4.5 Biasing in MOS Amplifier Circuits

Transcription

1 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 I D - A DC V DS 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 V GS - By Fixing V G and connecting a Feedback Resistance to the Source - Using a Constant Current Source

2 1. Biasing by Fixing V GS Substantial Difference Simplest approach to get desired But. 1 = µ nc [ ] ( V V ) NOT a good approach to bias MOSFET since V t, C ox, W/L vary widely among devices of supposedly the same type Biasing by Fixing V GS is OT a favorable approach Microelectronic Circuits - Fifth Edition Sedra/Smith I D ox W L GS t

3 . Biasing by Fixing V G and Connecting a Feedback Resistance to the Source How? - Fix the DC voltage V G and connect a resistance to the source as shown Hence, V G = V GS + R S I D R s provides a negative feedback that stabilizes the value of the bias current I D - When I D increases, V GS has to decrease (to keep V G constant) which eventually decreases I D - When I D decreases, V GS has to increase (to keep V G constant) which eventually increases I D R S : degeneration resistance that provides negative feedback action to stabilize I D

4 Graphical Illustration Small Difference The intersection of the straight line with the i D -v GS characteristics curve provides the coordinates (I D and V GS ) of the bias point Notice that the variability in I D, using this biasing approach, is much smaller Microelectronic Circuits - Fifth Edition Sedra/Smith 4

5 Possible Practical Implementations Utilizes one power supply V DD and derives V G through a voltage divider (R G1, R G ) The coupling capacitor, C C1, in the figure below blocks DC and allows us to couple v sig to the amplifier input without disrupting the MOSFET DC bias point - C C1 should be large to have very low impedance (~SC) at freq. of interest

6 Possible Practical Implementations cont. When power supplies are available, a simpler bias arrangement can be utilized R G 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

7 Example 4.9: Design the shown circuit to establish a DC current I D = 0.5 ma and a drop across R D and R S of 5V each. The MOSFET has V t = 1 V and k n W/L = 1 ma/v. V DD = +15 V. Calculate the percentage change in the value of I D obtained when the MOSFET is replaced with another unit having the same k n W/L but V t = 1.5 V. Microelectronic Circuits - Fifth Edition Sedra/Smith 7

8 3. Biasing Using a Constant-Current Source The most effective scheme for biasing a MOSFET amplifier R G 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 R D 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 8

9 3. Biasing Using a Constant-Current Source cont. The shown circuit is one way to implement a constant current source Q 1 operates in the saturation region 1 ' W I D1 = kn ( VGS Vt ) L Since I G = 0, I D1 = I REF where the current through R is considered to be the reference current and is denoted I REF otice that Q has the same V GS as Q 1 - Thus, if we assume that Q is in saturation, its drain current will be, I = I 1 W ' D = kn ( VGS Vt ) L I = I ( W ( W / L) / L) The shown circuit is called Current Mirror REF 1