EE247 Lecture 6 Summary last lecture Continuous time filters continued Opamp MOSFET RC filters Gm C filters Frequency tuning for continuous time filters Trimming via fuses or laser Automatic on chip filter tuning Continuous tuning Master slave tuning Periodic off line tuning Systems where filter is followed by ADC DSP existing hardware can be used to periodically update filter freq response EECS 247 Lecture 6 Filters 2007 H K Page 1 Summary Lecture 5 Continuous time filters Effect of integrator non idealities on continuous time filter behavior Effect of integrator finite DC gain non dominant poles on filter frequency response Integrator non linearities affecting filter maximum signal handling capability harmonic distortion and intermodulation distortion Effect of integrator component variations and mismatch on filter response need for frequency tuning Frequency tuning for continuous time filters Frequency adjustment by making provisions to have variable R or C Various integrator topologies used in filters Opamp MOSFET C filters Opamp MOSFET RC filters to be continued today EECS 247 Lecture 6 Filters 2007 H K Page 2 Improved MOSFET C Integrator Vi 2 IX1 IX 2 Vi Vi 2 Cox W V gs1 Vgs3 L IX1 C M3 ID M4 4 ID2 M2 W V gs1 Vgs3 Vi L M1 I D3 W V V Cox Vgs1 Vgs3 i i L 2 2 W V V I X 2 Cox Vgs3 Vgs1 i i L 2 2 G ID1 M1 2 3 4 IX2 W V Vi V i V L gs3 th 4 2 I X 1 I D1 I D3 I D3 Cox I X 1 I X 2 Cox VG3 VG1 W Vds V Vth Vds L gs 2 W Vi Vi I D1 Cox Vgs1 Vth L 4 2 I D Cox Vout C equal W L No threshold dependence Linearity achieved in the order of 50 70dB Ref Z Czarnul Modification of the Banu Tsividis Continuous Time Integrator Structure IEEE Transactions on Circuits and Systems Vol CAS 33 No 7 pp 714 716 July 1986 EECS 247 Lecture 6 Filters 2007 H K Page 3 R MOSFET C Integrator VG2 VG1 R M1 C M3 R M2 M4 Vi 2 Vi 2 Vout C Improvement over MOSFET C by adding resistor in series with MOSFET Voltage drop primarily across fixed resistor small MOSFET Vds improved linearity reduced tuning range Generally low frequency applications Ref U K Moon and B S Song Design of a Low Distortion 22 kHz Fifth Order Bessel Filter IEEE Journal of Solid State Circuits Vol 28 No 12 pp 1254 1264 Dec 1993 EECS 247 Lecture 6 Filters 2007 H K Page 4 R MOSFET C Lossy Integrator R2 VG1 R1 VG2 C M3 R1 M2 M4 Vi 2 Vi 2 M1 Vout C R2 Negative feedback around the non linear MOSFETs improves linearity but Compromises frequency response accuracy Ref U K Moon and B S Song Design of a Low Distortion 22 kHz Fifth Order Bessel Filter IEEE Journal of Solid State Circuits Vol 28 No 12 pp 1254 1264 Dec 1993 EECS 247 Lecture 6 Filters 2007 H K Page 5 Example Opamp MOSFET RC Filter 5th Order Bessel MOSFET RC LPF 22kHz bandwidth THD 90dB for 4Vp p 2kHz input signal Suitable for low frequency low Q applications Significant improvement in linearity compared to MOSFET C Needs tuning Ref U K Moon and B S Song Design of a Low Distortion 22 kHz Fifth Order Bessel Filter IEEE Journal of Solid State Circuits Vol 28 No 12 pp 1254 1264 Dec 1993 EECS 247 Lecture 6 Filters 2007 H K Page 6 Operational Amplifiers Opamps versus Operational Transconductance Amplifiers OTA Opamp OTA Voltage controlled voltage source Voltage controlled current source Output in the form of current High output impedance In the context of filter design called gm cells Cannot drive R loads Good for SC gm C filters Typically less complex compared to opamp higher freq potential Typically lower power Output in the form of voltage Low output impedance Can drive R loads Good for RC filters OK for SC filters Extra buffer adds complexity power dissipation EECS 247 Lecture 6 Filters 2007 H K Page 7 Integrator Implementation Transconductance C Opamp Transconductance C C Vin Vin Gm Gm Vo GmC Intg Vo GmC OTA Intg Vo Vin EECS 247 o s G whe r e o m C Lecture 6 Filters 2007 H K Page 8 Gm C Filters Simplest Form of CMOS Gm C Integrator Transconductance element formed by the source coupled pair M1 M2 All MOSFETs operating in saturation region M1 Vin Current in M1 M2 can be varied by changing Vcontrol Cint g M10 Vo M2 Vcontrol Ref H Khorramabadi and P R Gray High Frequency CMOS continuous time filters IEEE Journal of Solid State Circuits Vol SC 19 No 6 pp 939 948 Dec 1984 EECS 247 Lecture 6 Filters 2007 H K Page 9 Simplest Form of CMOS Gm C Integrator AC Half Circuit Cint g Cint g Vin M1 Vo M2 Vcontrol M10V control EECS 247 2Cint g2Cint g Vin Vo M2 M1 M10 Lecture 6 Filters Vcontrol Vo Vin M1 2Cintg AC half circuit 2007 H K Page 10 Gm C Filters Simplest Form of CMOS Gm C Integrator Use ac half circuit small signal model to derive transfer function Vo Vin M 1 2 Vo gm Vin 2Cint g s Vo g M 1 2 m Vin 2Cint g s 2Cintg AC half circuit Vo o Vin s o M1 Vin M 1 2 gm g m Vin Vo 2Cintg CGS 2 Cint g Small signal model EECS 247 Lecture 6 Filters 2007 H K Page 11 Gm C Filters Simplest Form of CMOS Gm C Integrator MOSFET in saturation region Cox W 2 Vgs Vth Id 2 L Gm is given by I W V V M 1 M 2 gm d Cox gs th Vgs L Id 2 Vgs Vth Vin 1 2 W 1 2 Cox I d L 2 M1 Cint g M10 Vo M2 Vcontrol Id varied via Vcontrol gm tunable via Vcontrol EECS 247 Lecture 6 Filters 2007 H K Page 12 2nd Gm C Filters Order Gm C Filter Use the Gm cell to build a 2nd order bandpass filter Vin Cint g M1 M10 EECS 247 Lecture 6 Filters Vo M2 Vcontrol 2007 H K Page 13 2nd Order Bandpass Filter I in Vo VR R VL L VC C IR I L IC V1 R R V1 1 V2 1 Vo 1 R sC R 1 1 1 Vin sL V3 Vo 1 R C 2 L R R R 1 s1 1 1 s 2 Vin EECS 247 Lecture 6 Filters 2007 H K Page 14 2nd Order Integrator Based Bandpass Filter VBP VBP 2s Vin s2 s 1 1 2 2 1 R C 2 L R R 0 1 1 2 1 Q 1 1 LC 2 Vin From matching point of v iew desirable 1 2 1 Q R 0 R EECS 247 s s1 1Q R 1 1 Lecture 6 Filters 2007 H K Page 15 2nd Order Integrator Based Bandpass Filter VBP First implement this part With transfer …
View Full Document
Unlocking...