Download - Expt 3 Op-Amps
-
7/29/2019 Expt 3 Op-Amps
1/27
Operational Amplifiers
Op-Amps are possibly the mostversatile linear integrated circuits usedin analog electronics.
The Op-Amp is not strictly an element;it contains elements, such as resistorsand transistors.
However, it is abasic building block,
just like R, L, and C. We treat this complex circuit as ablack
box.
-
7/29/2019 Expt 3 Op-Amps
2/27
EXPT 3STUDY OF BASIC OP-AMP CIRCUITS1. Inverting
2. Non-inverting
3. Voltage follower
4. Scale changer
5. Summer/adder
6. Differential
-
7/29/2019 Expt 3 Op-Amps
3/27
The Op-Amp Chip
The op-amp is a chip, a small black box with 8
connectors or pins (only 5 are usually used).
The pins in any chip are numbered from 1
(starting at the upper left of the indent or dot)
around in a U to the highest pin (in this case 8).
741 Op Amp or LM351 Op Amp
-
7/29/2019 Expt 3 Op-Amps
4/27
Op-Amp Input and Output The op-amp has two inputs, an inverting input (-)
and a non-inverting input (+), and one output. The output goes positive when the non-inverting
input (+) goes more positive than the inverting (-)
input, and vice versa.
The symbols + anddo not mean that that you
have to keep one positive with respect to the other;
they tell you the relative phase of the output.
(Vin
=V1-V
2)
A fraction of a millivolt
between the input
terminals will swing
the output over its full
range.
-
7/29/2019 Expt 3 Op-Amps
5/27
Powering the Op-Amp
Since op-amps are used as amplifiers, they need
an external source of (constant DC) power. Typically, this source will supply +15V at +V and
-15V at -V. We will use 9V. The op-amp will
output a voltage range of of somewhat less
because of internal losses.
The power supplied determines the
output range of the op-amp. It can
never output more than you put in.
Here the maximum range is about28 volts. We will use 9V for the
supply, so the maximum output
range is about 16V.
-
7/29/2019 Expt 3 Op-Amps
6/27
Op-Amp Intrinsic Gain
Amplifiers increase the magnitude of a signal by
multiplier called a gain -- A. The internal gain of an op-amp is very high. The
exact gain is often unpredictable.
We call this gain the open-loop gain orintrinsic
gain.
The output of the op-amp is this gain multiplied
by the input
5 6outopen loop
in
VA 10 10
V
21 VVAVAV olinolout
-
7/29/2019 Expt 3 Op-Amps
7/27
Op-Amp Saturation
The huge gain causes the output to change
dramatically when (V1-V2) changes sign.
However, the op-amp output is limited by the
voltage that you provide to it.
When the op-amp is at the maximum orminimum extreme, it is said to besaturated.
saturationnegativeVVthenVVif
saturationpositiveVVthenVVif
VVV
out
out
out
21
21
-
7/29/2019 Expt 3 Op-Amps
8/27
Feedback
Negative Feedback
As information is fed back, the output becomes more stable. Output tends
to stay in the linear range. The linear range is when Vout=A(V1-V2) vs.
being in saturation.
Examples: cruise control, heating/cooling systems
Positive Feedback
As information is fed back, the output destabilizes. The op-amp tends to
saturate.
Examples: Guitar feedback, stock market crash
Positive feedback was used before high gain circuits became available.
-
7/29/2019 Expt 3 Op-Amps
9/27
Op-Amp Circuits use Negative Feedback
Negative feedback couples the output backin such a way as to cancel some of the
input.
Amplifiers with negative feedback dependless and less on the open-loop gain and
finally depend only on the properties of the
values of the components in the feedbacknetwork.
The system gives up excessive gain to
improve predictability and reliability.
-
7/29/2019 Expt 3 Op-Amps
10/27
Typical Op Amp Circuit
V+ and V- power the op-amp
Vin is the input voltage signal
R2 is the feedback impedance
R1 is the input impedance
Rload is the load U2
uA741
+3
-2
V+
7
V-
4
OUT6
OS11
OS25
R1
1kRload
1k
V+ 9Vdc
V-
-9Vdc
0
0
0
0
0
Vout
R2 10k
Vin
FREQ = 1k
VA MPL = .2
VOFF = 0
V
V
-
7/29/2019 Expt 3 Op-Amps
11/27
in
f
in
in
f
outR
RAV
R
RV
1.The Inverting Amplifier
-
7/29/2019 Expt 3 Op-Amps
12/27
g
f
ing
f
out
R
RA
VR
R
V
1
1
2.The Non-Inverting Amplifier
-
7/29/2019 Expt 3 Op-Amps
13/27
Op-Amp Analysis
We assume we have an ideal op-amp:
infinite input impedance (no current at inputs) zero output impedance (no internal voltage losses)
infinite intrinsic gain
instantaneous time response
-
7/29/2019 Expt 3 Op-Amps
14/27
Golden Rules of Op-Amp Analysis
Rule 1: VA = VB The output attempts to do whatever is necessary tomake the voltage difference between the inputs zero.
The op-amp looks at its input terminals and swings
its output terminal around so that the externalfeedback network brings the input differential to zero.
Rule 2: IA = IB = 0
The inputs draw no current
The inputs are connected to what is essentially an
open circuit
-
7/29/2019 Expt 3 Op-Amps
15/27
1) Remove the op-amp from the circuit and
draw two circuits (one for the + and one for
theinput terminals of the op amp).
2) Write equations for the two circuits.
3) Simplify the equations using the rules for opamp analysis and solve for Vout/Vin
Steps in Analyzing Op-Amp Circuits
Why can the op-amp be removed from the circuit?
There is no input current, so the connections at the
inputs are open circuits.
The output acts like a new source. We can replace it
by a source with a voltage equal to Vout.
-
7/29/2019 Expt 3 Op-Amps
16/27
Analyzing the Inverting Amplifier
inverting input (-):
non-inverting input (+):
1)
-
7/29/2019 Expt 3 Op-Amps
17/27
:
:)1
Inverting Amplifier Analysis
in
f
in
out
f
out
in
inBA
A
f
outB
in
Bin
R
R
V
V
R
V
R
VVV
V
R
VV
R
VV
R
Vi
0)3
0:
:)2
-
7/29/2019 Expt 3 Op-Amps
18/27
Analysis of Non-Inverting Amplifier
g
f
in
out
g
gf
in
out
out
gf
g
inBA
out
gf
g
B
inA
R
R
V
V
R
RR
V
V
VRR
RVVV
V
RR
RV
VV
1
)3
:
:)2
Note that step 2 uses a voltage
divider to find the voltage at VB
relative to the output voltage.
:
:)1
-
7/29/2019 Expt 3 Op-Amps
19/27
inoutBA
inBoutA
VVthereforeVV
VVVVanalysis
,
]2]1:
1
in
out
V
V
3.The Voltage Follower
-
7/29/2019 Expt 3 Op-Amps
20/27
-
7/29/2019 Expt 3 Op-Amps
21/27
We can use a voltage follower to convert this realvoltage source into an ideal voltage source.
The power now comes from the +/- 15 volts to the
op amp and the load will not affect the output.
-
7/29/2019 Expt 3 Op-Amps
22/27
5.Adders
121
21
2
2
1
1
R
R
VV
VthenRRif
RV
RVRV
fout
fout
-
7/29/2019 Expt 3 Op-Amps
23/27
Weighted Adders
Unlike differential amplifiers, adders are
also useful when R1 R2.
This is called a Weighted Adder A weighted adder allows you to combine
several different signals with a different
gain on each input.
You can use weighted adders to build audio
mixers and digital-to-analog converters.
-
7/29/2019 Expt 3 Op-Amps
24/27
Analysis of weighted adder
I2
I1
If
2
2
1
1
2
2
1
1
2
2
1
1
2
22
1
1121
0
R
V
R
VRV
R
V
R
V
R
V
VVR
VV
R
VV
R
VV
R
VVI
R
VVI
R
VVIIII
fout
f
out
BA
AA
f
outA
f
outAf
AAf
-
7/29/2019 Expt 3 Op-Amps
25/27
6.Differential (or Difference) Amplifier
in
f
in
f
outR
RAVV
R
RV
)( 12
-
7/29/2019 Expt 3 Op-Amps
26/27
Analysis of Difference Amplifier(1)
:
:)1
-
7/29/2019 Expt 3 Op-Amps
27/27
in
fout
outinffout
inf
in
inf
f
fin
f
BA
out
inf
in
inf
fB
fin
inf
fin
outinf
B
fin
f
out
inBB
fin
f
A
f
outB
in
B
R
R
VV
V
VRVRVRVRR
RV
RR
RV
RR
RVV
VRR
RVRR
RV
RR
RRRR
VRVR
V
RR
R
V
R
V
VVforsolve
VRR
RV
R
VV
R
VV
i
12
1212
1
11
2
1
:
11:)3
:
:)2
Analysis of Difference Amplifier(2)
Note that step 2(-) here is very much
like step 2(-) for the inverting amplifier
and step 2(+) uses a voltage divider.