Miller-OTA Opamp design In AMIS CMOS 07 by Roman Prokop

Simple Miller-OTA Opamp with follower All MOSes should work in saturation region – then their parameters are following: NA – substrate doping ~ X .1016 cm-3

Simple Miller-OTA Opamp AC hand calculation AC small signal linearized model

Simple Miller-OTA Opamp AC hand calculation Redrawing - simplification

Simple Miller-OTA Opamp AC hand calculation – A0=? small ~1

Simple Miller-OTA Opamp AC hand calculation – fp1= Simple Miller-OTA Opamp AC hand calculation – fp1=? We know, where it is Follower neglected

Simple Miller-OTA Opamp AC hand calculation – fp1=?

Simple Miller-OTA Opamp AC hand calculation – fp1=?

Simple Miller-OTA Opamp AC hand calculation – fp1=? 3 possibilities a) No R, no C; G=0 Confirmation of the transfer function without R&C

Simple Miller-OTA Opamp AC hand calculation – fp1=? b) No R, only C; G=jωC A0

Simple Miller-OTA Opamp AC hand calculation – fp1=? c) R & C (R added); G=(R+1/jωC)-1 A0 R - negligible Pole without changes Zero is moved if R=1/gm7  fZ=∞

Simple Miller-OTA Opamp AC hand calculation GBW – Gain band width =?

Simple Miller-OTA Opamp AC hand calculation First non-dominant pole -> stability =? 1st non-dominant pole decides about stability. if fND1 > GBW  stable We have 3 ND poles. We are interested in the lowest one. ad 1) C1 is small (high f) C1 shorts the V1 to the ground

Simple Miller-OTA Opamp AC hand calculation First ND pole ad 2) the most usual case At this frequency we expect CC is a short we get diode with gm7 >> other G Stability condition - approx. 3 < If there is no close other pole !!! estimate !!!

Simple Miller-OTA Opamp AC hand calculation First ND pole ad 3) caused by load capacitance It can appear if Cload is bigger capacitance Then expecting Cload >> ΣCds,Cdg

Simple Miller-OTA Opamp AC hand calculation SR – Slew rate Input goes rail to rail  all IB current flows either through M1, M5, M6 or through M2 ICC:=min (IB,I4) Usually I4 > IB  depends on IB

Simple Miller-OTA Opamp DC hand calculation DC input range

Simple Miller-OTA Opamp DC hand calculation DC input range Be careful for temperature and process worst case

Simple Miller-OTA Opamp DC hand calculation – structure offset This systematic offset usually appears when Vds5 ≠ Vds6 Vds6 depends on Vgs7 To suppress the offset

Simple Miller-OTA Opamp DC hand calculation Matching offset The first stage gives the most significant contribution to the offset. Contribution of the second stage is negligible because of the first stage gain.  Usually sufficient for hand calculation Result is valid for 1σ statistical result - use value (4σ ÷6σ) for offset calculation

Matching AMIS CMOS07 parameters - NMOS Carefully: units mV, μm, %

Matching AMIS CMOS07 parameters - PMOS Carefully: units mV, μm, %

Simple Miller-OTA Opamp Hand calculation - Conclusion - AC Stability condition - approx. 3 <

Simple Miller-OTA Opamp Hand calculation - Conclusion - DC DC input range Matching offset

Simple Miller-OTA Opamp Simulation - AC Possible tested parameters: A0 - DC gain GBW – Gain bandwidth fp1 - The first pole frequency ~ fND1 - The first non-dominant pole frequency AM, PM – Gain margin, Phase margin

Simple Miller-OTA Opamp Simulation - DC Possible tested parameters: OFFSET - Input asymmetry - systematic offset - matching offset CM – DC input range

Simple Miller-OTA Opamp Simulation – DC input range Possible tested parameters: OFFSET - Input asymmetry - systematic offset - matching offset CM – DC input range

Good luck !!!