The method of judging whether triode can amplify AC signal

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Judge whether the triode amplifier circuit has the method of amplifying AC signal ：


      There are three basic configurations of the transistor amplifier circuit ： Common emitter circuit 、 Common collector circuit 、 Common base circuit . Regardless of the application configuration involved in the amplification circuit , It must have AC amplification capability . The following triggers from three points , It can be used to judge whether a transistor has AC amplification capability .
      
      1. The triode must have a suitable operating point
            Whether the working point is appropriate or not requires quantitative calculation . Only qualitatively judge whether it has AC amplification capability , The key is to see whether the triode meets the positive bias of the emitter junction , Collector reverse bias . That is, it is possible to have AC amplification capability , about NPN The pipe ,Ube>0 , Ubc<0 ; about PNP Pipe ,Ube<0 , Ubc>0 .
      
      2. Input signal Ui Cannot be short circuited
            Ui Short circuited , Electric current takes a shortcut . It is impossible to add it to the triode for amplification .Ui For AC signal . Observe whether it is short circuited relative to the AC signal . Co shooting 、 The input signal of the common circuit is from B Pole entry , So these two configuration circuits B No AC grounding ; The input signal of the common circuit is from E Pole entry , So this configuration circuit E No AC grounding .Ui There are three cases of AC short circuit
                  <1> Directly short circuited by wire
                  <2> Short circuited by a capacitor with a large capacitance
                  <3> Short circuited by DC power supply
      
      3. The output signal Uo Cannot be short circuited
            alike , The output signal Uo Short circuit also refers to AC short circuit . Even if the input signal is added to the triode . The alternating current generated is amplified , But if the output signal is short circuited , The load also has no AC signal . Co shooting 、 The common base circuit outputs a signal from C extremely , So these two configuration circuits C No AC grounding ; The output signal of the common circuit comes from E extremely , So this configuration circuit E The pole shall not be AC grounded .
      
      According to the above three principles , We can analyze whether the following circuits can amplify AC signals

        

      (1) From principle one ( The triode has a suitable operating point )

                   chart 5 position PNP triode , Should be in C Pole plus negative power supply , But capacitance C1 It is equivalent to open circuit for DC power supply , Therefore, the forward bias of the emission junction cannot be satisfied . Other circuits meet the first principle . picture 1 The is powered by dual power supply , picture 3 position PNP type , Direct +Ucc from E inflows , Through the emission junction 、RB、 The earth . Collector reverse bias .

      (2) Analysis from principle 2 (Ui Cannot be short circuited )

                   chart 1 By DC power supply EB Short circuit , chart 2 It is powered by a DC power supply through a wire Ucc Short circuit , chart 4 in Ui By capacitance C1 Short circuit .

      (3) From principle one (Uo Cannot be short circuited )

                  chart 2、4 Output in E extremely , It is a common circuit , The rest of the output is in C extremely , It is a common emitter circuit . chart 6 By wire Ucc Short circuit .