ENGI242/ELEC 222 Op Amps
April 2004
Introduction to Op Amps
ENGI 242
ELEC 222
Basic Op-Amp
The op-amp is a differential amplifier with a
very high open loop gain 25k ≤ AVOL ≤ 500k (much higher for FET inputs)
high input impedance
500kΩ ≤ ZIN ≤ 10MΩ
low output impedance
25Ω ≤ RO ≤ 100Ω
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Op-Amp Equivalent Circuit
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Op-Amp Specifications – DC Offset Parameters
• Even though the input voltage is 0, there will be an output.
This is called offset. The following can cause this offset:
– Input Offset Voltage
– Output Offset Voltage due to Input Offset Current
– Total Offset Voltage Due to Input Offset Voltage and Input Offset
Current
– Input Bias Current
•
See lm301.pdf or mc1741c.pdf for sample specification sheets
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General Op-Amp Specifications VIO
• Input Offset Voltage VIO
– The voltage that must be applied to the input terminals of an op amp to
null the output voltage
– Typical value is 2mV with a max of 6mV
– When operated open loop, must be nulled or device may saturate
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General Op-Amp Specifications IIO
• Input Offset Current
– The algebraic difference between the two input currents
– These are base currents and are usually nulled
– Typical value IIO 20 nA with a max of 200nA
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Technique to Null VO
• Short Input terminals to ground
• Connect potentiometer between compensation pins with wiper to VEE
– Potentiometer is usually a 10 turn device
• Connect meter to output and adjust potentiometer for VO = 0
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General Op-Amp Specifications CMRR
AD =
ACM =
VO
V IN
V OCM
V CM
⎛ AD ⎞
CMRR = 20 log ⎜
⎟
⎝ ACM ⎠
• Common Mode Rejection Ratio
– The ratio of the differential voltage gain (AD) to the common mode gain
(ACM)
– ACM is the ratio between the differential input voltage (VINCM) applied
common mode, and the common mode output voltage (VOCM)
– it can exceed minimum is 70db with a typical value of 90 db
– in properly designed circuit, it may exceed 110db
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General Op-Amp Specifications
•
Input Bias Current
– The average of the currents that flow into the inverting and
noninverting terminals
– Typical values rage from 7nA to 80 nA
•
IB =
IB+ + IB2
Differential Input Resistance
– Also know as the input resistance
– Resistance seen looking into the input terminals of the device
– Runs from a low of 2MΩ for an LM741 to a high of 1012Ω for
FET input devices
•
Output resistance
– Resistance between the output terminal ad ground
– Typical values are 75Ω or less
•
Input Capacitance
– The equivalent capacitance measured at either the inverting or
noninverting terminal with the other terminal connected to
ground
– May not be on all spec sheets
– Typical value for LM741 is 1.4pF
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General Op-Amp Specifications
• Power Supply Range
– May be differential or single ended
– Max is ± 22V
• Output Voltage Swing
– Range of output voltage
– Depends on power supply voltage used (typically about 85% to 90%)
– Usually about ±13.5V for a power supply voltage of ±15V
• Slew Rate
– The maximum rate of change in the output voltage in response to an input
change
– Depends greatly on device, higher is better (output resonds faster to input
changes)
– For LM741 it is .5V/µs while for the LM318 it is 70V /µs
• Gain Bandwidth Product
– The bandwidth of the device when the open loop voltage gain is 1
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Op Amp Equivalent Circuit
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Op-Amp Gain
• Op-Amps have a very high gain. They can be
connected open- or closed loop.
• Open-loop (AVOL) refers to a configuration where
there is no feedback from output back to the input
• AVOL may exceed 10,000
• Closed-loop (AVCL) configuration reduces the gain
In order to control the gain of an op-amp it must
have negative feedback
• Negative feedback will reduce the gain and
improve many characteristics of the op-amp
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Typical Op Amp Frequency Response
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Change in AV with Feedback
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Virtual Ground
Since ZIN is very
high, we assume no
current can flow into
any lead of the op
amp
When the noninverting input pin is
at ground, the
inverting input pin is
at 0V
The equivalent circuit.
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Practical Op-Amp Circuits
Typical Op-amp circuit configurations include the:
• Unity Gain Buffer (Voltage Follower)
• Inverting Amplifier
• Noninverting Amplifier
• Summing Amplifier
• Integrator
• Differentiator
Note: the integrator and differentiator are considered active filters
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Unity Gain Buffer (Follower)
VO
V1
VO = V1
AV =
AV = 1
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Inverting Op Amp
The input is applied to the inverting (-) input
the non-inverting input (+) is grounded
RF is the feedback resistor, and is connected from the output to the
inverting input
This is called negative feedback
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Inverting Op Amp
We assume that no current
enters the inverting terminal
II- < 100nA
VD ≈ 0V
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VO
I IN R F
= VS
IIN R 1
RF
AV = R1
AV =
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Inverting Op-Amp Gain
Closed Loop Gain is controlled by the external resistors:
RF and R1
VO
IIN RF
AV =
= VS
IIN R1
RF
AV = R1
For Unity Gain: AV is -1 and RF = R1 AV = -
RF
= -1
R1
The minus sign denotes a 180 degree phase shift between input and output
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Inverting Op Amp Compensated for Ibias
R is used to compensate
for difference in IBIAS+
and IBIAS-
AV = April 2004
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RF
R1
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Inverting Op-Amp
A
This configuration achieves
high gain with a smaller
range of resistor values than
the basic inverter
V-
V+
R2 RF ⎞
⎛ R2 + RF
AV = - ⎜
+
⎟
R1
R1 R3 ⎠
⎝
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Inverting Amplifier with High Zin
Use a Unity Gain Buffer to obtain a very high input resistance with an
inverting amplifier
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Inverting Amplifier for Low RL
Use a Unity Gain Buffer to obtain a very high input resistance to
drive a low impedance load
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Noninverting Amplifier
R2 ⎞
⎛
VO = Vin ⎜ 1 +
⎟
R1
⎝
⎠
VO
R
2 ⎞
⎛
AV =
= ⎜1 +
⎟
Vin
R1 ⎠
⎝
V- = V+ = vi
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Noninverting Op Amp Compensated for IBIAS
Rbias is used to compensate for difference in IBIAS+ and IBIASApril 2004
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Differential (Difference) Amplifier
V1
V2
AV = -
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A
A
VO
=
V2 - V1
-
R2
R1
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Differential Amplifier Output
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Instrumentation Amplifier Buffered Input
AV = -
R1 = R2, RF1 = RF2
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RF
R1
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Instrumentation Amplifier
R1 = R2, RF1 = RF2
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RA ⎞
⎛ RF ⎞ ⎛
AV = - ⎜
⎟⎜1 + 2
⎟
1
R
RB ⎠
⎝
⎠⎝
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Inverting Summing Amplifier
By applying KCL to the
multiple inputs, we can
consider the contribution of
each source individually
IF + I- = I1 + I2 + I3
but I- W 0
∴IF = I1 + I2 + I3
VO = -IF RF
RF
⎛ RF
VO = - ⎜
V1 +
V2 +
R2
⎝ R1
V2
⎛ V1
VO = - RF ⎜
+
+
R2
⎝ R1
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RF
⎞
V3 ⎟
R3
⎠
V3 ⎞
⎟
R3 ⎠
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Non-inverting Summing Amplifier
Perform a source transformation
for each input
Sum the current sources and find
RTH for the resistances
VIN+ = IT RTH
V2
V3 ⎞
⎛ V1
VIN + = ⎜
+
+
⎟ RTH
R2
R3 ⎠
⎝ R1
where RTH = R1 // R2 // R3
⎛ VIN + ⎞
VO = ⎜
⎟ ( RIN + RF )
⎝ RIN ⎠
VO
RF ⎞
⎛
AV =
= ⎜1 +
⎟
VIN +
RIN ⎠
⎝
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Integrator
The output is the integral of
the input
This circuit is a low-pass
filter circuit, and is used and
sensor conditioning circuits
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vo(t) = −
1
v1(t)dt
RC ∫
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Differentiator
The differentiator takes the
derivative of the input
This circuit is a high-pass
filter circuits
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vo(t) = − RC
ENGI 242/ELEC 222 Op Amps
dv1(t)
dt
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Comparator
•
•
•
•
High Gain Op Amp
Operated Open Loop
Designed to compare an input to a reference voltage
Gives output (digital level) to indicate if input is above or below reference
– Circuit designed to give VOSAT and –VOSAT only
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Comparator Operation Example
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LM 311 Comparator
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Window Comparator
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Determine the Output
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Block Diagram of 555
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Astable Multivibrator
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555 Used as an Astable Multivibrator
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Schmidt Trigger 7414
• A Schmidt trigger (a comparator with Hysteresis) is a bistable digital (twostate) device
• It accepts virtually any analog input and provides a logic 0 or 1 output
– A typical use is to take distorted digital signals (due to RC time constant of
transmission line) and provide a used to square-wave output
– Can be used to eliminate noise near reference point that would cause problems
in analog comparators
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Hysteresis
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