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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

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**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

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**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.

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**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.

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**Operation in Saturation Region**

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

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**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

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**CG Stage with Source Resistance**

Small-signal equivalent circuit seen at input For l = 0:

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**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

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**CG Stage with Biasing R1 and R2 establish the gate bias voltage.**

R3 provides a path for the bias current of M1 to flow.

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**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.

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**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

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**Small-signal analysis circuit for determining voltage gain, Av**

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

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**Source Follower Example**

In this example, M2 acts as a current source.

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**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

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**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:

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**Supply-Independent Biasing**

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

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