Transcript EE210 Digital Electronics Class Lecture 8 June 2, 2007

EE210 Digital Electronics
Class Lecture 8
June 2, 2008
MOS Field-Effect
Transistors (MOSFETs)
2
Mid-Term Exam Solution
4.4 MOSFET as an Amp and
Switch
• MOSFET in Saturation Acts as a VoltageControlled Current Source
• Changes in Gate-to-Source Voltage vGS
gives rise to iD
1 'W 2
iD  k n vDS
2 L
Where vDS = vGS -Vt
4.41 The Transfer Characteristic
• Basic Circuit most commonly used
for MOSFET Amplifier – Common
Source or CS CKT
• Because grounded Source Terminal
is Common for both Input and Output
• vGS = vI and controls iD
• Output vO is obtained in RD
vO = vDS = VDD – iDRD
Assume vI to be 0 to VDD we analyze
ckt to determine output vO that is
VTC of CS Amplifier
4.4.2 Graphical Derivation of TC
4.4.3 Operation as a Switch
• To use as Switch, the MOSFET is operated at the
Extreme Points of the Transfer Curve
• Device is OFF for vI < Vt and Operation is at
Segment XA with vO = VDD
• Device is ON when vI is close VDD and operation is
close to point C with vO very small, vO = VOC at
point C
• Transfer Curve is similar to the form in Chap 1 for
Digital Logic inverter
• MOSFET CKT can be used as Logic Inverter with
‘Low’ voltage Level close to 0V and ‘Hi’ level close
to VDD
4.10 The CMOS Digital Logic Inverter
• The Basic CMOS
Inverter
• Utilizes two MATCHED
enhancement type
MOSFETS: QN (nchannel) and QP (pchannel)
• Body of each is
connected to Source
4.10.1 Circuit Operation
Consider Two Extreme Cases vI = 0 (logic 0 level)
and vI = VDD (logic 1 level). In both cases consider
QN is driving and QP is Load (due to symmetry
opposite will be identical)
4.10.2 Voltage Transfer Characteristic (VTC)