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Transcript
Subject Name: LINEAR IC’s AND APPLICATIONS Subject Code:10EC46 Prepared By: Aparna.P / MP Priyadarshini Department: Electronics and Communication Date:14-5-2015 5/11/2017 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. 5/11/2017 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°. 5/11/2017 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. 5/11/2017 Frequency compensation • • • • 5/11/2017 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 • • • 5/11/2017 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 • • 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. 5/11/2017 Upper cutoff Frequency • 5/11/2017 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 5/11/2017 Zin mode compensation 5/11/2017