– as source of “infinite” output current
– Connection to GND to emphasize KCL for OpAmp
OutPut Current
• On Example Find Power-Conditions for Isrc’s
 find Voltage relative to GND Between
– 3m Source and 0.5k Resistor
– 5m Source and 8k Resistor
Picture
3 kΩ
0.5 kΩ
+
3 mA
8 kΩ
5 mA
2 kΩ
Vo
1 kΩ
Io
−
Ideal  Ri   Ro  0
Ao  