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Miller-OTA Opamp design
In AMIS CMOS 07 by Roman Prokop
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Simple Miller-OTA Opamp with follower
All MOSes should work in saturation region – then their parameters are following: NA – substrate doping ~ X cm-3
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Simple Miller-OTA Opamp AC hand calculation
AC small signal linearized model
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Simple Miller-OTA Opamp AC hand calculation
Redrawing - simplification
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Simple Miller-OTA Opamp AC hand calculation – A0=?
small ~1
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Simple Miller-OTA Opamp AC hand calculation – fp1=
Simple Miller-OTA Opamp AC hand calculation – fp1=? We know, where it is Follower neglected
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Simple Miller-OTA Opamp AC hand calculation – fp1=?
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Simple Miller-OTA Opamp AC hand calculation – fp1=?
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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
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Simple Miller-OTA Opamp AC hand calculation – fp1=?
b) No R, only C; G=jωC A0
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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=∞
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Simple Miller-OTA Opamp AC hand calculation GBW – Gain band width =?
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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
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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 !!!
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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
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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
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Simple Miller-OTA Opamp DC hand calculation DC input range
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Simple Miller-OTA Opamp DC hand calculation DC input range
Be careful for temperature and process worst case
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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
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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
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Matching AMIS CMOS07 parameters - NMOS
Carefully: units mV, μm, %
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Matching AMIS CMOS07 parameters - PMOS
Carefully: units mV, μm, %
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Simple Miller-OTA Opamp Hand calculation - Conclusion - AC
Stability condition - approx. 3 <
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Simple Miller-OTA Opamp Hand calculation - Conclusion - DC
DC input range Matching offset
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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
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Simple Miller-OTA Opamp Simulation - DC
Possible tested parameters: OFFSET - Input asymmetry - systematic offset - matching offset CM – DC input range
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Simple Miller-OTA Opamp Simulation – DC input range
Possible tested parameters: OFFSET - Input asymmetry - systematic offset - matching offset CM – DC input range
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Good luck !!!
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