UE22EC343AB3_Unit2_Consolidated.pdf

LINEAR INTEGRATED CIRCUITS Dr Shashidhar Tantry Electronics and Communication Engineering Unit 2 Simultaneous equations with m < 1 Linear op amp transfer function is limited to equation of straight line y = +/-mx+/-b Unit 2 Non inverting attenuator with zero offset, positive of...

LINEAR INTEGRATED CIRCUITS Dr Shashidhar Tantry Electronics and Communication Engineering Unit 2 Simultaneous equations with m < 1 Linear op amp transfer function is limited to equation of straight line y = +/-mx+/-b Unit 2 Non inverting attenuator with zero offset, positive offset and negative offset Zero offset Positive offset Negative offset Unit 2 Non inverting attenuator with zero offset, positive offset and negative offset Zero offset Positive offset Negative offset Unit 2 Inverting attenuator with zero offset, positive offset and negative offset Zero offset Positive offset Negative offset Unit 2 Inverting attenuator with zero offset, positive offset and negative offset Zero offset Positive offset Negative offset Unit 2 Development of non ideal op amp equations Concept of DC errors and AC errors Inaccuracies can be minimized using negative feedback Stability is usually an criteria when operating frequency is high Internally compensated and externally compensated to improve stability Unit 2 Development of non ideal op amp equations Error indicator, proportional to signal and inversely When loop gain is large, proportional to loop gain  Unit 2 Development of non ideal op amp equations Non Inverting amp From 1 and 2, After simplification, In the form of closed loop function, --------- 1 VB calculated based on resistor divider from VOUT --------- 2 Unit 2 Development of non ideal op amp equations Non Inverting amp For measurement, break the loop, apply test signal at one end and measure voltage at the other end Unit 2 Development of non ideal op amp equations Inverting amp Open loop gain is different compared to non inverting amp Loop gain is same compared to non inverting amp Unit 2 Development of non ideal op amp equations Inverting amp For measurement, break the loop, apply test signal at one end and measure voltage at the other end Unit 2 Development of non ideal op amp equations Differential amp Unit 2 Development of non ideal op amp equations Differential amp Loop gain is same compared to non inverting amp and inverting amp Unit 2 Practical aspects Unit 2 Practical aspects When open loop gain is high When open loop gain is high Unit 2 Practical aspects Unit 2 Practical aspects Unit 2 Voltage feedback op amp compensation Background Oscillations are considered as boundary between stability and non stability Poor stability circuit exhibits ringing and overshoot Phase margin is one measure for stability Compensation provides patch between stability and performance Compensation network is by RC network  Unit 2 Voltage feedback op amp compensation Internal compensation A capacitor C connected between input and output for compensation, called internal compensation capacitor Plot of internally compensated op amp Unit 2 Voltage feedback op amp compensation Internal compensation Plot of internally compensated op amp, measure of phase margin with damping ratio and overshoot (Data sheet for TL03X) Unit 2 Importance of external compensation High frequency noise reduction Improve phase margin Reduce overshoot Compensation can be tailored to the circuit requirement Unit 2 Dominant pole compensation Output capacitor added Combination of output capacitor and output impedance forms dominant pole Equivalent circuit after loop break Loop broken for loop gain calculations Unit 2 Dominant pole compensation Apply Thevenin’s theorem to separate Zo and CL Calculate Vreturn voltage Zth ZF Unit 2 Dominant pole compensation When (ZF+ZO) >> ZO Unit 2 Dominant pole compensation Loop gain is equal to Unit 2 Dominant pole compensation Phase margin around 45 degrees Unit 2 Dominant pole compensation Closed loop transfer function is given by Loop gain Which represents gain of non inverting amp Unit 2 Gain compensation Loop gain parameter and closed loop parameters are related Example, Change non inverting amp gain from 2 to 10 Loop gain will reduce by -14db Improvement observation from the Bode plot Unit 2 Lead compensation C is because of parasitic capacitance Break the loop for loop gain calculation Unit 2 Lead compensation Contribution of open loop op amp Unit 2 Lead compensation Zero placed near pole 2 RF has to larger compared to RG in parallel with RF  Unit 3 Lead compensation When a is infinity, transfer function can be seen as, Transfer function of inverting amp is given by Transfer function is given by ------10 ----12 ----11 Unit 2 Lead compensation Behavior on bode plot for 10, 11,12 Unit 2 Compensated attenuation Stray capacitance is added due to PCB trace This circuit is unstable because of three poles Loop gain Unit 2 Compensated attenuation Compensation capacitor is added parallel to feedback resistor Loop gain Unit 2 Compensated attenuation Compensation capacitor is added, acts like open loop gain two pole system Gain plot Unit 2 Compensated attenuation Closed loop gain of inverted amp does not change. Capacitor has not effect on gain Closed loop gain Unit 2 Lead lag compensation R and C used for compensation Compensation circuit adds Pole and Zero loop gain Unit 2 Lead lag compensation Gain plot Unit 2 Lead lag compensation Ideal closed loop gain calculated Unit 2 Lead lag compensation Ideal closed loop gain calculated Substitute Vth and Rth Unit 2 Lead lag compensation Unit 3 Comparison of compensation schemes Scheme Advantages Disadvantages Internal compensation No need for extra Under certain load component capacitance, it is unstable Dominant pole Suitable for high load Load capacitance make op compensation capacitance amp to ring Gain compensation Good in terms of stability Gain reduces Lead compensation Increases bandwidth Reduces closed loop gain Compensated attenuator Useful scheme when stray Needs matching two RC capacitance seen at time constants inverting input Lead lag compensation Increased bandwidth More external components Unit 2 Textbook : Op Amp for Everyone : Bruce Carter and Ron Mancini Fifth Edition 2017 THANK YOU Dr Shashidhar Tantry Electronics & Communication Engineering [email protected]