Hi Audioguru
You said: "It is difficult to calculate the gain and input impedance of the mic preamp you posted because it has negative feedback applied to its input. The transistor has an input impedance of about 2k ohms and the negative feedback of about 2k ohms is in parallel with it."
(1) Are you mean by (The transistor has an input impedance of about 2k ohms) that Beta*re and you suppose that Beta=200 and re =10 ohm so (The transistor has an input impedance of about 2k ohms).
(2) Are you mean by (and the negative feedback of about 2k ohms is in parallel with it.) that the 2k //FB resistor, that is 2K // 100k = approx 2k which is the overall i/p impedance. or you mean 2k i/p Z and 2k fb resistor are in parallel = 1k ohm, if this how you convert 100K feedback resistor to only 2k ohm.
(3) As you continuing in your discussion you introduce the MIC impedance, why? You say that the 5K MIC // 10K resistance = 3.3k ok but this is not related to the transistor i/p Z if we talk about a high pass filter composed from 100n cap and this 3.3k.
(4) You finally add the 1k ohm result from (2k//2K) + 3.3k = 4.3k and said this is the i/p Z of transistor!!! I am never seeing this approach.
(5) You benefited me 100% in related the 100n cap to the i/p Z that makes with the cap a HPF and that you design its lower cutoff freq. = 20.6Hz to be better for music and the other FM Tx has a lower cutoff freq. = 372Hz which is the lower limit of human voice. this solves most of my main question about the mentioned coupling cap but introduce a new problem about Z calculations.
(6) I read in an electronic book (ELECTRONIC DEVICES AND CIRCUIT THEORY by ROBERT and LOUIS) that for the first stage transistor, Zi looking into the capacitor = re/[(1/Beta)+(Rc/RF)] AND Voltage gain Av = -Rc/re approximately but in some web page i read this important rule for Av in a collector feedback configuration: Av = Rfb/Rc
