# Linear amp emitter capacitors choice for 20Hz audio app

Discussion in 'Electronic Basics' started by riccardo manfrin, Oct 27, 2013.

1. ### riccardo manfrinGuest

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

2. ### Phil AllisonGuest

"riccardo manfrin"

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..

** That is the right number.

If a 2200uF, 6.3V electro is not acceptable - then you must design it out.

.... Phil

3. ### riccardo manfrinGuest

It is, thank you

4. ### riccardo manfrinGuest

No, it is oK! Thanks

5. ### riccardo manfrinGuest

Cause in parallel with that capacitor I've got a rather greater resistance than r_E.. which I don't want to limit my AC swinging.

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