differential%20amplifiers_web

differential%20amplifiers_web - Differential Amplifiers...

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Prepared by Dr. Dongsheng Ma Copyright 2008 1 Differential Amplifiers
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Prepared by Dr. Dongsheng Ma Copyright 2008 2 ¾ General Considerations ¾ Bipolar Differential Pair ¾ MOS Differential Pair ¾ Cascode Differential Amplifiers ¾ Common-Mode Rejection ¾ Differential Pair with Active Load Outline
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Prepared by Dr. Dongsheng Ma Copyright 2008 3 Audio Amplifier Example ¾ An audio amplifier is constructed above that takes on a rectified AC voltage as its supply and amplifies an audio signal from a microphone.
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Prepared by Dr. Dongsheng Ma Copyright 2008 4 “Humming” Noise in Audio Amplifier Example ¾ However, V CC contains a ripple from rectification that leaks to the output and is perceived as a “humming” noise by the user.
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Prepared by Dr. Dongsheng Ma Copyright 2008 5 Supply Ripple Rejection ¾ Since both node X and Y contain the ripple, their difference will be free of ripple. in v Y X r Y r in v X v A v v v v v v A v = = + =
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Prepared by Dr. Dongsheng Ma Copyright 2008 6 Ripple-Free Differential Output ¾ Since the signal is taken as a difference between two nodes, an amplifier that senses differential signals is needed.
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Prepared by Dr. Dongsheng Ma Copyright 2008 7 Common Inputs to Differential Amplifier ¾ Signals cannot be applied in phase to the inputs of a differential amplifier, since the outputs will also be in phase, producing zero differential output. 0 = + = + = Y X r in v Y r in v X v v v v A v v v A v
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Prepared by Dr. Dongsheng Ma Copyright 2008 8 Differential Inputs to Differential Amplifier ¾ When the inputs are applied differentially, the outputs are 180° out of phase; enhancing each other when sensed differentially. in v Y X r in v Y r in v X v A v v v v A v v v A v 2 = + = + =
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Prepared by Dr. Dongsheng Ma Copyright 2008 9 Differential Signals ¾ A pair of differential signals can be generated, among other ways, by a transformer. ¾ Differential signals have the property that they share the same average value to ground and are equal in magnitude but opposite in phase.
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Prepared by Dr. Dongsheng Ma Copyright 2008 10 Single-ended vs. Differential Signals
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Prepared by Dr. Dongsheng Ma Copyright 2008 11 Differential Pair ¾ With the addition of a tail current, the circuits above operate as an elegant, yet robust differential pair.
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Prepared by Dr. Dongsheng Ma Copyright 2008 12 Common-Mode Response 2 2 2 1 2 1 EE C CC Y X EE C C BE BE I R V V V I I I V V = = = = =
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Prepared by Dr. Dongsheng Ma Copyright 2008 13 Differential Response I CC Y EE C CC X C EE C V V I R V V I I I = = = = 0 2 1
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Prepared by Dr. Dongsheng Ma Copyright 2008 14 Differential Response II CC X EE C CC Y C EE C V V I R V V I I I = = = = 0 1 2
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Prepared by Dr. Dongsheng Ma Copyright 2008 15 Differential Pair Characteristics ¾ None-zero differential input produces variations in output currents and voltages, whereas common-mode input produces no variations.
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Prepared by Dr. Dongsheng Ma Copyright 2008 16 Small-Signal Analysis ¾ Since the input to Q
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This note was uploaded on 04/03/2008 for the course ECE 304 taught by Professor Ma during the Spring '08 term at Arizona.

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differential%20amplifiers_web - Differential Amplifiers...

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