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Файл:Basics of electronics. Study aid
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Applications: Battery-powered (3/5 V) products
Test equipment
Communication
Industrial controls
Automotive sensors
3. High Speed
Bandwidth BW > 50 MHz
Slew rate > 100 V/s
These devices are designed for high frequency applications.
Applications: Video and Imaging
Military/Aerospace
Wireless/wired communications
4. Low Power
Supply Current IS < 0.5 mA/op-amp
These op-amps optimized for low power consumption and can operate
at low power-supply voltages (i. e., ±1.5V d.c.)
5. Micropower
Supply Current IS < 200 A
These op-amps are used for low power consumption.
6. High Output Power
Output current I
>100 mA
out
Devices that operate at high d.c. power supply voltages.
Applications: High Voltage Instrumentation
Programmable Power Supplies
Electrostatic Transducers and Deflection
Audio Amplifiers and Servo Drivers
7. Low Noise
Low noise op-amps provide voltage noise densities below 5 nV/rtHz, yet
still offer high speed, precision, low power and tiny packages. Low noise opamps offer tested, guaranteed limits over the full operating temperature range
for voltage noise (both wideband and low frequency), current noise and more.
8. Buffers
These op-amps have internal 100 % negative feedback. Their voltage
gain is very close to unity.
9. Comparators
These are devices that have no negative feedback networks and
therefore are saturated with very low (V) input signal voltages. Comparators
are used to compare input signal levels.
101
9.2. Basic Op-Amp Circuit Applications
As we have an understanding of the op-amp’s characteristics, let us put
it to use in some basic circuit applications. To begin with, we will examine
the comparator circuit.
The Open-Loop Comparator Circuit
Figure 9.12 shows how the op-amp can be used as a comparator, which
is a circuit that is used to detect changes in voltage level. In fig. 9.12(a), the
noninverting input (+) of the op-amp is grounded and the input signal is
applied to the op-amp’s inverting input (–). Referring to the associated
waveforms, you can see that when the input swings positive in relation to the
positive input (which is zero), the output of the amplifier goes into the
immediate saturation due to a very large gain of the op-amp.
+V
cc
+V
cc
V
out
V
V
R1
ref
R2
+V
cc
V
in
in
V
out
-V
cc
-V
cc
(a) (b)
Fig. 9.12. Open-loop comparator circuits: (a) zero referenced (b) with a positive
reference voltage
For example, if the op-amp had a voltage gain of 25,000 (Av = 25,000),
even a small input of –25 mV would make the op-amp drive its output to
625 V (|V
| = Vin Av = 25 mV 25,000 = 625 V). Since the maximum
out
possible positive output voltage cannot exceed the positive supply voltage
(+V), the output goes to its maximum positive limit, which is equal to the +V
supply voltage. When the input swings positive, the amplifier is driven
immediately into its opposite state (cutoff), and the output goes to its
maximum negative limit, which is equal to the –V supply voltage.
Figure 9.12(b) shows how a voltage divider, made up of R
and R2, can
1
be used to supply the inverting input (–) of the op-amp with a reference
voltage (V
) that can be determined by using the voltage-divider formula.
ref
R
VV
ref CC
2
RR
12
.
102
V
m
V
in
V
m
V
in
V
ref
V
V
-V
sat
sat
0
t
m
V
out
+
0
t
-
(a) (b)
Fig. 9.13. Comparator circuit waveforms
V
V
-V
sat
sat
0
t
m
V
out
+
0
t
-
Referring to the associated waveforms in fig. 9.13(b), you can see that
whenever the a.c. input signal is more positive than the reference voltage, the
output is positive. On the other hand, whenever the a.c. input signal is less
than the reference voltage, the output is negative.
9.2.1. The Inverting Amplifier
The op-amp is usually operated in either the open-loop mode or closed-
loop mode. With the previously discussed comparator circuit, the op-amp
was operating in its open-loop mode because there was no signal feedback
from output to input. In most instances, the op-amp is operated in the closedloop mode, in which there is signal feedback from output back to input. This
feedback signal is always out of phase with the input signal and therefore
opposes the original signal, and thus it is called the ‘degenerative or negative
feedback’. The negative feedback, however, is necessary in nearly all op-amp
circuits for the following reasons:
1. As the op-amp has such an extremely high gain, even a very small
input signal will be amplified to a very large signal, which drives the op-amp
out of its linear region and into saturation and cutoff. The negative feedback
lowers the op-amp’s gain, and therefore controls the op-amp to prevent the
output waveform distortion.
2. A high gain can make the amplifier go into the oscillation due to the
positive feedback. The negative feedback prevents an amplifier from going
into the oscillation by reducing the op-amp’s gain.
103
3. The open-loop gain of an op-amp can have a very large range of
values for the same device. For example, the 741’s open-loop gain can be
anywhere from a minimum of 25,000 to 200,000. The negative feedback in
the op-amp circuit will reduce the gain to a consistent value so that the same
part can be used to provide the same response.
R2
I
FB
R1
I
V
in
in
I
out
I
load
V
R
out
L
Fig. 9.14. Op-amp inverting amplifier circuit
Figure 9.14 shows how an op-amp can be connected as an inverting
amplifier circuit, which produces an amplified output signal that is 180° out
of phase with the input signal. Looking at the output voltage sign (–V
notice that the negative symbol preceding V
is used to indicate the 180°
out
out
),
phase inversion between input and output. In this circuit arrangement, the
input signal (Vin) is applied through the input resistor (R1) to the inverting
input (–) of the op-amp, while the noninverting input (+) is connected to
ground. A feedback loop is connected from the output back to the inverting
input via the feed-back resistor R2.
Take a closer look at the closed-loop feedback system that occurs within
this amplifier circuit. If the applied input voltage is zero volts (Vin = 0 V), the
differential input signal (which is the difference between the op-amp’s ‘+’ and
‘–’ inputs) is 0 V, because both the inverting and noninverting inputs are at 0 V.
A differential input of zero volts therefore will generate an output of zero volts.
If the input signal goes positive toward +5 V, the output (V
) swings
out
negative due to the internal op-amp circuit phase inversions. This negative
output voltage swing is applied back to the inverting input via R2 to
counteract the original positive input change.
If a small voltage, measured at the inverting input with respect to the
noninverting input, is assumed to exist, the amplifier output voltage will be of
opposite polarity and can always increase in value (with infinite output
available) until the voltage between the inputs becomes infinitesimally small.
When the amplifier output is fed back to the inverting input, the output
voltage always takes on the value required to drive the signal between the
inputs toward zero.
104
The two summing point restraints are so important that they are called
‘Golden Rules’:
1. No current flows into either input terminal of the ideal operational
amplifier.
2. When the negative feedback is applied around the ideal operational
amplifier, the differential input voltage approaches zero.
Then the input current Iin flows through R1 in the direction shown in
fig. 9.14 and, because the input bias current of op-amp is equal to zero, I
also flows through R
.
2
The inverting input voltage is ‘virtually’ zero, because the voltage
between the inputs is zero.
The voltage drop across R
V
is equal to the input voltage.
1
We can find current Iin:
V
With
0V,
VIRIR R
out FB in
I
22 2
in
.
in
R
1
V
in
.
R
1
Then the gain of the inverting amplifier can be calculated with
in
VR
G
out
VR
in
2
,
1
and we can see that the value of G is independent of the open-loop voltage
gain A (when it is very large).
This circuit is simply an inverting amplifier but it is sometimes called a
scale changer or simply, a scaler.
R1 R2
I
cc
cc
I
out
FB
I
load
V
R
out
L
V
V
cc1
cc2
V
in
+V
0V
-V
Fig. 9.15. Positive input voltage is applied to the inverting amplifier
105
R1 R2
I
cc
cc
I
out
FB
I
load
V
R
out
L
V
V
cc1
cc2
V
in
virtually
zero
0V
Fig. 9.16. Negative input voltage is applied to the inverting amplifier
9.2.2. The Noninverting Amplifier
+V
-V
Figure 9.17 shows how an op-amp can be connected as a noninverting
amplifier circuit, which produces an amplified output signal without the
phase inversion. We can see that the input voltage (Vin) is applied to the opamp’s noninverting input (+), and therefore the output voltage (V
) is in
out
phase with the input. To achieve negative feedback, the output is applied
back to the inverting input (–) of the op-amp via the feedback network
formed by R2 and R1.
R1 R2
I
FB
I
load
V
R
out
L
) is
out
I
in
V
in
Fig. 9.17. Noninverting amplifier circuit
I
out
Now let us take a closer look at the closed-loop negative feedback
system that occurs within this amplifier circuit. The output voltage (V
proportionally divided across R2 and R1, with the feedback voltage (VR1)
developed across R1, being applied to the inverting input of the op-amp, as
shown in fig. 9.17. As V
is in phase with Vin, the feedback voltage (VR1) is
out
also in phase with Vin, and therefore these two in-phase inputs to the op-amp
106
are common-mode input signals. As a result, feedback will be degenerative.
R
However, because V
difference between V
is slightly larger than Vin, there will be a small
out
and VR1, and this differential input will be amplified.
in
To summarize, the noninverting amplifier provides negative feedback by
feeding back an in-phase common-mode signal, and this degenerative
feedback decreases the op-amp’s gain to
prevent the output waveform distortion,
prevent the amplifier oscillation, and
reduce the gain of the op-amp to a consistent value.
A very small microvolt difference between Vin and VR1 is amplified
fig. 9.17); however, as there is such a very small difference between
(see
these two input signals, it can be said that
`
VV
in R
.
1
Because the closed-loop voltage gain of any op-amp is equal to
the gain of the noninverting amplifier can also be calculated with
V
out
V
.
1
R
G
By using the voltage-divider formula, we can develop a formula for
calculating V
R1
.
R
VV
1
1
RR
12
out
.
By rearranging the above formula as follows:
and since
VV G
1out R
, the noninverting amplifier’s gain can also be
VRR R
out
VR R
11 1
R
12 2
calculated with the formula
R
2
R
.
1
G
1
G
V
out
V
in
1
As the output voltage of an amplifier is equal to the product of input
voltage and gain (
VGV
out in
), we can add Vin to the previous closed-loop
gain formula in order to calculate output voltage.
R
VV
out in
2
1.
R
1
107
R1 R2
I
cc
cc
I
FB
out
I
load
V
R
out
L
V
V
cc1
cc2
V
+V
in
-V
Fig. 9.18. Positive input voltage is applied to the noninverting amplifier
R1 R2
I
cc
I
FB
out
I
load
V
R
out
L
V
V
cc1
cc2
V
+V
in
-V
cc
Fig. 9.19. Negative input voltage is applied to the noninverting amplifier
With the inverting amplifier, the input impedance is determined by the
input resistor (R
connected because V
). With the noninverting amplifier, there is no resistor
in
is applied directly into the very high input impedance
in
of the op-amp. As a result, the noninverting amplifier circuit has extremely
high input impedance.
The open-loop comparator and closed-loop inverting and noninverting
amplifier circuits are just three of many op-amp application circuits. In the
next section of this chapter, we will examine many other typical op-amp
circuit applications.
Additional Op-Amp Circuit Applications
The operational amplifier flexibility and characteristics make it the ideal
choice for a wide variety of circuit applications. In fact, as the op-amp is the
most frequently used linear IC, it is safe to say that you will find several opamps in almost every electronic system. In this section we will concentrate
108
on the operation and characteristics of all of the most frequently used op-amp
circuit applications.
The Voltage-Follower Circuit
Figure 9.20 shows how an op-amp can be connected to form a
noninverting voltage-follower circuit. Using the noninverting closed-loop
gain formula discussed previously, we can calculate the voltage gain of this
circuit.
The gain of the noninverting circuit of fig. 9.17 is
VR
G
out
VR
in
2
1
1
and output and input are in phase.
If R
= 0 or/and R1 = , we will have
2
In this case the circuit is called a
0
111.
G
R
1
R
2
voltage follower or buffer. The circuit
is shown in fig. 9.20.
V
out
V
in
Fig. 9.20. Voltage follower or buffer
General characteristics of buffer: voltage gain = 1;
input impedance
output impedance
;
0;
current gain
power gain
;
.
With a gain of unity, the output voltage is equal to the input voltage – so
what is the advantage of this circuit? The answer is the op-amp
characteristics of a high input impedance and a low output impedance.
Similar to the BJT’s emitter-follower and the FET’s source-follower, the opamp voltage-follower circuit derives its name from the fact that the output
voltage follows the input voltage in both polarity and amplitude. This circuit
is therefore ideal as a buffer, interfacing a high-impedance source to a lowimpedance load.
109
9.2.3. The Summing Amplifier Circuits
Inverting Adder
The summing amplifier circuit, or adder amplifier, consists of two or
more input resistors connected to the inverting input of an op-amp as shown
in fig. 9.21. This circuit sums or adds all of the input voltages, and therefore
the output voltage is:
R1
V1
R2
R4
V2
R3
V3
V
out
Fig. 9.21. Three inputs inverting adder
Using Ohm’s law (V = R · I), we can also calculate the input current
with the formula
V
V
i
R
1
1
2
,
R
1
0
V
V
;
i
2
3
3
R
3
as inverting input voltage is ‘virtually’ zero ( ).
In the same way
i
2
and
iiii
4123
,
as input bias current is zero.
Since the inverting input is the node where all the currents from the
inputs add together to provide the current through R
, this node is sometimes
4
called the summing junction.
With
Therefore
0V,
VVVV
out
RRR
RR R
If all resistors are equal, then
ViR
44 4
12 3
.
4out
123
VVVV
123out
110
.
.
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