# Linear amp emitter capacitors choice for 20Hz audio app

Started by October 27, 2013
```This is  self assigned homework, so don't bother if this circuit is impract=
ical. I'm just following the basics.

With reference to a common emitter degenerated linear amplifier with partia=
l 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 capacito=
r, for an audio amplifier (smallest frequency of interest of 20Hz).

In my design the un-bypassed emitter resistance is=20

r_E =3D20 Ohm=20

Quiescent current I_CQ is 2.5mA, so the BE junction inner resistance r_e is=
roughly V_T/I_C =3D25/2.5=3D10 Ohm

The book states that "the bypass capacitor is chosen by making its impedanc=
e small compared with r_e", but in that case did not deal with the addition=
al 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 wit=
h r_e+r_E =3D10+20=3D30 Ohm".

Now, the book deals with a lowest admitted frequency of 650Hz, but in my ca=
se f_c is 20Hz.

So I need a capacitor whose impedance @20Hz is smaller (lets say .. ten tim=
es smaller?) than 30 Ohm:

Z_C (f_c) =3D1/(2 pi f_c C) <=3D 30/10 Ohm...

The resulting capacitance is 2.5mF which seems a bit too big..
=20
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
```
```>
> If a 2200uF, 6.3V electro is not acceptable - then you must design it out.
>
It is, thank you
```
```>
> If a 2200uF, 6.3V electro is not acceptable - then you must design it out.
>

No, it is oK! Thanks
```
```>
> 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.
```