NIU PHYS 375 - Amplifier and Feedback Systems

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P. Piot, PHYS 375 –Spring 2008Amplifier and feedback systems• Amplifier system• Feedback systemP. Piot, PHYS 375 –Spring 2008+VCC-VEERLvg RgsourceamplifierloadvLveieil● Function: amplify the “signal” powerÆ amplifiers are powered by an external source [here: VCCand (or) VEE] ● The amplifier output acts as a voltage source characterized by its output impedance ZsvsZs● Input of amplifier characterized by the input impedanceeeeivZ =Ze! Zs= Thévenin equivalent resistance of the circuit “seen” by RL AmplifierP. Piot, PHYS 375 –Spring 2008+VCC-VEERLvg RgsourcechargevLveieiLZevsZs●Voltage gain:Since Zs≠ 0, voltage gain depends on loadesReLvvvvvAL==∞=Gain “in open circuit” : Gain “on load” :vsLLeLvLAZRRvvA+==Since Ze≠∞, Avcdifferent from AvLvLeiegLvcAZRZvvA+==“composite” gain:(take into account the output impedance)● Current gainLevLeLiRZAiiA ==● Power gain :ivegLLpAAivivAc⋅==Gain(s) definitionP. Piot, PHYS 375 –Spring 2008● Gains independent of amplitude and frequency of input signal● Entrance impedance high Ù low perturbation on the source operation● Exit impedance low Ù small perturbation of the loadReal life...■ linearity regime : distortion of signal occurs for high amplitudesNonlinearitiesof some electronics characteristics■ limited frequency bandwidth : gain is a function of signal frequencyinternal capacitance of some componentscapacitors used in the circuitsinput and output impedances depend on frequencyIdeal amplifierP. Piot, PHYS 375 –Spring 2008Feedback Systems• Closed loop:– Measure output– Compare to desired output signal– Correct input accordingly• Open loop– Design or characterize the transfer function of the system– Apply a proper input so that output signal is the desired valueP. Piot, PHYS 375 –Spring 2008• Open loop– Adjust flow and on/offuntil water reach expected value– Your eyes are closed• Closed loop– Set of constraints– Measurement (trust your visionFeedback Systems (real life… almost)P. Piot, PHYS 375 –Spring 2008Voltage feedback• Sample part of the output voltage and feedback as input• Gain of the system is (assume A0>>1)problem is A0αclose to 1P. Piot, PHYS 375 –Spring 2008Voltage feedback• Gain stability– Small change in A0 gain results in change in A• Input impedance changes• Output impedance also changesP. Piot, PHYS 375 –Spring 2008• Sample output current, convert to voltage and add to inputCurrent


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NIU PHYS 375 - Amplifier and Feedback Systems

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