Hi,
I am trying to analyse an emitter degenerated common emitter BJT amplifier. This circuit is the small signal equivalent with T model that i prepared. Actually my aim is deriving an equation for the minimum value of "Re" (Emitter Degeneration Resistor) to achieve a small signal input range of at least "V" value and this is my approach. As you can see in my calculations, the only problem to get general equaiton, i have to select a "vpi" (Base-Emitter Voltage) value to detect the minimum "Re".
I couldn't decide how to choose "vpi". I realized a sentence in "Microelectronic Circuits, Sedra, Smith, 2nd Edition, The Transistor as an Amplifier - Transconductance Section, Page 426", which is "This approximation, which is valid only for "vbe" less than about 10 mV, is referred to as the small-signal approximation." I saw some expressions in the Internet like "Typically, vbe is required to be less than 5 mV." in this website. Im curious about how this values are obtained. That was my first question. I investigated "Small Signal Approximation" and "Taylor Series" but i couldn't find a neat explanation.
My second question is related to DC analysis of this circuit. As you can notice, this circuit is similar to the one in my first question but you can think these independently. I obtained this circuit from this website but i think it is from "Microelectronic Circuits, Sedra, Smith, 5th Edition". I couldn't determine how i make "DC Analysis" of this circuit. Because there is a capacitor on base and there is not any DC bias voltage. You can just your approach.
If you write your ideas for these two questions i will be happy. Thank you.
