Electronic Circuits for Mechatronics ELCT 609 Lecture 7: MOS-FET Amplifiers Assistant Professor Office: C3.315 E-mail: eman.azab@guc.edu.eg 1
Enhancement N-MOS Modes of Operation Mode V GS I DS V DS Cutoff (OFF) V GS <V T Its Value is irrelevant ' W K k n L Triode (Linear) Saturation V GS >V T k n : Process Trans-conductance Parameter [ma/v 2 ] W: Channel Width L: Channel Length W/L: Aspect Ratio 2
Enhancement N-MOS in Saturation Notes: In Saturation region: As V DS increases, the channel length L decreases (Channel Length modulation) C W I V V 2 L L n ox DS GS T 2 C W I V V 1 V 2L 2 n ox DS GS T DS λ: Process Technology Parameter [V -1 ] V A : Early Voltage [V] 3
Enhancement N-MOS in Saturation Notes: Channel Length modulation can be modeled by a resistor across the MOS Drain and Source 1 IDS 1 rds VDS K VGS VT 2 For DC Analysis (neglect r ds ) C W I V V 2L 2 n ox DS,Sat GS T 2 r ds I 1 DS,Sat I V A DS,Sat The ideal value for r ds is infinity (Why?) 4
MOS-FET Large Signal Model 5
N-MOS Large Signal Model MOS-FET in Saturation can be modeled as a voltage controlled current source This model is useful for circuit analysis: Hybrid Model K i v V 2 2 DS GS T Note that this model can be used Only for Saturation Mode 6
MOS-FET Analog Amplifiers Design and Analysis 7
MOS Amplification Mode We will use the I-V characteristics of MOS to achieve the voltage amplification v V i R o DD D D K i D (vi V T) 2 2 Figures from Sedra and Smith 8
MOS Amplification Mode We can easily separate DC and AC Signals (Superposition) Under the assumption that the AC signal amplitude is very small MOS must work in Saturation to avoid signal distortion Input terminals of the amplifier are Gate/Source Output terminals of the amplifier are Drain/Source Equivalent Circuit for Small Signal Analysis g i KV V D m GS T v gs g m 2KI DS 2IDS V V GS T 9
Common Source Amplifier Input Terminal: Gate Output Terminal: Drain DC Biasing: Current Source What is the use of C s, C c1 and C c2? What is the value of DC Drain Current I DS? Find the Voltage gain, Input and Output Resistances? 10
Common Source Amplifier To calculate the DC current, we will Consider the Capacitors Ideal. (Z c = at DC) Draw the Circuit for DC Signals ONLY (I DS =I) 11
Common Source Amplifier To calculate the voltage gain, We will assume the capacitor impedance is Short Circuit Draw the Circuit for AC Signals ONLY (Small Signal Model) 12
Common Source Amplifier Draw the Equivalent Small Signal model for the Circuit v A g (r / /R / /R ) out G v m ds D L vsig Rsig RG R 13
Common Source Amplifier Input and Output Resistance are derived using Thevenin Theorem R in v i x x R G v R R / /r x out D ds ix 14
Common Source Amplifier Notes on Common Source: Inverting Amplifier High Input Resistance High Output Resistance Gain is dependent on Output Resistance (LOAD) 15
Common Gate Amplifier Input Terminal: Source Output Terminal: Drain DC Biasing: Current Source What is the use of C c1 and C c2? What is the value of DC Drain Current I DS? Find the Voltage gain, Input and Output Resistances? 16
Common Gate Amplifier To calculate the DC current, we will Consider the Capacitors Ideal. (Z c = at DC) Draw the Circuit for DC Signals ONLY (I DS =I) 17
Common Gate Amplifier To calculate the voltage gain, We will assume the capacitor impedance is Short Circuit Draw the Circuit for AC Signals ONLY (Small Signal Model) 18
Common Gate Amplifier Draw the Equivalent Small Signal model for the Circuit For Simplicity we will consider r ds = A v vout g m(r D / /R L) v 1 g R sig m sig 19
Common Gate Amplifier Input and Output Resistance are derived using Thevenin Theorem R in 1 g m R out R D Notes on Common Gate: Non-Inverting Amplifier Low Input Resistance (Suitable Input Current) High Output Resistance Gain is dependent on Output Resistance (LOAD) 20
Common Drain Amplifier Input Terminal: Gate Output Terminal: Source DC Biasing: Current Source What is the use of C c1 and C c2? What is the value of the DC Drain Current I DS? Find the Voltage gain, Input and Output Resistances? 21
Common Drain Amplifier To calculate the DC current, we will Consider the Capacitors Ideal. (Z c = at DC) Draw the Circuit for DC Signals ONLY (I DS =I) Figure from Sedra and Smith 22
Common Drain Amplifier To calculate the voltage gain, We will assume the capacitor impedance is Short Circuit Draw the Circuit for AC Signals ONLY (Small Signal Model) 23
Common Drain Amplifier Draw the Equivalent Small Signal model for the Circuit A v vout g m(r ds / /R L) RG v 1 g (r / /R ) R R sig m ds L sig G 24
Common Drain Amplifier Input and Output Resistance are derived using Thevenin Theorem R in 1 R r / / g RG out ds m Notes on Common Drain: Non-Inverting Amplifier Unity Gain (Buffer) Low Output Resistance Gain is dependent on Output Resistance (LOAD) 25
Summary Amplifier Type Common Source Common Gain Common Drain Voltage Gain High Intermediate Voltage Gain Polarity Unity Gain or Less Negative Positive Positive Input Resistance High Low High Output Resistance High High Low 26
Assignment Common Source with Source Resistance Find the voltage gain, Input and Output Resistance? 27