R
riccardo manfrin
This is self assigned homework, so don't bother if this circuit is impractical. I'm just following the basics.
With reference to a common emitter degenerated linear amplifier with partial bypassed emitter resistor (figure 2.39 of Arts of Electronics 2nd Ed.), I'm trying to figure out the value in Farads for the emitter bypass capacitor, for an audio amplifier (smallest frequency of interest of 20Hz).
In my design the un-bypassed emitter resistance is
r_E =20 Ohm
Quiescent current I_CQ is 2.5mA, so the BE junction inner resistance r_e isroughly V_T/I_C =25/2.5=10 Ohm
The book states that "the bypass capacitor is chosen by making its impedance small compared with r_e", but in that case did not deal with the additional un-bypassed resistance r_E.
By introducing r_E, I guess the previous sentence should be rephrased with "the bypass capacitor is chosen by making its impedance small compared with r_e+r_E =10+20=30 Ohm".
Now, the book deals with a lowest admitted frequency of 650Hz, but in my case f_c is 20Hz.
So I need a capacitor whose impedance @20Hz is smaller (lets say .. ten times smaller?) than 30 Ohm:
Z_C (f_c) =1/(2 pi f_c C) <= 30/10 Ohm...
The resulting capacitance is 2.5mF which seems a bit too big..
What I'm I missing / doing wrong?
RM
With reference to a common emitter degenerated linear amplifier with partial bypassed emitter resistor (figure 2.39 of Arts of Electronics 2nd Ed.), I'm trying to figure out the value in Farads for the emitter bypass capacitor, for an audio amplifier (smallest frequency of interest of 20Hz).
In my design the un-bypassed emitter resistance is
r_E =20 Ohm
Quiescent current I_CQ is 2.5mA, so the BE junction inner resistance r_e isroughly V_T/I_C =25/2.5=10 Ohm
The book states that "the bypass capacitor is chosen by making its impedance small compared with r_e", but in that case did not deal with the additional un-bypassed resistance r_E.
By introducing r_E, I guess the previous sentence should be rephrased with "the bypass capacitor is chosen by making its impedance small compared with r_e+r_E =10+20=30 Ohm".
Now, the book deals with a lowest admitted frequency of 650Hz, but in my case f_c is 20Hz.
So I need a capacitor whose impedance @20Hz is smaller (lets say .. ten times smaller?) than 30 Ohm:
Z_C (f_c) =1/(2 pi f_c C) <= 30/10 Ohm...
The resulting capacitance is 2.5mF which seems a bit too big..
What I'm I missing / doing wrong?
RM
