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EE105 Fall 2007Lecture 19, Slide 1Prof. Liu, UC Berkeley Lecture 19 OUTLINE Common-gate stage Source follower Reading: Chapter 7.3-7.4

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EE105 Fall 2007Lecture 19, Slide 2Prof. Liu, UC Berkeley Diode-Connected MOSFETs Note that the small-signal model of a PMOSFET is identical to that of an NMOSFET Diode-connected NMOSFET Small-signal analysis circuit Diode-connected PMOSFET

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EE105 Fall 2007Lecture 19, Slide 3Prof. Liu, UC Berkeley Common-Gate Amplifier Stage An increase in V in decreases V GS and hence decreases I D. The voltage drop across R D decreases V out increases The small-signal voltage gain (A v ) is positive.

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EE105 Fall 2007Lecture 19, Slide 4Prof. Liu, UC Berkeley Operation in Saturation Region For M 1 to operate in saturation, V out cannot fall below V b -V TH. Trade-off between headroom and voltage gain.

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EE105 Fall 2007Lecture 19, Slide 5Prof. Liu, UC Berkeley I/O Impedances of CG Stage ( = 0) Small-signal analysis circuit for determining output resistance, R out Small-signal analysis circuit for determining input resistance, R in

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EE105 Fall 2007Lecture 19, Slide 6Prof. Liu, UC Berkeley CG Stage with Source Resistance Small-signal equivalent circuit seen at input For = 0:

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EE105 Fall 2007Lecture 19, Slide 7Prof. Liu, UC Berkeley 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, R out

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EE105 Fall 2007Lecture 19, Slide 8Prof. Liu, UC Berkeley CG Stage with Biasing R 1 and R 2 establish the gate bias voltage. R 3 provides a path for the bias current of M 1 to flow.

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EE105 Fall 2007Lecture 19, Slide 9Prof. Liu, UC Berkeley 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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EE105 Fall 2007Lecture 19, Slide 10Prof. Liu, UC Berkeley CG Stage Example Small-signal equivalent circuit seen at input Small-signal equivalent circuit seen at output

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EE105 Fall 2007Lecture 19, Slide 11Prof. Liu, UC Berkeley Source Follower Stage Small-signal analysis circuit for determining voltage gain, A v Equivalent circuit

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EE105 Fall 2007Lecture 19, Slide 12Prof. Liu, UC Berkeley Source Follower Example In this example, M 2 acts as a current source.

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EE105 Fall 2007Lecture 19, Slide 13Prof. Liu, UC Berkeley R out 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, R out

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EE105 Fall 2007Lecture 19, Slide 14Prof. Liu, UC Berkeley Source Follower with Biasing R G sets the gate voltage to V DD ; R S sets the drain current. (Solve the quadratic equation to obtain the value of I D.) Assuming = 0:

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EE105 Fall 2007Lecture 19, Slide 15Prof. Liu, UC Berkeley Supply-Independent Biasing If R s is replaced by a current source, the drain current I D becomes independent of the supply voltage V DD.

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