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The shaded area shows the consistency
Op-Amp Imperfections in The Linear Range of Operations
Gain and Bandwidth Limitations
 Ideal op amps have infinite open-loop gain magnitude (AoL is infinite), but the gain
of a real op amp is finite and a function of frequency
 dc open-circuit differential voltage gain is typically between 104 to 106
 The bandwidth is usually limited by the designer to prevent oscillations from
feedback, by a process is called frequency compensation
The open loop gain function
of an op-amp usually has a
single dominant pole and is
given as:
 AOL (f)
 A0OL
 fBOL
 AOL (f)
- open-loop gain as function of frequency
- dc open-loop gain
- open-loop break frequency
- constant up to fBOL then it rolls off at 20
dB/decade
 The product of gain A0OL and FBOL is ft, and is called
the gain-bandwidth product
ft is defined as the frequency where the gain of the
amplifier becomes 1
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Gain-Bandwidth Limitations
Assuming infinite input impedance
and zero input current
Voltage across resistor R1
(2.26)
Definition of open-loop gain
Hence from (2.27)
Thus,
Therefore, the closed-loop gain
and
(2.27)
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For an ideal op amp
But
In the limit of AOL tending to infinity,
This is the same result as before
Op amp Closed-loop gain is given as,
Putting
We have,
Defining
we have
and
which is very similar in form to the open loop gain
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Closed-Loop Bandwidth


Substituting,
fBCL A0CL = fBOL A0OL= ft  Gain Bandwidth product is constant
But we know that
and
Therefore
This same formula applies to a non-inverting as well as an inverting amplifier
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Gain Bandwidth Product
Note that you are trading off the high gain of the op-amp for a higher bandwidth
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Closed-Loop Gain Versus Frequency
b
A0CL
A0CL
(dB)
fBCL
1
0.999990
0
4 MHz
9.9990
20
400 kHz
40
40 kHz
0.1
0.01 99.90
Not that at ft the gain becomes 1
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Large Signal Operation
Output Voltage Swing
Output voltage cannot
exceed the maximum
supply voltage. It also
depends on the type of opamp design to some
extent.
Output Current Limits
Clipping of the output voltage occurs when either the output voltage or the output current
reaches its limit
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Example on output voltage clipping
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