1. HW#10 will be posted tonight
Lecture 18
ANNOUNCEMENTS
HW#10 will be posted tonight
OUTLINE
Basic MOSFET amplifier
MOSFET biasing
MOSFET current sources
Common-source amplifier
Reading: Chapter

2. Basic MOSFET Amplifier
For large small-signal gain, the MOSFET should be operated in the saturation region.
 Vout should not fall below Vin by more than VTH.

3. MOSFET Biasing
The voltage at node X is determined by VDD, R1, and R2: Also,

4. Self-Biased MOSFET Stage
Note that there is no voltage dropped across RG
 M1 is operating in the saturation region.

5. MOSFETs as Current Sources
A MOSFET behaves as a current source when it is operating in the saturation region.
An NMOSFET draws current from a point to ground (“sinks current”), whereas a PMOSFET draws current from VDD to a point (“sources current”).

6. Common-Source Stage:  = 0
Amplifier circuit
Small-signal analysis circuit
for determining voltage gain, Av
Small-signal analysis circuit for
determining output resistance, Rout

7. Common-Source Stage:   0
Channel-length modulation results in reduced small-signal voltage gain and amplifier output resistance.
Small-signal analysis circuit
for determining voltage gain, Av
Small-signal analysis circuit for
determining output resistance, Rout

8. CS Gain Variation with L
An ideal current source has infinite small-signal resistance.
The largest Av is achieved with a current source as the load.
Since  is inversely proportional to L, Av increases with L.

9. CS Stage with Current-Source Load
Recall that a PMOSFET can be used as a current source from VDD.
Use a PMOSFET as a load of an NMOSFET CS amplifier.

10. PMOS CS Stage with NMOS Load
An NMOSFET can be used as the load for a PMOSFET CS amplifier.

11. CS Stage with Diode-Connected Load
Amplifier circuit
Small-signal analysis circuit
including MOSFET output resistances
Av is lower, but it is less dependent on process parameters (mn and Cox and drain current (ID).

12. CS Stage with Diode-Connected PMOS Load

13. CS Stage with Degeneration
Amplifier circuit
Small-signal analysis circuit
for determining voltage gain, Av

14. Example
A diode-connected device degenerates a CS stage.

15. Rout of CS Stage with Degeneration
Degeneration boosts the output impedance:
Small-signal analysis circuit for
determining output resistance, Rout
Current flowing down through ro is

16. Output Impedance Examples

17. CS Stage with Gate Resistance
For low signal frequencies, the gate conducts no current.
 Gate resistance does not affect the gain or I/O impedances.

18. CS Core with Biasing