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Subject Name: LINEAR IC’s AND APPLICATIONS
Subject Code:10EC46
Prepared By: Aparna.P / MP Priyadarshini
Department: Electronics and Communication
Date:14-5-2015
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Circuit Stability
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The signal voltage V s is amplified by a
factor of R2/R1 and phase shifted through
-180 for an inverting amplifier..
Output is divided by the feedback factor to
produce feedback voltage Vf.
Because of the feedback, every operational
amplifier is a potential oscillator.
To prevent oscillation external
components are added .
Such networks are referred to as frequency
compensation networks.
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Single stage amplifier
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For the single stage amplifier , the voltage
gain decreases at high frequency due to
the construction of transistor or the stray
capacitance.
From the frequency response curve, the
gain decreases by -20dB/decade after the
pole frequency fp.
The phase shifts of each stage also added
together to give the total phase shift for the
opamp which is additional to the normal 180 ° phase shift that occurs for an opamp.
The difference between 360° and the
actual phase shift at Mβ = 1 is referred to
as phase margin .
For stability, minimum phase margin
should be equal to 45°.
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Operational amplifier
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Each of the three stages of the op amp has
its own particular upper cut off frequency
with a fall of rate of 6dB/octave .
Also each stage has a maximum phase
shift of -90°.
The three responses are combined together
to give straight line approximation of
gain/frequency and phase/frequency
response of the opamp as shown in the
figure.
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Frequency compensation
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Phase lag and phase lead are the two most used
frequency compensation networks,
From fig , at lower frequencies v2 lags by v1 since
Xc1>>R2 .
At higher frequencies the circuit reduces to a resitive
network and it introduces some attenuation.
A phase lag network stabilizes an amplifier by reducing
the open loop gain, so that frequency at which Mβ = 1
occurs when the phase shift is too small to cause
instability.
Frequency compensation
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From the figure its noted that CR network introduces a
phase lead.
The phase lead network is used to cancel out the
unwanted phase lag in the opamp.
This increase the phase margin at Mβ = 1 as well as the
stability of the circuit.
Frequency compensation
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A small capacitance connected between
the input and output terminals of an
inverting amplifier stage behaves as a
much larger in put capacitance because of
the miller effect.(capacitance
amplification).
This can be used in the lag or lead network
to provide compensation for the op amp.
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Upper cutoff Frequency
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The frequency at which the closed loop gain falls to 3dB below the normal
mid gain frequency ie the frequency at which M=Av implies that open
loop phase shift is always 90.
SLEW RATE
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Zin mode compensation
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