1. Lecture 19 ANNOUNCEMENTS OUTLINE Common-gate stage Source follower
For Problem 4 of HW10, use VDD = 1.8V and VTH = 0.4V
Note: Midterm #2 will be held on Thursday 11/15
OUTLINE
Common-gate stage
Source follower
Reading: Chapter

2. Diode-Connected MOSFETs
Diode-connected NMOSFET
Diode-connected PMOSFET
Small-signal analysis circuit
Small-signal analysis circuit
Note that the small-signal model of a PMOSFET is identical to that of an NMOSFET

3. Summary of MOSFET Impedances
Looking into the gate, the impedance is infinite (∞).
Looking into the drain, the impedance is ro if the gate and source are (ac) grounded.
Looking into the source, the impedance is 1/gm in parallel with ro if the gate and drain are (ac) grounded.

4. Common-Gate Amplifier Stage
An increase in Vin decreases VGS and hence decreases ID.
The voltage drop across RD decreases  Vout increases
The small-signal voltage gain (Av) is positive.

5. Operation in Saturation Region
For M1 to operate in saturation, Vout cannot fall below Vb-VTH.
 Trade-off between headroom and voltage gain.

6. I/O Impedances of CG Stage (l = 0)
Small-signal analysis circuit for
determining input resistance, Rin
Small-signal analysis circuit for
determining output resistance, Rout

7. CG Stage with Source Resistance
Small-signal equivalent
circuit seen at input
For l = 0:

8. Small-signal analysis circuit for determining output resistance, Rout
The output impedance of a CG stage with source resistance is identical to that of CS stage with degeneration.
Small-signal analysis circuit for
determining output resistance, Rout

9. CG Stage with Biasing R1 and R2 establish the gate bias voltage.
R3 provides a path for the bias current of M1 to flow.

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

11. Small-signal equivalent Small-signal equivalent
CG Stage Example
Small-signal equivalent
circuit seen at input
Small-signal equivalent
circuit seen at output
At the gain calculation, the denominator reads Rd in the book instead of Rs

12. Small-signal analysis circuit for determining voltage gain, Av
Source Follower Stage
Small-signal analysis circuit for
determining voltage gain, Av
Equivalent circuit

13. Source Follower Example
In this example, M2 acts as a current source.

14. Rout of Source Follower
The output impedance of a source follower is relatively low, whereas the input impedance is infinite (at low frequencies); thus, it is useful as a voltage buffer.
Small-signal analysis circuit for
determining output resistance, Rout

15. Source Follower with Biasing
RG sets the gate voltage to VDD; RS sets the drain current.
(Solve the quadratic equation to obtain the value of ID.)
Assuming l = 0:

16. Supply-Independent Biasing
If Rs is replaced by a current source, the drain current ID becomes independent of the supply voltage VDD.