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EC04 804(C) Analog MOS Circuits Guide line Prepared by Vishnu N V Sincere Thank

EC04 804(C) Analog MOS Circuits Guide line Prepared by Vishnu N V Sincere Thanks to Sindhu.L (Lecturer in ECE) GEC Sreekrishnapuram www.edutalks.org Introduction* *indicates out of syllabus CMOS circuit design consists of defining i/ps,o/ps hand calculations, ckt simulations, layout of the ckt etc. CMOS ICs are fabricated on thin circular slices of Si called wafers. Each wafer contains individual chips or “die”. www.edutalks.org Introduction* www.edutalks.org MOSFET symbols* www.edutalks.org MODULE 1 Reference “CMOS Circuit Design Layout, and Simulation” Baker,Li,Boyce Page no 165-184 www.edutalks.org Analog MOS models Low frequency model. • Study Expressions for Threshold voltage VTHN,ID,beta MOS in saturation (Refer fig9.1 &9.2) • When MOSFET is in saturation VDS>VGS-VTHN www.edutalks.org High Frequency Model (Page 171) To obtain high frequency model add capacitance to the low frequency model • Capacitance b/w gate and Source Cgs • Capacitance b/w gate and drain Cgd • Capacitance b/w drain and source diffusion region Cdb,Csb • Capacitance of gate over field region www.edutalks.org Temperature effects in MOSFET Threshold voltage and Trance conductance parameter changes with temperature. www.edutalks.org Noise in MOSFETS The noise generators in MOSFETS are due to • Thermal noise and • Flicker(1/f “one over f noise The rms thermal noise current is generated by • the effective channel resistance(3/2.1/gm) • Parasitic drain, source, gate and substrate resistances (RD,RS,RG,RB) www.edutalks.org Noise in MOST…….. www.edutalks.org Noise cont……… www.edutalks.org Module 2 Reference “CMOS Circuit Design Layout, and Simulation” Baker,Li,Boyce Page no 427-488 www.edutalks.org Current sources and Sinks www.edutalks.org Current sources and Sinks www.edutalks.org .. www.edutalks.org Simple source.. www.edutalks.org Simple source.. www.edutalks.org Simple source… www.edutalks.org Simple source… www.edutalks.org Simple source… www.edutalks.org Simple source… www.edutalks.org Cascode source To increase the output resistance of a current source or sink. www.edutalks.org Cascode source www.edutalks.org Triple Cascode www.edutalks.org Other Current sources or Sinks The basic current mirror can be improved significantly with negative feedback such as • Wilson current mirror • Regulated cascode Both offer stable current values for wide voltage swings and enhanced output impedance. Regulated cascode offer higher degree of o/p impedance & current stability. www.edutalks.org Wilson current source www.edutalks.org Regulated cascode  Regulated cascode Offers significant advantages over others 1. O/p impedance is significantly higher 2. Minimum value of Vo is lower than most of the other configurations. 3. Higher degree of current stability. www.edutalks.org References In CMOS circuits we derive reference voltages from power supplies by • Resistor and MOSFET • MOSFET only voltage references • Band gap voltage references www.edutalks.org Voltage References Voltage divider has the advantage of • Simplicity • Temperature insensitivity • Process insensitivity But • In order to reduce the power dissipation the resistors must be made large. Large resistance require large area in the dieNot practical . www.edutalks.org Voltage References The voltage divider formed between resistor and MOSFET is the same ckt used for a bias in the current mirror Ref Page 463 A MOSFET only Reference is a voltage divider formed between an n and p MOSFETs. It has the advantage that the layout can be small. www.edutalks.org Biasing schemes for voltage references The resistor-resistor, resistor-MOS,MOS only voltage references are very sensitive to the power supply voltages and temperature. So to reduce these the circuits must be properly biased. www.edutalks.org Current source self biasing THREE TYPES • Threshold voltage referenced Self –biasing • Diode referenced Self Biasing • Thermal voltage referenced Self Biasing www.edutalks.org Band gap References Band gap references combine the +ve Temp Coefficient of the thermal voltage with the –ve Temp Coefficient of the of the diode forward voltage in a circuit to achieve a voltage reference with a zero Temp Coefficient. Once we have a temperature independent voltage reference, with the use of an op-amp we can generate multiples of this reference. www.edutalks.org to be continued…….. www.edutalks.org uploads/Litterature/ study-guide 23 .pdf