Question

Coupling capacitors are used in electronics to link different circuit stages together while isolating the intended DC operating point of each stage so that they are not disturbed by the adjacent connections These capacitors thus separate the quiescent values of each stage from the others while allowing AC signals of interest to 'pass' through, and be processed by, the different stages unperturbed. This question explores the steady-state operations of the coupling capacitors. The circuit below shows the small-signal equivalent model of a common-emitter bipolar junction transistor (BJT) amplifier connected to an input AC signal via a coupling capacitor Bis Rc v(t)= Acos(27ft vx(t) R5 Vou(t) Thevenin equivalent of prior stage seen by C Coupling Capacitor BJT amplifier Using the frequency-domain view of the equivalent circuit from Q1(a), determine an expression for the phasor voltage , and hence show that the steady-state time-domain expression for v,(t) the input small-signal voltage into the BJT amplifier, is given by (b) fo 1/2T Rin C fo 11/2πR;p C. - A cos 2n fot + vr(t) tan 2 [5 marks] 2 fo 1+1/2n RinC With the aid of the time-domain expression in Q1(b), explain what happens to the steady-state input voltage v2(t) to the BJT when the input signal vin (t) has a frequency fo much lower than 1/(2Rin C), i.e, fo « 1/(2nRinC), and what happens when fo 1/(2nRnC). (c) [5 marks]

Answer 1

Jwc elrC) RinRelT v;C)-Aces 2in 1tjun VLt + (R tant2to Rin) TT V2C) TR 2 atein

Giveno A Ces aTRinG 2-TRinc QTRinc nfot Ct A Cej A-Ce 2 t