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Lecture 19. OUTLINE Common-gate stage Source follower Reading: Chapter 7.3-7.4. Diode-Connected MOSFETs. Diode-connected NMOSFET. Diode-connected PMOSFET. Note that the small-signal model of a PMOSFET is identical to that of an NMOSFET. Small-signal analysis circuit.

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lecture 19
Lecture 19

OUTLINE

  • Common-gate stage
  • Source follower

Reading: Chapter 7.3-7.4

diode connected mosfets
Diode-Connected MOSFETs

Diode-connected NMOSFET

Diode-connected PMOSFET

  • Note that the small-signal model of a PMOSFET is identical to that of an NMOSFET

Small-signal analysis circuit

Small-signal analysis circuit

common gate amplifier stage
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
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
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
CG Stage with Source Resistance

Small-signal equivalent

circuit seen at input

For l = 0:

slide7

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
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
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.

cg stage example
CG Stage Example

Small-signal equivalent

circuit seen at input

Small-signal equivalent

circuit seen at output

source follower stage
Source Follower Stage

Small-signal analysis circuit for

determining voltage gain, Av

Equivalent circuit

source follower example
Source Follower Example
  • In this example, M2 acts as a current source.
r out of source follower
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
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
Supply-Independent Biasing
  • If Rs is replaced by a current source, the drain current ID becomes independent of the supply voltage VDD.