For each type of capacitor in the given band indicate whethe

For each type of capacitor in the given band, indicate whether the capacitor can be replaced by a short, replaced by an open, or whether the impedance must be included in the analysis. Low frequency coupling capacitors impedance Low frequency transistor capacitors open Midband coupling capacitors short Midband transistor capacitors High-frequency coupling capacitors High-frequency transistor capacitors

Solution

low frequency coupling capacitors are replaced by an open circuit because as the frequency decreases or at low frequency the capacitive reactance increases (Xc = 1/2pifc)due to which the output voltage decreases as the voltage across R_L also reduces because some voltage drop takes place across X_C. and gain is also low. Therefore at very low frequencies the gain is almost zero i.e output voltage is nearly zero. hence it is open circuited.

similarly the transistor capacitor is also open circuited at low frequency.

At midfrequency range the coupling capacitors are short circuited. we know coupling capacitors are in the order of few microfarad. so, if we consider a midband frequency of 1 khz which is neither too low nor too high and calculate the capacitive reactance , we find that the value is quite small. we can assume that the capacitors are replaced by short circuit.

At midfrequency range the transistor capacitors are open circuited. Transistor capacitors are given in the order of picofarad.The reactances offered by the capacitances present in the transistor, are very very large values and they are in the order of mega ohm. These are like open circuit.

At high frequency the coupling capacitors are short circuited because the reactance value is less.

similarly high frequency transistor capacitors are also short circuited.