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Operational Amplifiers

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Operational Amplifiers

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    1. Operational Amplifiers

    2. Important Parameters

    3. One-Stage Op Amps

    4. ICMR, OCMR In open loop configuration,----Range of voltages for which circuit can operate even if input dc or output dc shifts

    5. ICMR

    6. ICMR in open loop Configuration

    7. DC Voltage Range Available For Designing Opamp in Unity Gain Configuration In feed back mode Range of voltages which can be chosen as input/ output dc to configure opamp in feedback mode LARGER THE RANGE, EASY TO DESIGN

    8. DC Voltage Range Available For Designing Opamp in Unity Gain Configuration

    9. ICMR in Unity Gain Configuration

    10. Feed back stabilizes DC bias to a particular value Remember, in feedback, circuit shall always come to designed voltage value even if input dc level shifts Because change in voltages can cause Vgs of transistors to change hence changing Ibias which triggers a corrective action if vin dc inc. node x inc. node X will force node Y to inc. Causing Vgs of M2 to inc. so I2 inc. which makes node Y to fall because Iss will not allow a change Hence, Feedback circuit can work at a particular voltage at node Y

    11. Cascode Op Amps

    13. Unity Gain One Stage Cascode-difficult to bias

    14. OCMR, ICMR(telescopic) in feedback mode- OCMR, ICMR(telescopic) in feedback mode--- DC Voltage Range available in Unity Gain Configuration of opamp. In this range, feedback causes dc levels at output / input to stabilize to its designed value even under fluctuations

    16. Single-Ended Output Cascode Op Amps

    17. Triple Cascode

    18. Folded Cascode Op Amps

    19. Folded Cascode Stages (cont.)

    20. Folded Cascode (cont.)

    21. Folded Cascode (cont.)

    22. OCMR, ICMR IN FEEDBACK

    24. GAIN SLIGHTLY LESS THAN TELESCOPIC POWER DISSIPATION HIGHER

    25. Telescopic vs. Folded Cascode Pole

    26. POLE AT FOLDING POINT POLE FREQUENCY LOWER THAN TELESCOPIC (possibly)

    27. Example Folded-Cascode Op Amp

    28. Two-Stage Op Amps

    30. Single-Ended Output Two-Stage Op Amp

    32. Output Impedance Enhancement With Feedback

    33. Gain Boosting in Cascode Stage

    34. Differential Gain Boosting

    36. Differential Gain Boosting (cont.)

    37. OCMR, ICMR Voutmax.= Vdd-Vgs-2Vov. =3-1.3-0.3= 2.4V Voutmin.= Vgs5+2Vov. =1.6V Vx min= Vgs5+Vov.=1.3V Vinmin= Vx min.+Vt=2.3V Vinmin= Vgs1+Vov= 1.3 Vinmax= Vgs1+Vov+Vt= Vxmax+Vt= 2.8v

    39. Differential Gain Boosting (cont.)

    41. Fully differential circuits Drawback common mode level can not be a stable desired value due to process variations

    42. Well defined common mode level

    43. Common-Mode Feedback

    44. Differential Pair with FB, yet no loss of gain

    45. Vocm ?Process variation dependence If w/L of M3 reduces, Vx reduces? Vsg3 increases?making I1=I3

    46. Common-Mode Feedback (cont.)

    47. Can we use Feed Back As CORRECTION? [o/p to i/p ] Feed back here does not correct this problem---why? If M3 w/L reduces due to process variation? Vx reduces to equalize I1=I3 if Vx dec. Vgs1 decreases ? Vp dec.?so M5 goes into linear We need other correction method.

    49. High Gain Amp Model

    51. How to sense?

    53. Common-Mode Feedback (cont.)

    56. Resistive Sensing

    57. Remedy---Source-Follower Sensing

    58. CMFB Example in folded cascode

    59. Alternative CMFB for Folded Cascode

    62. Returning CMFB with Triode Devices in folded cascode

    64. Drawbacks

    65. CMFB using Triode circuit

    66. Forcing desired Reference voltage

    67. CMFB Triode Example with Reference (cont.)

    68. Differential Pair with LCMFB

    69. Analysis of amplifier with CMFB circuit 3 analysis required Does Acm increase after including cmfb thus degrading CMRR? What is the condition for Voc= Vref? What is the condition for loop to be stable?

    71. Complete circuit

    72. Acmwithout feedback

    73. (Details from grey meyer)-- ---[Acm=voc/vic] without CMFB

    74. Acmwith CMFB

    75. (Details from grey meyer)--[Acm=voc/vic] with CMFB

    76. Details from grey meyer)-- --- [Acmf = voc/vic] with CMFB

    77. (Details from grey meyer)--Gain [Voc/Vz= voc/vcmc] without CMFB

    78. (Details from grey meyer)-- -Voc/Vz

    79. Condition for Voc= Vref

    80. Loop gain Loop gain= Voc/ Vref= Af

    81. For Vref=Voc

    82. (Details from grey meyer)--

    83. Condition for feedback loop to be stable

    84. For feedback loop to be stable

    85. (Details from grey meyer)--

    86. (Details from grey meyer)--

    87. To stabilize CMFBreduce gm by splitting (Details from grey meyer)

    89. Rail-rail ICMR

    90. Constant Gm Circuits

    91. Slew Rate Slew Rate (SR) limit: Real OpAmp has a maximum rate of change of the output voltage magnitude limit SR can cause the output of real OpAmp very different from an ideal one if input signal magnitude is too high Affects settling time of OPAMP

    92. Normal Settling

    93. R-C charging

    98. Why? Origin of slewing

    99. ?V largeOPAMP slew

    100. High To Low Transition

    101. Slew Rate

    102. Slewing Undesirable because Limits the speed of OPAMP Can not be eliminated Remedy ---- Estimate max. speed that can be obtained Then make slew rate large How? provide additional current boosting,

    103. Estimation of Full Power Bandwidth Full Power bandwidth: the range of frequencies for which the OpAmp can produce an undistorted sinusoidal output with peak amplitude equal to the maximum allowed voltage output

    105. Estimation of slew rate

    106. Slewing in Telescopic Op Amp

    107. Differential Slew Rate Positive slew rate---large positive step at input Negative slew rate----large negative step at input

    108. Folded-Cascode Slewing

    109. Folded-Cascode (cont.)

    110. Constraint on Ip Ip > Iss

    111. Slewing Recovery if Ip < Iss

    112. Slewing Recovery (cont.)

    113. Two stage cmos opamp With RC Compensation

    117. Normal OTAclass AB Operation

    118. Charging output

    119. Parameters

    120. Adaptive biasing with LCMFB

    121. Boosting current when large I/P

    122. OPERATION OF LCMFB When input is small, M1, M2 , M6, M7 carry equal current (Icm/2) and x and y are at same potential When i/p is largeM1 carries all current , M2 cuts off. M7 discharges y causing M7, M8 to cut off . This makes I1> I2. A differential current (Id= I1-I2)) flows through R1= R2=R. Vx > Vy. Vz remains constant. But Vx=Vgs5 increases ? so I5 increases from I5 to ( I5 + ?) So I4 through CL increases But largest current corresponds to largest Vgs5= Vzcm+IssR only

    123. Operation

    124. Operation

    125. CE for charging output in slew mode

    126. Impact of LCMFB on Small signal behaviour

    127. Technique to further improve Slew Rate Adaptive bias

    128. Adaptive biasing for pmos diff.amp

    129. Adaptive biasing ---Nmos diff amplifier

    130. Operation of adaptive bias Small sig mode M1, M2 carry same current Iss/2 Large sig. mode--2 source follower M6, M7 always carry Ib current as their Vgs always remain same M8, M9 can carry large current (Ib+I) as their Vgs can vary. For large input, M2 is cut off, M1 has large Vgs. So it carries large current(> Iss), which is sunk by M9

    131. VB can be low to obtain low power consumption under low level inputs.

    132. Impact on AC behaviour

    133. Ac behaviour

    134. With RC Compensation

    135. ICMR

    136. Systematic Offset

    137. Random Offset

    143. PSRR OF A CIRCUIT

    147. PSRR Calculationssingle stage

    148. Cascode amp.

    149. PSRR Calculations

    154. 2 stage CMOS OP AMP

    156. Without Cc

    158. With Cc

    159. Only second stage

    161. Noise

    162. Noise 2 sources---- Noise coupled to input signal Small current and voltage fluctuations that are generated with in the device Performance parameter----signal to noise ration (SNR)

    163. Origin of device noise Existence of noise is due to the fact the charge is not continuous but is carried in discrete amounts equal to electron charge Thus noise is associated with fundamental processes in integrated circuit devices so it can not be removed

    164. Why should we study noise Because noise represents a lower limit to the size of electrical signal (min. detectable signal) that can be amplified by a circuit without significant deterioration in signal quality Noise results in upper limit to the useful gain of an amplifier because if gain is increased without limit, then due to noise fluctuations at output node, transistors may go to linear region

    165. Noise-Random signal Value of noise signal cannot be predicted at any time even if past values are known

    166. If microphone drives a resistive load, More heat will be generated in case b. ?Average value of ac signals

    167. How to estimate Noise? Observe noise for a long time Using measured results, prepare a statistical model Extract useful properties (here, noise power) from this model that can be predicted Use noise power for doing noise analysis

    168. Average Power Average power delivered by a periodic

    169. Noise power

    170. How to find Average power Square the signal Area under the waveform is calculated Normalize the area to T Pav expressed in V2

    171. Noise content Noise content varies with frequency Noise power spectral density is obtained i.e to find the magnitude of low and high noise components

    172. How to obtain Noise spectrum

    173. Noise spectrum

    174. Types of noise Thermal noise

    175. Representation of thermal noise

    176. MOSFET noise---thermal noise Noise generated in the channel

    177. MOS---flicker noise

    179. Representation

    180. MOS noise

    181. Noise corner frequency

    182. Computation of Noise in circuits

    183. Uncorrelated noise sources Noise produced by resistor is independent of noise produced by transistor

    184. Output noise / Hz

    185. output noise/ Hz for comparison Drawbacks of using output noise for comparison Consider two amplifiers of gain A1, A2 Amp1 has Vout= 1V, Vn= v30nV/ vHz. Amp2 has Vout= 3V, Vn= v60nV/ vHz. Which is better? Difficult to make comparison

    186. A2 generates more noise, but has higher gain A1 has low gain but generates less noise

    187. Comparison Parameter Signal to noise ratio---how large is signal in comparison to noise Should be large Input referred noise voltage (indep. Of gain) fictitious quantity as it can not be measured at the input This indicates how small an input the circuit can detect Should be small

    188. Representation

    190. How to reduce input referred noise voltage gm1 should be maximized

    191. 2nd circuit

    192. How to reduce input referred noise voltage

    193. Frequency response

    194. Total output noise

    195. SNR---signal power to noise power

    196. Csacode amplifier

    197. Resistive load differential amp

    202. Active load diff amp

    206. Vx

    207. Vy

    211. Noise bandwidtheasy way to compare multipole systems

    212. Input signal noise 2 steps strategy--- Use fully differential circuits with high CMRR Use Negative feedback ---signal to noise ration (SNR)

    215. Method-1 Vout= 100Vin + Vn SNR = 100vin/ vn

    217. Implementation

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