Linear Integrated Circuits - Unit 3 - PDF

LINEAR INTEGRATED CIRCUITS Dr Shashidhar Tantry Electronics and Communication Engineering Unit 3 Current feedback op amps Current feedback amplifier Current feedback refers to any closed-loop configuration in which the error signal used for feedback is in the form of a current. A c...

LINEAR INTEGRATED CIRCUITS Dr Shashidhar Tantry Electronics and Communication Engineering Unit 3 Current feedback op amps Current feedback amplifier Current feedback refers to any closed-loop configuration in which the error signal used for feedback is in the form of a current. A current feedback op amp responds to an error current at one of its input terminals, rather than an error voltage, and produces a corresponding output voltage The transfer function of a transimpedance amplifier is expressed as a voltage output with respect to a current input the open-loop "gain", vO/iIN, is expressed in ohms Unit 3 Current feedback op amps Summary of points in current feedback op amps Non inverting and Inverting gain circuits use the same formula for the gain that is -RF/RG and 1+RF/RG Use RF as recommended by the data sheet, RG can be as per the gain Use noninverting circuit, otherwise it will load source Never put capacitance across feedback resistor If only DC gain required, preferably use voltage feedback Current feedback op amps are choice for high speed and high current requirements For filters, voltage feedback op amp is preferred Current feedback op amps are best for high speed and high current circuits  Unit 3 Current feedback amplifier model High impedance here Input Buffer Output Buffer(neglected for analysis) Low impedance Low impedance here Model Unit 3 Development of stability equations Stability measurement done by loop gain Feedback circuit is seen at negative input terminal Break the loop, apply test signal and measure output Loop gain Unit 3 Development of stability equations Break the loop, apply test signal and measure output Z is the transimpedance of the op amp which is nothing but gain in terms of impedance Loop gain I2 = I1(ZG+ZB)/ZG Unit 3 Development of stability equations VTO = I1Z This is Loop gain  Unit 3 Noninverting current feedback amplifier VOUT = IZ I = I 1 – I2 I1 = VA/ZG I2 = (VOUT – VA )/ZF Circuit is nothing but non inverting amplifier VA = VIN - IZB I = (VA/ZG ) - (VOUT – VA )/ZF Unit 3 Noninverting current feedback amplifier Unit 3 Noninverting current feedback amplifier Identical to Voltage Feedback amplifier Unit 3 Inverting current feedback amplifier I + I2 = I 1 I + (VIN –VA )/ZG ) = (VA – VOUT )/ZF -VA = IZB VOUT = IZ Circuit is nothing but inverting amplifier Unit 3 Inverting current feedback amplifier Unit 3 Inverting current feedback amplifier Identical to Voltage Feedback amplifier Unit 3 Stability analysis Stability equation (loop gain) Typical values Gain and Phase equations for loop gain Unit 3 Stability analysis Open loop Gain is measured in terms of Z which trans-impedance Loop gain too measured in terms of impedance Assumed open loop gain to have two poles Loop gain reduces gain. As a result phase margin improves Unit 3 Stability analysis Summary of points RF is usually recommended by the manufacturer RF selection based on stability and bandwidth requirement Stability equation Different form When ZB = 0 and ZF = RF Aβ = Z/RF Unit 3 Selection of feedback resistor First method : Key points RF defines stability of the amplifier It is important to select RF properly RF value range given by the manufacturer If different value to be used, results of loop gain are extrapolated assuming linear relation Loop gain 1 value Loop gain 2 value New value of feedback resistor  Unit 3 Selection of feedback resistor Second method We can also use graphical method to select RF value For a given gain, calculate RF and from RF calculate Bandwidth of the amplifier Table represents different gain RF and Bandwidth values Graph taken from the data sheet of CFA  Unit 3 Stability and input capacitance Stray capacitance gets introduced across input resistor Hence ZG becomes reactive From 1 and 2, loop gain is given by, Loop gain is given by, Z Loop gain added with one pole ZB = RB ZG = RG Unit 3 Stability and feedback capacitance Unit 3 Stability and feedback capacitance  Unit 3 Stability and feedback capacitance Stray capacitance gets introduced across feedback resistor Hence ZF becomes reactive From 1 and 2, loop gain is given by, Loop gain is given by, Loop gain added with one pole and one zero, ZB = RB ZG = RG Unit 3 Stability and feedback capacitance Gain plot with and without addition of poles and zeros  Unit 3 Stability with input capacitance and feedback capacitance ZF becomes reactive and ZG also becomes reactive Loop gain equation becomes, To make circuit to cancel pole and zero, we need to keep Unit 3 Comparison between VFA and CFA - Precision Traditional op amps are voltage feedback type whereas current feedback amps are recent VFA input comes from differential pair Differential pair are perfectly matched VBE are matched Q1 and Q2 are matched Transistor current gains are matched Matched for layout equality Thermal balancing Trimming Unit 3 Comparison between VFA and CFA - Precision CFA inputs are at different impedance levels CFA Input impedance for inverting and non inverting are different Common mode rejection is not good Overall precision is bad compared to VFA Unit 3 Comparison between VFA and CFA - Bandwidth VFA bandwidth not good VFA Loop gain decreases for an increase in bandwidth Error is directly related to the bandwidth Unit 3 Comparison between VFA and CFA - Bandwidth CFA bandwidth good CFA No low frequency pole Loop gain does not depend on closed loop gain Unit 3 Comparison between VFA and CFA - Stability VFA stability Stability depends on a, RG and RF Stray capacitance does not change stability significantly CFA stability Stability depends on RF Stary capacitance play role in stability significantly Unit 3 Comparison between VFA and CFA - Impedance VFA impedance Input impedance is high and matched at both terminals Even in CMOS input impedance is high CFA impedance Not high compared to VFA Two input impedances are not matched Because of low input impedance at inverting terminal Unit 3 Comparison Table on equations Unit 3 Fully Differential op amp Differential input (Two inputs), Differential input(Two inputs), single output(One output) differential output(One output) 2 inputs and 1 output 2 inputs and 2 outputs Unit 3 Fully Differential op amp Unit 3 Fully Differential op amp Output 1 Output 2 Differential input(Two inputs), differential output(One output) Typical circuit inside fully differential amplifier Unit 4 Fully Differential op amp Two closed loop feedbacks are required for feedback circuit Loop is from inverting input to non inverting output and non inverting input to inverting output Assume 180 degree phase difference between two inputs and two outputs Both loops to be matched Ideal fully differential Here, each loop is a inverting type op amp gain is  Unit 3 Fully Differential op amp Single Ended to Differential Conversion In some applications, it is necessary to have two output, which can be derived from single input Input at inverting terminal Input at non inverting terminal Gain of single ended to differential is In many cases, fully differential output is expressed as (Vout+ - Vout-) Unit 3 Fully Differential op amp Single Ended to Differential Conversion Gain plot when RF is equal to RG that is when gain is one. Value of differential gain is always equal to one which is VIN = VOUT+ - VOUT- Unit 3 Fully Differential op amp Concept of VOCM A new pin is added called VOCM VOCM is called voltage output common mode This sets output common mode voltage, output voltage swings with reference to this voltage This is specific to TI chips Unit 3 Fully Differential op amp Instrumentation amplifier Instrumentation amplifier with VOCM concept at second stage Unit 3 Fully Differential op amp Differential low pass filter Unit 3 Fully Differential op amp Differential high pass filter Unit 3 Op Amp Noise Theory Noise is a purely random signal Instantaneous value can not be predicted at any point of time Noise is created either internally or externally Instantaneous values are either positive voltage or negative voltage These can be plotted as Gaussian probability function Noise types Thermal noise and shot noise follow Gaussian function Unit 3 Op Amp Noise Theory σ is the standard deviation and it is root mean square value of noise current or voltage Noise floor is the level of noise when there is no input given Unit 3 Op Amp Noise Theory Example 1 If two noise sources are 2V rms, total value will be Signal to Noise Ratio Example 2 If one noise source is 10V and another 1V rms, When there are multiple noise sources, total noise total value will be will be Higher noise source dominates Unit 3 Op Amp Noise Theory Noise Units Normally expressed as rms volts(amps) per root hertz Usually noise measured over a frequency band Unit 3 Op Amp Noise Theory Output noise = Gain * Input noise Unit 3 Op Amp Noise Theory Unit 3 Op Amp Noise Theory Shot Noise Also called as quantum noise Caused by random movement of charge carriers in a conductor Few random electrons get energy and move randomly These noise independent of temperature These are always associated with current flow This noise is flat when plotted for spectral density They represent imperfections in a metal Unit 3 Op Amp Noise Theory Shot Noise Unit 3 Op Amp Noise Theory Unit 3 Op Amp Noise Theory Thermal Noise Also called as Johnson’s noise It is due to thermal agitation of electrons It has uniform spectral density Unit 3 Op Amp Noise Theory Unit 3 Op Amp Noise Theory Thermal Noise Lowering temperature reduces thermal noise Unit 3 Op Amp Noise Theory Flicker Noise Also called as 1/f noise It is present in all active and passive elements It is in crystal structure this can be reduced by better structure This noise Increases with decrease in frequency It is associated with DC currents Unit 3 Op Amp Noise Theory Burst Noise Also called as popcorn noise It is characterised by high frequency pulses It’s amplitude is several times the thermal noise Can be heard at frequency in around 100hertz in a speaker Unit 3 Op Amp Noise Theory White Noise Unit 3 Op Amp Noise Theory Avalanche Noise Occurs at p n junction area Occurs when strong reverse field is applied to the junction Electrons will get kinetic energy and move, resulting in a current Unit 3 Op Amp Noise Theory Noise Colours Colours describe type of noise and its frequency dependency Unit 3 Op Amp Noise Theory Op amp Noise Op amp noise described in a graph At low frequency, it is 1/f noise At high frequency, it is white noise Unit 3 Op Amp Noise Theory Op amp Noise The point where 1/f noise is same as white noise is called noise corner frequency Unit 3 Op Amp Noise Theory Op amp noise model Unit 3 Reference: Op Amp for Everyone : Bruce Carter and Ron Mancini Fifth Edition 2017 Design with operational amplifiers and analogue integrated circuits by Sergio Franco THANK YOU Dr Shashidhar Tantry Electronics & Communication Engineering [email protected]