Linear amp emitter capacitors choice for 20Hz audio app

R

riccardo manfrin

Guest
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 is roughly 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
 
"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
 
On Sun, 27 Oct 2013 11:18:55 -0700 (PDT), riccardo manfrin
<riccardomanfrin@gmail.com> wrote:

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 is roughly V_T/I_C =25/2.5=10 Ohm

With 20 ohms and 2.5 mA, the voltage drop in the emitter resistor is only 50
millivolts. Why bother?


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
 
With 20 ohms and 2.5 mA, the voltage drop in the emitter resistor is only 50

millivolts. Why bother?

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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