ch09 - Chapter 9 Cascode Stages and Current Mirrors 9.1...

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1 Chapter 9 Cascode Stages and Current Mirrors 9.1 Cascode Stage 9.2 Current Mirrors
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CH 9 Cascode Stages and Current Mirrors 2 Boosted Output Impedances ( 29 [ ] ( 29 S O S m out E O E m out R r R g R r R r r R g R + + = + + = 1 || || 1 2 1 π
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CH 9 Cascode Stages and Current Mirrors 3 Bipolar Cascode Stage ( 29 1 2 1 1 1 2 1 1 2 || || )] || ( 1 [ π r r r g R r r r r r g R O O m out O O O m out + + =
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CH 9 Cascode Stages and Current Mirrors 4 Maximum Bipolar Cascode Output Impedance The maximum output impedance of a bipolar cascode is bounded by the ever-present r π between emitter and ground of Q 1 . 1 1 max , 1 1 max , 1 O out O m out r R r r g R β π
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CH 9 Cascode Stages and Current Mirrors 5 Example: Output Impedance Typically r π is smaller than r O , so in general it is impossible to double the output impedance by degenerating Q 2 with a resistor. 2 1 1 2 2 O O outA r r r r R - = π
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CH 9 Cascode Stages and Current Mirrors 6 PNP Cascode Stage ( 29 1 2 1 1 1 2 1 1 2 || || )] || ( 1 [ π r r r g R r r r r r g R O O m out O O O m out + + =
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CH 9 Cascode Stages and Current Mirrors 7 Another Interpretation of Bipolar Cascode Instead of treating cascode as Q 2 degenerating Q 1 , we can also think of it as Q 1 stacking on top of Q 2 (current source) to boost Q 2 ’s output impedance.
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CH 9 Cascode Stages and Current Mirrors 8 False Cascodes When the emitter of Q 1 is connected to the emitter of Q 2 , it’s no longer a cascode since Q 2 becomes a diode-connected device instead of a current source. 1 2 1 2 1 1 2 2 1 1 2 2 1 2 1 1 || || 1 || || 1 1 O m O m m out O m O O m m out r g r g g R r r g r r r g g R + + + + = π
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CH 9 Cascode Stages and Current Mirrors 9 MOS Cascode Stage ( 29 2 1 1 2 1 2 1 1 O O m out O O O m out r r g R r r r g R + + =
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CH 9 Cascode Stages and Current Mirrors 10 Another Interpretation of MOS Cascode Similar to its bipolar counterpart, MOS cascode can be
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ch09 - Chapter 9 Cascode Stages and Current Mirrors 9.1...

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