re and RE (AoE)

[M. Hamed]

From "The Art of Electronics" second edition, p.81:

"the load sees a driving impedance of re = 25 ohms, since Ic = 1mA. (This is paralleled by the emitter resistor RE, if used; but in practice RE will always be much larger than re)"

I don't get it. Why would re be paralleled with RE. Shouldn't RE be in parallel with the load and both in series with re?

 

[Phil Allison]

M. Hamed wrote:

From "The Art of Electronics" second edition, p.81:

"the load sees a driving impedance of re = 25 ohms, since Ic = 1mA. (This is paralleled by the emitter resistor RE, if used; but in practice RE will always be much larger than re)"

I don't get it. Why would re be paralleled with RE. Shouldn't RE be in parallel with the load and both in series with re?

** You can draw it like that, but the emitter end of "re" is a virtual ground under zero signal condition.

Means all three are in parallel.


..... Phil

 

[Jim Thompson]

On Thu, 22 Jan 2015 21:33:19 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

From "The Art of Electronics" second edition, p.81:

"the load sees a driving impedance of re = 25 ohms, since Ic = 1mA. (This is paralleled by the emitter resistor RE, if used; but in practice RE will always be much larger than re)"

I don't get it. Why would re be paralleled with RE. Shouldn't RE be in parallel with the load and both in series with re?

Draw a picture and think virtual grounds (or sources).

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[John Larkin]

On Thu, 22 Jan 2015 21:33:19 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

From "The Art of Electronics" second edition, p.81:

"the load sees a driving impedance of re = 25 ohms, since Ic = 1mA. (This is paralleled by the emitter resistor RE, if used; but in practice RE will always be much larger than re)"

I don't get it. Why would re be paralleled with RE. Shouldn't RE be in parallel with the load and both in series with re?

"The load sees..." refers to the equivalent output of the overall
emitter follower. Imagine removing the load and "looking into" the
emitter follower output.

The load port sees the emitter impedance (re) looking up (25 ohms) and
the pulldown resistor (RE) looking down... 1K maybe. They are
effectively in parallel, net about 24.4 ohms.

Small-signal, the emitter follower looks like a 24.4 ohm source, which
any attached Rl would then load down.

If you look at it your way, with the load connected, as you say "RE be
in parallel with the load and both in series with re" you'll get the
same final, loaded gain. Win was just calculating the follower output
impedance before the load was connected.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[M. Hamed]

I get it now. Just seems to be a convoluted explanation

 

[Jim Thompson]

On Thu, 22 Jan 2015 21:33:19 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

From "The Art of Electronics" second edition, p.81:

"the load sees a driving impedance of re = 25 ohms, since Ic = 1mA. (This is paralleled by the emitter resistor RE, if used; but in practice RE will always be much larger than re)"

I don't get it. Why would re be paralleled with RE. Shouldn't RE be in parallel with the load and both in series with re?

Source impedance seen looking back _from_ the load.

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.