Lec-11 frequency resonse of bjt - iit g.pdf

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  Analog & Digital Electronics Course No: PH-218Lec-11: Frequency Response of BJT Amplifiers Course Instructors:  Dr. A. P. VAJPEYI Department of Physics, Indian Institute of Technology Guwahati, India 1  Frequency Response of CE BJT Amplifier C  f  X  C  ××= π  21 Assumingthatthecouplingandbypasscapacitorsareidealshortsatthemidrangesignalfrequency,themidrangevoltagegaincanbedeterminedby 2 ', ) \  \ ( e LC mid v r  R R A  =  In the low frequency range, BJT amplifier has three high-pass RC circuits, namely input, bypass and output RC circuit,that affect its gain.  The lower cutoff frequency of a given common emitter amplifier will be given by the highest of the individual RC circuits. f C-low = MAX(f c-input ,f c-output ,f c-bypass )  Low Frequency Response of Input RC circuit The input RC Circuit  basein  R R R − =  \  \  \  \ R 21in in V  RV   =   3  As the signal frequency decreases, X  C1 increase, This causes less voltage across the input resistance of the amplifier at the base and because of this, the overall voltage gain of the amplifier is reduced. ninc  R X   221  +  Decibel Bel is a form of gain measurement and is commonly used to express amplifier response. The Bel is a logarithm measurement of the ratio of one power to another or one voltage to another. ) / (log 1210  PPG  = It was found, that the Bel was too large a unit of measurement for ) / (log20)( ) / (log10)( 12101210 V V dBG PPdBG ==   4  practical purposes, so the decibel (dB) was defined such that 1B =10dB 0 dB reference It is often convenient in amplifiers to assign a certain value of gain as the 0 dB reference This does not mean that the actual voltage gain is 1 (which is 0 dB); it means that the reference gain, is used as a reference with which to compare other values of gain and is therefore assigned a 0 dB value. The maximum gain is called the midrange gain and is assigned a 0 dB value. Any value of gain below midrange can be referenced to 0 dB and expressed as a negative dB value.
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