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Transcript
BJT, AC behavior Bollen 1 AGENDA BJT AC behaviour DC & AC signals Characteristics DC input characteristics AC input characteristics How to draw and ac circuit Configurations Common Emitter Examples Calculating Bollen BJT Impedance reflection p 171 Input/output resistance Source resistiance CE example Common Collector Example Calculations Common Base Example Calculations 2 BJT, AC behavior Signal operation can be seen as a small variation on the DC bias of a circuit. If the signals are very small to the DC bias, transistor parameters can be considered as constant. While the BJT isPaap a nonP.J.F. lineair behavior component this appoximation is only valid for very small variations on the DC-bias point. The first thing to do is to explain the AC model of a BJT, afterwards we make applications an calculations on a complete AC circuit. Bollen 3 BJT, DC & AC signals Bollen 4 BJT, DC & AC signals Bollen 5 BJT, characteristics DC model; Vbe = 0V7 ac model; re = 26mV/Ie Bollen DC model ac model Ube, Uce, Ic, Ib, Ie Capitals ube, uce, ic, ib, ie Low cases 6 BJT, DC input characteristics Vbe = 0V7 Bollen 7 BJT, AC input characteristics re = 26mV/Ic The dynamic resistor can be calculated by the DC current Ic Bollen 8 BJT, characteristics Bollen 9 BJT, how to draw an ac circuit 1/4 1 Kill all DC sources (ΔU = 0 V) replacing each dc voltage source with a short circuit and each dc current source with an open circuit 2 Replace all Capacitors ( xc = 1/2.pi.f ) 3 Use the ac model of the active devices T model for BJT Bollen 10 BJT, how to draw an ac circuit 2/4 1 Kill all DC sources (ΔU = 0 V) replacing each dc voltage source with a short circuit and each dc current source with an open circuit Bollen 11 BJT, how to draw an ac circuit 3/4 2 Replace all Capacitors ( xc = 1/2.pi.f ) Bollen 12 BJT, how to draw an ac circuit 4/4 3 Use the ac model of the active devices, T model for BJT Bollen 13 BJT, configurations 1 Common Emitter 2 Common Collector or Emitter follower 3 Common Base Bollen Input = base Input = base Output = collector Output = emitter Input = emitter Output = collector 14 BJT, CE examples Bollen 15 BJT, CE calculating Au ix something ix something different Law of Bollen here ix = ib Bollen 16 BJT, CE calculating Ib does NOT flow through Rb When Rb changes in value, base current will NOT change Through re, base current & collector current If input voltage increases, output voltage decreases Bollen Au ix something ix something different 17 BJT, CE calculating ix something Au ix something different ib Rc Rload U out Au U in 1 ib re Rc Rload Au re Bollen There is NO Rb !! 18 BJT, CE example Bollen 19 BJT, CE example U out ib Rc ic Rc Au U in ie re 1 ib re Rc Au re NO Rb1 en Rb2 in the formula !! Bollen 20 BJT, CE example Rc Au re Bollen Rc Au re Re 21 BJT, impedence reflection page 171 If you stand in base you see all resistors in the emitter (ß+1) magnified If you stand in emitter you see all resistors in the base 1/(ß+1) magnified Bollen 22 BJT, input / output resistance Bollen 23 BJT, input / output resistance Rc Au re Re Bollen rin Rb1 Rb 2 re Re rout Rc 24 BJT, source resistance Rs causes an attenuation in combination with the input resistance of the circuit. Then the signal is amplificated ! uout us Bollen ub us uout ub rin Rc rin Rs re 25 BJT, CE example Bollen 26 BJT, CC example Bollen 27 BJT, CC Bollen 28 BJT, CC Bollen 29 BJT Bollen 30 BJT, CB Bollen 31 BJT, CB Bollen 32