Chapter 5portal.unimap.edu.my/portal/page/portal30/Lecture... · Electronic Devices and Circuit...
Transcript of Chapter 5portal.unimap.edu.my/portal/page/portal30/Lecture... · Electronic Devices and Circuit...
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Chapter 5
Operational Amplifiers
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Operational Amplifiers
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Ch.10 Summary
The Basic Op-Amp
Operational amplifier (Op-amp): A high gain differential
amplifier with a high input impedance (typically in M) and
low output impedance (less than 100).
Note the op-amp has two inputs and one output.
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Ch.10 Summary
Op-Amp Gain
Op-Amps can be connected in open-loop or closed-loop
configurations.
Open-loop: A configuration with no feedback from the op-amp output
back to its input. Op-amp open-loop gain typically exceeds 10,000.
Closed-loop: A configuration that has a negative feedback path from
the op-amp output back to its input. Negative feedback reduces the
gain and improves many characteristics of the op-amp.
• Closed-loop gain is always lower than open-loop gain.
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Ch.10 Summary
Inverting Op-Amp
The input signal is applied to
the inverting (–) input
The non-inverting input (+)
is grounded
The feedback resistor (Rf)
is connected from the output
to the negative (inverting)
input; providing negative
feedback.
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Ch.10 Summary
Inverting Op-Amp Gain
Gain is set using external resistors: Rf and R1
The negative sign denotes a 180 phase shift between input and output.
1R
R
V
VA f
i
ov
11
1
R
RA
RR
fv
f
Gain can be set to any value
by manipulating the values of
Rf and R1.
Unity gain (Av = 1):
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Ch.10 Summary
Common Op-Amp Circuits
Inverting amplifier
Noninverting amplifier
Unity follower
Summing amplifier
Integrator
Differentiator
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Ch.10 Summary
Inverting/Noninverting Amplifiers
1
1
VR
RV f
o
Inverting Amplifier Noninverting Amplifier
1
1
)1( VR
RV f
o
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Ch.10 Summary
Unity Follower
1VVo
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Ch.10 Summary
Summing Amplifier
Because the op-amp
has a high input
impedance, the
multiple inputs are
treated as separate
inputs.
3
3
2
2
1
1
VR
RV
R
RV
R
RV fff
o
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Ch.10 Summary
Integrator
The output is the
integral of the input;
i.e., proportional to the
area under the input
waveform. This circuit
is useful in low-pass
filter circuits and sensor
conditioning circuits.
dttvRC
tvo )(1
)( 1
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Ch.10 Summary
Differentiator
The differentiator
takes the derivative
of the input. This
circuit is useful in
high-pass filter
circuits.
dt
tdvRCtvo
)()( 1
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Ch.10 Summary
DC-Offset Parameters
Input offset voltage
Input offset current
Input offset voltage and input offset current
Input bias current
Even when the input voltage is zero, an op-amp can have an
output offset. The following can cause this offset:
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Ch.10 Summary
Input Offset Voltage (VIO)
The specification sheet for an op-amp indicates
an input offset voltage (VIO).
The effect of this input offset voltage on the
output can be calculated with
1
1
R
RRVV f
IOo(offset)
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Ch.10 Summary
Input Offset Current (IIO)
The input offset current (IIO) is specified in the specifications for
an op-amp.
The effect of IIO on the output offset voltage can be calculated
using:
)()()( IOIO to Ioffset dueo to Voffset dueooffset o V V V
If there is a difference between the dc bias currents generated by
the same applied input, this also causes an output offset voltage:
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Ch.10 Summary
Total Offset Due to VIO and IIO
Op-amps may have an output offset voltage due to VIO
and IIO. The total output offset voltage equals the sum of
the effects of both:
)()()( IOoIOoo to Ioffset dueV to Voffset dueVoffsetV
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Ch.10 Summary
Input Bias Current (IIB)
A parameter that is related to input offset current (IIO) is called
input bias current (IIB)
The input bias currents are calculated using:
The total input bias current is the average of the
two:
2IO
IBIB
III
2IO
IBIB
I II
2
IBIB
IB
III
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Ch.10 Summary
Frequency Parameters
An op-amp is a wide-bandwidth amplifier. The
following factors affect the bandwidth of the op-
amp:
Gain
Slew rate
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Ch.10 Summary
Gain and Bandwidth
The op-amp’s high
frequency response is
limited by its internal
circuitry. The plot shown
is for an open loop gain
(AOL or AVD). This means
that the op-amp is
operating at the highest
possible gain with no
feedback resistor.
In the open loop mode, an op-amp has a narrow bandwidth. The
bandwidth widens in closed-loop mode, but the gain is lower.
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Ch.10 Summary
Slew Rate (SR)
Slew rate (SR): The maximum rate at which an
op-amp can change output without distortion.
The SR rating is listed in the specification sheets
as the V/s rating.
s) V/(in Δt
ΔVSR o
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Ch.10 Summary
Maximum Signal Frequency
The slew rate determines the highest
frequency of the op-amp without distortion.
where VP is the peak voltage
pπV
SRf
2
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Ch.10 Summary
General Op-Amp Specifications
Other op-amp ratings found on specification
sheets are:
Absolute Ratings
Electrical Characteristics
Performance
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Ch.10 Summary
Absolute Ratings
These are
common
maximum ratings
for the op-amp.
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Ch.10 Summary
Electrical Characteristics
Note: These ratings are for specific circuit conditions, and they often
include minimum, maximum and typical values.
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Ch.10 Summary
CMRR
One rating that is unique to op-amps is CMRR or common-mode
rejection ratio.
Because the op-amp has two inputs that are opposite in phase
(inverting input and the non-inverting input) any signal that is
common to both inputs will be cancelled.
Op-amp CMRR is a measure of the ability to cancel out common-
mode signals.
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Ch.10 Summary
Op-Amp Performance
The specification sheets
will also include graphs
that indicate the
performance of the op-
amp over a wide range of
conditions.
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Op-Amp Applications
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Ch.11 Summary
Common Op-Amp Applications
Constant-gain amplifier
Voltage summing
Voltage buffer
Controlled sources
Instrumentation circuits
Active filters
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Ch.11 Summary
Constant-Gain Amplifier
Inverting amplifier
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Ch.11 Summary
Constant-Gain Amplifier
Noninverting amplifierNoninverting amplifier
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Ch.11 Summary
Multiple-Stage Gains
The total gain (3-stages) is given by:321 AAAA
or:
321
1R
R
R
R
R
RA fff
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Ch.11 Summary
Voltage Summing
3
3
2
2
1
1
VR
RV
R
RV
R
RV fff
o
The output is the sum
of individual signals
times the gain:
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Ch.11 Summary
Voltage Buffer
The unity gain amplifier shown
is commonly referred to as a
voltage buffer or a voltage
follower.
Any amplifier with no gain or loss is called a unity gain
amplifier.
The advantages of using a unity gain amplifier:
• Very high input impedance
• Very low output impedance
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Ch.11 Summary
Instrumentation Circuits
Some examples of instrumentation circuits using op-amps:
Display driver
Instrumentation amplifier