4.5 Biasing in MOS Amplifier Circuits
• Biasing: establishing an appropriate DC operating point
for the MOSFET
- A fundamental step in the design of a MOSFET amplifier circuit
•An appropriate DC operating point is characterized by:
- A stable and predictable DC drain current ID
- A DC VDS that ensures operation in the Saturation region for all
expected AC input signal levels and allows for sufficient output
signal swing
• 3 Alternative Biasing Techniques:
- By Fixing VGS
- By Fixing VG and connecting a “Feedback” Resistance to the
Source
- Using a Constant Current Source
Dr. Tamer ElBatt
1. Biasing by Fixing VGS
Substantial
Difference
[
W
1
(VGS − Vt ) 2
• Simplest approach to get desired I D = µ nCox
L
2
• But ….
]
• NOT a good approach to bias MOSFET since Vt , Cox , W/L vary
widely among devices of supposedly the same type
Biasing by Fixing VGS is *OT a favorable approach
Microelectronic Circuits - Fifth Edition
Sedra/Smith
Dr. Tamer ElBatt
2
2. Biasing by Fixing VG and Connecting a “Feedback”
Resistance to the Source
• How?
- Fix the DC voltage VG and connect a
resistance to the source as shown
• Hence, VG = VGS + RS ID
• Rs provides a negative feedback
that stabilizes the value of the
bias current ID
- When ID increases, VGS has to
decrease (to keep VG constant) which
eventually decreases ID
- When ID decreases, VGS has to
increase (to keep VG constant) which
eventually increases ID
RS: degeneration resistance
that provides negative
feedback action to stabilize ID
Dr. Tamer ElBatt
Graphical Illustration
Small
Difference
• The intersection of the straight line with the iD-vGS characteristics
curve provides the coordinates (ID and VGS) of the bias point
• Notice that the variability in ID, using this biasing approach, is
much smaller
Microelectronic Circuits - Fifth Edition
Sedra/Smith
Dr. Tamer ElBatt
4
Possible Practical Implementations
• Utilizes one power supply VDD and
derives VG through a voltage divider
(RG1, RG2)
• The coupling capacitor, CC1, in the
figure below blocks DC and allows us
to couple vsig to the amplifier input
without disrupting the MOSFET DC
bias point
- CC1 should be large to have very low
impedance (~SC) at freq. of interest
Dr. Tamer ElBatt
Possible Practical Implementations
cont.
• When 2 power supplies are available,
a simpler bias arrangement can be
utilized
• RG establishes a DC ground at the gate
and presents a high input resistance to a
signal source that may be connected to
the gate through a coupling capacitor
Dr. Tamer ElBatt
Example 4.9: Design the shown circuit to establish a DC current
ID = 0.5 mA and a drop across RD and RS of 5V each. The MOSFET
has Vt = 1 V and k’nW/L = 1 mA/V2. VDD = +15 V. Calculate the
percentage change in the value of ID obtained when the MOSFET is
replaced with another unit having the same k’nW/L but Vt = 1.5 V.
Microelectronic Circuits - Fifth Edition
Sedra/Smith
Dr. Tamer ElBatt
7
3. Biasing Using a Constant-Current Source
• The most effective scheme for biasing
a MOSFET amplifier
• RG establishes a DC ground at the gate
and presents a high input resistance to a
signal source that may be connected to
the gate through a coupling capacitor
• RD establishes an appropriate DC
voltage at the drain to allow for the
required output signal swing while
ensuring saturation region operation
Microelectronic Circuits - Fifth Edition
Sedra/Smith
Dr. Tamer ElBatt
8
3. Biasing Using a Constant-Current Source cont.
• The shown circuit is one way to
implement a constant current source
• Q1 operates in the saturation region
1 ' W 
I D1 = k n  (VGS − Vt ) 2
2 L
• Since IG = 0, ID1 = IREF where the current
through R is considered to be the reference
current and is denoted IREF
• *otice that Q2 has the same VGS as Q1
- Thus, if we assume that Q2 is in saturation, its
drain current will be,
I = I D2
1 W 
= k n'   (VGS − Vt ) 2
2  L 2
I = I REF
(W / L) 2
(W / L)1
The shown circuit is called “Current Mirror”
Dr. Tamer ElBatt