LECTURE 11 - THE OP-AMP
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LECTURE 11 - THE OP-AMP • Op-Amp: “Operational Amplifier” • Circuit Symbol: 8 7 6 5 1 2 3 4 LM 741 + _ Power S u pp ly - Power S up p ly + OUT • Reality:
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LECTURE 11 - THE OP-AMP. Op-Amp: “Operational Amplifier” Circuit Symbol:. Reality:. Differential Amplifier. Circuit Model in linear region. V 0. V +. +. A. AV 1. R i. . V . +. . +. +. V 1. V 0. But if A ~ , is the output infinite?. “Very high gain” . . . - PowerPoint PPT Presentation
Transcript of LECTURE 11 - THE OP-AMP
LECTURE 11 - THE OP-AMP
• Op-Amp: “Operational Amplifier”
• Circuit Symbol:
8 7 6 5
1 2 3 4
L M 7 4 1
+_ PowerSupply -
PowerSupply + O U T
• Reality:
THE DIFFERENTIAL AMPLIFIER
+
+
V0AV1
+
V1
Ri
Circuit Model in linear region
)VV(AV0
+
AV+
V
V0
Differential Amplifier
“Differential” V0 depends only on difference (V+ V-)
“Very high gain” A But if A ~ , is the output infinite?
The output cannot be larger than the supply voltages. It will limit or “clip” if we attempt to go too far. We call the limits of the output the “rails”.
WHAT ARE I-V CHARACTERISTICS OF AN ACTUAL HIGH-GAIN DIFFERENTIAL AMPLIFIER ?
Example: Amplifier with gain of 105, with max V0 of 3V and min V0 of 3V.
VIN(V)1 2 3
V0 (V)
0.1
0.2
3 2 1
.2
(a)I-V near origin
3
(b)I-V over wider range
VIN(V)10 20 30
V0 (V)
1
30 20 10
21
23
upper “rail”
lower “rail”
+ V0
+
VIN
• Circuit model gives the essential linear part
• But V0 cannot rise above some physical voltage related to the positive power supply VCC (“ upper rail”) V0 < V+RAIL
• And V0 cannot go below most negative power supply, VEE i.e., limited by lower “rail” V0 > V-RAIL
THE VOLTAGE FOLLOWER
Negative feedback Stabilizes the output
NEGATIVE FEEDBACK
V0+
VIN
NEGATIVE FEEDBACK
Familiar examples of negative feedback:
Thermostat controlling room temperature
Driver controlling direction of automobile
Photochromic lenses in eyeglasses
Familiar examples of positive feedback:
Microphone “squawk” in room sound system
Mechanical bi-stability in light switches
Thermonuclear reaction in H-bomb
Fundamentally pushes
toward stability
Fundamentally pushes toward
instability or bi-stability
EASY WAY TO GET ANSWERFOR OP-AMP CIRCUITS
“Ideal Op-Amp Technique”:
With ideal op-amp technique we can analyze all sorts of negative feedback
VV )1(
0i
0i )2(
IN
IN
Why?(a) RIN large by design
(b) V+ V() voltage difference across RIN 0
V0
+
iIN(-)
A
iIN(+)
Why?V0 CANNOT , BUT A V+ V() in order that V0 = A (V+ V)
Finite infinite must be zero
EXAMPLES USING IDEAL OP-AMP TECHNIQUE
Vin R2
R1
Vout
+
_
Vp
Vn
I
The non-inverting amplifier
EXAMPLES USING IDEAL OP-AMP TECHNIQUE
???????????????????
Vin
R2
R1
Vout
+
_
Vn
Vp
C
