© Electronics Recall Last Lecture The MOSFET has only one current, I D Operation of MOSFET – NMOS and PMOS – For NMOS, V GS > V TN V DS sat = V GS – V TN – For PMOS V SG > |V TP | V SD sat = V SG + V TP

© Electronics I D versus V DS (NMOS) or I D versus V SD (PMOS)

© Electronics NMOS o V TN is POSITIVE o V GS > V TN to turn on o Triode/non-saturation region o Saturation region o V DSsat = V GS - V TN PMOS o V TP is NEGATIVE o V SG > |V TP | to turn on o Triode/non-saturation region o Saturation region o V SDsat = V SG + V TP

© Electronics DC analysis of FET

© Electronics MOSFET DC Circuit Analysis MOSFET DC Circuit Analysis - NMOS  The source terminal is at ground and common to both input and output portions of the circuit.  The C C acts as an open circuit to dc but it allows the signal voltage to the gate of the MOSFET.  In the DC equivalent circuit, the gate current into the transistor is zero, the voltage at the gate is given by a voltage divider principle: V G = V TH = R 2 V DD R 1 + R 2 Use KVL at GS loop: V GS –V TH + 0 = 0 V GS = V TH

© Electronics MOSFET DC Circuit Analysis - NMOS 2.Assume the transistor is biased in the saturation region, the drain current: 3.Use KVL at DS loop I D R D + V DS – V DD = 0 4.Calculate V DSsat = V GS - V TN 1.Calculate the value of V GS 5.Confirm your assumption: If V DS > V DS (sat) = V GS – V TN, then the transistor is biased in the saturation region. If V DS < V DS (sat), then the transistor is biased in the non-saturation region.

© Electronics Calculate the drain current and drain to source voltage of a common source circuit with an n-channel enhancement mode MOSFET. Assume that R 1 = 30 k , R 2 = 20 k , R D = 20 k , V DD = 5V, V TN = 1V and K n = 0.1 mA/V 2 V DSsat = V GS – V TN = 2 – 1 = 1V, so, V DS > V DSsat, our assumption that the transistor is in saturation region is correct V TH = 20 5 = 2V hence V GS = V TH = 2V EXAMPLE:

© Electronics EXAMPLE The transistor has parameters V TN = 2V and K n = 0.25mA/V 2. Find I D and V DS V DD = 10V R D = 10k  R 1 = 280k  R 2 = 160k 

© Electronics Solution 2. Assume in saturation mode: I D = K n (V GS - V TN ) 2 So, I D = mA 3. KVL at DS loop: V DS = V DD – I D R D = 10 – (10) = 3.31 V 4. V DS sat = V GS – V TN = – 2 = V So, V DS > V DSsat, therefore, assumption is correct! 1. V TH = = V Answer: I D = mA and V DS = 3.31 V KVL at GS loop: V GS – V TH + 0 = 0  V GS = V TH

© Electronics MOSFET DC Circuit Analysis MOSFET DC Circuit Analysis - PMOS Different notation: V SG and V SD Threshold Voltage = V TP Use KVL at GS loop: V SG V TH – V DD = 0 V SG = V DD - V TH V G = V TH = R 2 V DD R 1 + R 2

© Electronics MOSFET DC Circuit Analysis MOSFET DC Circuit Analysis - PMOS  Assume the transistor is biased in the saturation region, the drain current:  Calculate V SD : Use KVL at DS loop: V SD + I D R D - V DD = 0  If V SD > V SD (sat) = V SG + V TP, then the transistor is biased in the saturation region.  If V SD < V SD (sat), then the transistor is biased in the non-saturation region. V SD = V DD - I D R D I D = K p (V SG + V TP ) 2

© Electronics 50 k  7.5 k  Calculate the drain current and source to drain voltage of a common source circuit with an p-channel enhancement mode MOSFET. Also find the power dissipation. Assume that, V TP = -1.1V and K p = 0.3 mA/V 2 Use KVL at SG loop: V SG – 5 = 0 V SG = 5 – 2.5 = 2.5 V Assume biased in saturation mode: Hence, I D = 0.3 ( 2.5 – 1.1) 2 = mA Calculate V SD Use KVL at SD loop: V SD + I D R D – 5 = 0 V SD = 5 - I D R D V SD = 5 – ( 7.5) = V 5V V SG > |V TP |

© Electronics V SD sat = V SG + V TP = 2.5 – 1.1 = 1.4V Hence, V SD < V SD sat. Therefore assumption is incorrect. The transistor is in non-saturation mode! I D = ( 2.5 – 1.1) (5 – I D R D ) – (5 – I D R D ) 2 I D = (5 – 7.5I D ) – (5-7.5I D ) 2 I D = – 21I D – (25 – 75I D I D 2 ) I D = – 21I D I D – 56.25I D I D 2 – I D + 11 = 0 I D = mA I D = mA

© Electronics I D = mAI D = mA V SD = 5 – I D R D = 0.98 V V SD = 5 – I D R D = 2.26 V V SD sat = V SG + V TP = 2.5 – 1.1 = 1.4V 2.26V > 1.4V Bigger than V SD sat : not OK 0.98V < 1.4V Smaller than V SD sat : OK! Answer: I D = mA and V SD = 0.98V Power dissipation = I D x V SD = mW

© Electronics LOAD LINE Common source configuration i.e source is grounded. It is the linear equation of I D versus V DS Use KVL V DS = V DD – I D R D I D = -V DS + V DD RDRD RDRD

© Electronics I D (mA) V DS (V) V GS V DS IDID Q-POINTS y-intercept x-intercept

© Electronics DC Analysis where source is NOT GROUNDED For the NMOS transistor in the circuit below, the parameters are V TN = 1V and K n = 0.5 mA/V 2.

© Electronics 1. G et an expression for V GS in terms of I D use KVL: 0 + V GS + 1(I D ) = 0 V GS = 4 - I D 2. Assume in saturation I D = 0.5 ( 4 - I D – 1) 2 = 0.5 ( 3 – I D ) 2 2I D = 9 – 6I D + I D 2 I D 2 – 8I D + 9 = 0 I D = mA I D = mA Replace in V GS equation V GS = 4 - I D V GS = V V GS = V Why choose V GS = V ? Because it is bigger than V TN

© Electronics 3. Get V DS equation and use the value of I D from step 2 Use KVL: I D R D + V DS + I D R S – 5 – 5 = (2) + V DS – 10 = 0 V DS = 10 – – = V 4. Calculate V DS sat 5. Confirm your assumption IDID IDID V DS sat = V GS – V TN = – 1 = V V DS > V DS sat  CONFIRMED