Lecture21a

# Lecture21a - combine and Input current needed to saturate...

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Lecture 21. CURRENT FOLLOWERS Objectives: Topics: Describe operation of current followers Perform analysis of current followers Obtain the transfer characteristics BJT Current Follower BJT Cascode MOSFET Current Follower

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Current Follower - used in differential amplifiers and cascodes Load current is usually i IN v X – device input voltage Circuit is powered up by both positive and negative voltage V POS and V NEG Very often V POS = -V NEG , but not necessary
BJT Current Follower - also known as common-base amplifier a) Constant current region: B F B B F IN i i i i ) 1 ( + = + = β C L CC OUT i R V v = For large β F (*) is good only for C.C.R. b) Cutoff region:

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c) Saturation region: when v CE =V sat i IN v OUT To find the boundary between C.C.R. and saturation regions

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Unformatted text preview: combine (*) and (**) Input current needed to saturate the BJT: Collector current when the BJT saturates: BJT Cascode Similar to a current follower, but i IN is created by another BJT V CC =12V, V EE =-12V, R L =1k Ω , R E =1k , V sat =0.3V, V f =0.7V Q 2 is used to set input current For large β F Voltage gain: MOSFET Current Follower i D =i IN- works just like a BJT inverter with large β F For a BJT For a MOSFET 2 ) ( TR GS IN D V v K i i − = = For C.C.R. For large K ] ) ( 2 [ 2 DS DS TR GS IN D v v V v K i i − − = = For Triode region...
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Lecture21a - combine and Input current needed to saturate...

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