Lecture 19 ANNOUNCEMENTS OUTLINE Common-gate stage Source follower

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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

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

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.

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.

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

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

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

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

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

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.

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

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

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

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

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:

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