OP AMP1
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Transcript of OP AMP1
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OP AMP
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Objectives of lecturer
• Describe how an ideal OP AMP
behaves
• Characteristics, Define voltage
gain
• Explain operation of op amp in
negatif feedback
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• Op amp has high-power and low power
device with a maximum power rating of
less than a watt.
• Some op amps are optimized for their
bandwidth, other for low input offsets,
others for low noise and so on.
• This is why the variety of commercially
available op amps is so large.
• Op amps use to the most basic active
component in analog systems.
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The following are the important properties common to
all operational amplifiers (OP-amps):
1. An operational amplifier is a multistage amplifier. The
input stage of an OP-amp is a differential amplifier stage.
An inverting input and a noninverting input.
2. A high input impedance (usually assumed infinite) at both
inputs.
3. A low output impedance (< 200 Ω).
4. A large open-loop voltage gain, typically 105.
5. The voltage gain remains constant over a wide frequency
range.
6. Very large CMRR (> 90 dB).
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Schematic Symbol of Operational Amplifier
• The basic operational amplifier has five terminals: two
terminals for supply voltages +V and –V; two input terminals
(inverting input and noninverting input) and one output
terminal.
• A signal applied to plus terminal will appear in the same
phase at the output as at the input. A signal applied to the
minus terminal will be shifted in phase 180° at the output
• In most cases, if only one input is required for an OP-amp
circuit, the input not in use will be shown connected to
ground. A single-input OP-amp is generally classified as
either inverting or noninverting.
• The OP-amp is produced as an integrated circuit (IC).
Because of the complexity of the internal circuitry of an OP-
amp, the OP-amp symbol is used exclusively in circuit
diagrams.
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Symbols for Op Amps
OpAmp uA741
LM111 LM324
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Terminal on an Op Amp
Non-inverting Input terminal
Inverting input terminal
Output terminal
Positive power supply (Positive rail)
Negative power supply (Negative rail)
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Op Amp Equivalent Circuit
vd = v2 – v1
A is the open-loop voltage gain v2
v1 Voltage controlled voltage source
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Output Voltage From OP-Amp
1.Voltage gain of OP-amp. • The maximum voltage gain of given OP-amp is AOL. Its value
is generally greater than 10,000.
• The actual gain (ACL) of an OP-amplifier is reduced when
negative feedback path exists between output and input.
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Output Voltage From OP-Amp
2. OP-Amp Input/Output Polarity Relationship
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3. Supply Voltages.
Output Voltage From OP-Amp
The supply voltages for an OP-amp are normally equal in
magnitude and opposite in sign e.g., ± 15V, ± 12V, ± 18V.
These supply voltages determine the limits of output voltage of
OP-amp. These limits, known as saturation voltages, are
generally given by;
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Characteristics of Op Amp
The ideal op amp represents a perfect voltage amplifier an is
often referred to as a Voltage-Controlled Voltage Source
(VCVS).
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741 • Is a standard and a classic op amp that has been available since the
1960s.
• In 1965 Fairchild semiconductor introduction the µA709 and improved
with µA741.
• Other 741 designs have appeared from various maufacturers,
motorola MC1741, National Semiconductor LM741 and so on.
• When no feedback path (or loop) is used, the voltage gain is
maximum and is called the open-loop voltage gain, designated Avol
• 741 has voltage gain so high, it becomes the inputs offsets can easily
saturate the op amp.
• This is why practical circuits need external components between the
input and output of an op amp to stabilize the voltage gain.
• For instance, in many applications negative feedback is used to adjust
the overall voltage gain to a much lower value in exchange for stable
linear operation..
• For instance, an input as small as 10 mikro Volt produces an output of
1 V.
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Frequency Gain Respone
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Bias and Offset • Op amp has input bias and offsets that produce an output error when there is
no input signal.
• In many application the output error signal is small enough to ignore. But
when the output error cannot be ignored, a designer can reduce it by using
equal base resistor. This elimetes the problem of bias current, but not the
offset curent or offset voltage.
• This is why it is best to elimate output error by using the nulling circuits given
on the datasheet.
• To neutralize the effect of input bias current (80 nA) flowing through this
source resistance, a discrete resistor of equal value is added to non inverting
input.
• To elimate the effect of an input offset
current of 20 nA an an input offset voltage
of 2 mV, the data sheet of 741C
recommends using a 10 k ohm
potensiometer between pins 1 and 5. by
adjusting this potensiometer with no input
signal, we can null or zero the output
voltage.
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CMRR
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Slew Rate
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Slew Rate • The slew rate of an OP-amp is a measure of how fast the
output voltage can change and is measured in volts per
microsecond (V/µs)
• If the slew rate of an OP-amp is 0.5V/µs, it means that the
output from the amplifier can change by 0.5 V every μs
• Since frequency is a function of time, the slew rate can be used
to determine the maximum operating frequency of the OP-amp
as follows
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the important points regarding the frequency response of
an OP-amp :
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Exercise
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Exercise Determine the maximum operating frequency for the circuit if the slew rate is 0.5 V/µs.
Solution : The maximum peak output voltage(Vpk) is approximately 8V. Therefore, maximum operating frequency (fmax) is given by;
= 9.95 kHz
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How to find these value
• Component datasheet
Many manufaturers have made these component freely available on the internet
Example LM 324 Op Amp
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dB
• Decibels
Since P = V2/R
10 log (P/Pref) or 20 log (V/Vref)
In this case:
20 log (Vo/Vin) = 20 log (A) = 100
A = 105 = 100,000
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Large Signal Voltage Gain = A
• Typical
– A = 100 V/mV = 100V/0.001V = 100,000
• Minimum
– A = 25 V/mV = 25 V/0.001V = 25,000
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OP-Amp with Negative Feedback
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OP-Amp with Negative Feedback
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to be continued next section