ECE 3254 PreLab 7 notes
Edited 9-24-07
Op Amps
R
out
AV
i
+
-
A
B
V
0
R
in
V
i
V
i
+
-
V
+
V
-
V
0
Op Amp Block Diagram and Equivalent Circuit
An ideal op amp has infinite gain (A = ∞), infinite input impedance (Rin = ∞), and zero Ω output
impedance (Rout = 0).
•Infinite gain means that the two input pins must have the same voltage - if there was any voltage
difference, infinite gain would peg the output against the power supply rail. A negative feedback
amplifier configuration uses the feedback network to set the amplifier circuit gain.
•Infinite input impedance means that the input current to the signal pins is zero because if
Rin = ∞, then Iin = Vin/∞ = 0.
•When the Op Amp is not ideal, the two input pins may not have exactly the same voltage, and
there may be some current flowing into or out of the input pins. For most of our measurements,
this will not be noticeable.
•Non-ideal characteristics may load the circuit, introduce noise, and reduce Rin and Gain.
Inverting Amplifier
The Op Amp ideal characteristics mean that
IR3 = 0 and Vpin 2 = Vpin 3.
These conditions mean that Vpin 3 = VR2 = 0, so
Vpin 2 = 0.
If Vpin2 = 0, Ii = Vg / R1
Then VR1 = Vg and
Ri (seen by the source) = Vg / Ii = R1
With the input current into pin 2 = 0, IR2 = Ii and
VR2 = Ii * R2.
Now what is that output voltage and circuit gain?
Because the Op Amp is an active circuit with feedback, the output current at pin 6 will be whatever is
necessary to produce the correct output voltage. Do not attempt to write a loop or node equation that
includes pin 6 and RL. If you look at the voltage across R2, you see that Vo = -VR2 because the pin 2 end
Prelab 7 Notes Page 1 of 2
