Subject Name: LINEAR IC’s AND APPLICATIONS Subject Code:10EC46 Prepared By: Aparna.P / MP Priyadarshini Department: Electronics and Communication Date:14-5-2015 9/6/2018

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. 9/6/2018

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°. 9/6/2018

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. 9/6/2018

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. 9/6/2018

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. 9/6/2018

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. 9/6/2018

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. 9/6/2018

SLEW RATE   9/6/2018

Zin mode compensation 9/6/2018