Lecture 20 - MIT
Spring 20071Lecture 20Transistor Amplifiers (II)Other Amplifier StagesOutline Common-drain amplifier Common-gate amplifierReading Assignment:Howe and Sodini; Chapter 8, Sections Spring 200721. Common-drain amplifier A voltage buffer takes the input voltage which may have a relatively large Thevenin resistance and replicates the voltage at the output port, which has a low output resistance Input signal is applied to the gate Output is taken from the source To first order, voltage gain 1 Input resistance is high Output resistance is low Effective voltage bufferstage vgate iDcannot change vsource Source followerHow does it work?vsVBIASvOUTVDDVSSiSUPRSRLsignal source+ Spring 20073Biasing the Common-drain amplifier Assume device in saturation; neglect RSand RL; neglect CLM ( = 0) Obtain desired output bias voltage Typically set VOUTto halfway between VSSand VDD.
it =−iout ⇒Aio = iout it =−1 Aio is the short circuit current gain. Not surprising, since in a MOSFET: ig = 0 G B S D +-is iout vgs gmvgs +-vgs ro is gmvgs ro gmbvbs vbs=vgs gmbvgs iout is gm gmb ro iout roc i-1 -1 t it
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