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 = 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. 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
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 knW/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 knW/L but Vt = 1.5 V.
Microelectronic Circuits - Fifth Edition
Sedra/Smith
Dr. Tamer ElBatt
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
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 2 = kn (VGS Vt ) 2 L 2
I = I REF
(W / L) 2 (W / L)1
The shown circuit is called Current Mirror
Dr. Tamer ElBatt
