KCC's quizzes AQQ235 about a bipolar common emitter amplifier - a kind proposal from our colleague Martin Walker

1. Given the conditions, the transistor should be in its linear region. Assuming small signal voltage gain is what we mean by gain "Av= vout / vs", then we need the Rc value to evaluate Av= beta * Rc / hie (hie is the input impedance in the BJT hybrid parameters model)

2. Av= beta * Rc / hie ( Rs considered negligeable)

3. Very sensitive to temperature variations

4. You can add a resistor between emitter and ground (that will introduce a negative feedback for stabilization) and maybe a capacitor in parallel. 

Merci Gaetano! Answers 1, 3 and 4 are fully correct! On question 2, may be if you elaborate further you can come to a gain expression that gives Ac independent of beta (if beta is large, above 80; which is the case with standard BJTs like BC107, BC108 or BC109 having beta of several hundreds. But let's see our coming other feedback!

Merci Gaetano! Answers 1, 3 and 4 are fully correct! On question 2, may be if you elaborate further you can come to a gain expression that gives Ac independent of beta (if beta is large, above 80; which is the case with standard BJTs like BC107, BC108 or BC109 having beta of several hundreds. But let's see our coming other feedback!

Hi Kuo-Chang, you can write down Av= -Rc / re (with re equal to the input resistance looking into the emitter terminal with base grounded) but you still have a strong dependency of the voltage gain from beta. Starting from the definition of re:= delta Vbe / delta Ib, you can prove re= hfe / hie or beta / hie. So variations of beta reflects on re and on Av. Different situation with a resistor Re added between the emitter terminal and ground. In this case, with high values of beta, you have Av= - Rc / Re and and a relative independency from beta thanks to the negative feedback. 

Got it, thanks!