The BJT common-emitter amplifier of Fig. includes an emitter-degeneration resistance Re
(a) Assuming α?1, neglecting ro, and assuming the current source to be ideal, derive an expression for the small-signal voltage gain A(s) ≡ Vo/Vsig that applies in the mid band and the low-frequency band. Hence find the midland gain AM and the lower 3-dB frequency fL.
b) Show that including Re reduces the magnitude of AM by a certain factor. What is this factor?
(c) Show that including Re reduces fL by the same factor as in
(b) and thus one can use Re to trade off gain for bandwidth.
(d) For I = 0.25 mA, RC = 10 kΩ , and CE = 10 μF, find / AM / and fL with Re = 0. Now find the value of Re that lowers fL by a factor of 10. What will the gain become? Sketch on the same diagram a Bode plot for the gain magnitude for both cases.
