1. Subject Name: LINEAR IC’s AND APPLICATIONS Subject Code:10EC46 Prepared By: Aparna.P / MP Priyadarshini Department: Electronics and Communication Date:
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2. Circuit Stability
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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3. Single stage amplifier
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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4. Operational amplifier
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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5. Frequency compensation
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.
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6. Frequency compensation
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.
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7. Frequency compensation
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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8. Upper cutoff Frequency
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.
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9. SLEW RATE
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10. Zin mode compensation
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