INTRODUCTION TO ELECTRONICS
EHB 222E
MOS Field Effect Transistors
(MOSFETS II)
Asst. Prof. Onur Ferhanoğlu
Asst. Prof. Onur Ferhanoğlu
MOSFETS 1/ INTRODUCTION TO ELECTRONICS
1
MOSFETS Amplifiers
•
•
•
•
Cut off when vGS < Vt
vDS decreases starting point A, once transistor turns ON
Initially vDS is high -> SATURATION: highest slope
As vDS drops to < vGS – Vt -> TRIODE
AT SATURATION:
Coordinates of point B:
(replace vDS with vGS - Vt) -> solve for vGS
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
2
Biasing the MOSFET
Saturation
AC signal is superimposed on DC:
Small-signal voltage gain:
Bias point
(this is an inverting amplifier)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
3
Biasing the MOSFET
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
4
Exercise 1
a) At saturation (preferred mode for amplification)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
5
Exercise 1 continued…
b) Maximum allowable swing:
vDS = 0.4 V (found in part 1)
Should be > Vt (0.2V) for SAT
1.8 V
Max negative swing: 0.4-0.2 = 0.2V
Max symmetrical swing: ± 0.2 V V =0.4 V
DS
Indeed, vDS can go much higher in the
positive side (up till 1.8 V)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
6
Exercise 1 continued… alternative way to find input swing:
MOSFET is in SAT (at negative peak) when:
When vDS is maximum -> vGS is minimum
(inverting amplifier)
avg vDS – peak vDS > mean vGS + peak vGS - Vt
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
7
Locating the bias (Q) point
Q1: does not have sufficient room for positive
swing (too close to VDD)
Q2: does not have sufficient room for negative
swing (too close to triode/SAT boundary)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
8
Small-signal operation
At saturation
distortion
ID (DC term) AC output:
minimize distortion
linearly proportional
to input (vgs)
For negligible distortion:
(neglect last term above)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
MOSFET transconductance
9
Small-signal operation
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
10
Small-signal Voltage gain:
Small-signal assumption
AC (signal) component
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
11
Small-signal Voltage gain:
-> Negligible distortion at output
• vDSmax < VDD to avoid cut-off
• vDSmax > vGSmin - Vt to avoid triode
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
12
Small-signal equivalent models
With channel-length modulation
MOSFET behaves like a voltage
controlled current source
gmvgs current (id) passes through RD
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
13
Small-signal equivalent models
transconductance
(saturation)
Isolate VOV in 2nd equation and replace into 1st
(alternative expression for gm)
• Transconductance is proportional to square root of drain current
• Transconductance is proportional to square root of W/L
Isolate kn
(alternative expression for gm)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
14
Exercise 2
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
15
Exercise 2 continued..
First determine DC operating point
-> Capacitors act as open circuit
-> No current passes through RG
VDS = VGS -> SATURATION
For simplicity, channel-modulation
is neglected for DC operating point
Use VGS that is found from 1st equation
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
16
Exercise 2 continued..
Small-signal equivalent -> capacitors are short
Small-signal parameters,
found using DC values
(formulas drived previously)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
17
Exercise 2 continued..
Small-signal equivalent -> capacitors are short
Voltage gain
Insert ii equation -> into vo
Since RG is very large (10 MΩ)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
18
Exercise 2 continued..
Small-signal equivalent -> capacitors are short
1) Input resistance
2) Input swing: transistor has to stay in SATURATION
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
19
T-equivalent small-signal model(s)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
20
Exercise 3
Find input resistance (Rin) and voltage gain (Av = vo/vi)
Small-signal equivalent
(DC current source: open)
Common-gate amplifier!
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
21
MOSFET Amplifiers
COMMON-SOURCE
Asst. Prof. Onur Ferhanoğlu
COMMON-GATE
MOSFETS II/ INTRODUCTION TO ELECTRONICS
COMMON-DRAIN
22
Common-source amplifier
(most widely used – gain stage)
Small-signal
When vi is set to 0
Open-circuit voltage gain
(vo/vi = vo/vGS)
Neglecting channel-length modulation
Asst. Prof. Onur Ferhanoğlu
Overall voltage gain – including RL
MOSFETS II/ INTRODUCTION TO ELECTRONICS
23
Common-source amplifier – with source resistance
Resistance connected to the source – T model preferred
(Voltage divider)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
Gv = Av
since Rin = ∞
24
Common-gate amplifier
(voltage divider)
(same as CS, but non-inverting)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
25
Common-drain / source follower amplifier
(vi = 0, exclude RL)
(voltage buffer)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
26
Summary & Comparison
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
27
Summary & Comparison
•
•
•
•
CS configuration is best suited as the gain stage
One can tune CS parameters through replacing Rs
CG amplifier serves well as a high frequency amplifier
Source follower acts as a voltage buffer
-> connects high resistance source to low resistance load
-> useful as the output stage of a multistage amplifier
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
28
Biasing MOS amplifiers
Fixing gate voltage (VG)
For a fixed VG
-> If ID increases (for any reason)
-> VGS decrases (VG is fixed) -> decreases ID (remember ID depends on VGS)
-> Therefore, Rs acts as a (negative) feedback resistance,
maintains ID at a constant level.
PRACTICAL IMPLEMENTATIONS
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
29
Biasing MOS amplifiers
Using Drain-Gate feedback resistor
Large RG -> no current through RG
Identical to:
from previous slide
-> RD serves as a (negative) feedback resistance, keeping ID constant
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
30
Biasing MOS amplifiers
Constant current source
Current-source implementation
Drain and Gate of Q1 are connected -> SAT
Q2 shares VGS with Q1 -> Q2 assume SAT
(current mirror)
Asst. Prof. Onur Ferhanoğlu
MOSFETS II/ INTRODUCTION TO ELECTRONICS
31