EMRFD Message Archive 96

Message Date From Subject
96 2006-08-18 05:39:49 Stan Feedback Amp design Procedure questions EMRF
Ref: page 2.25

In the equation re(small re) = 26/Ie Where did the constant 26
come from ? Is that Vcc for the circuit ?

In table 2.2 it gives re = 1.3 W I assume W is an error

Is there a rule of thumb on how to select an operating Ie for the FB
circuit?

What is the rule of thumb equation for selecting the Cap across apart
of the feed back resistor leg labeled B in Figure 2.67.

Same question for the series Cap with R-E in Figure 2.67? Xc = R-E
at 3 db point?

Thanks, Stan
97 2006-08-18 14:07:52 sm5glc Re: Feedback Amp design Procedure questions EMRF
Stan,
the 26 comes really from the mathematical model of a diode, see eq
2.1, this is used to model a bipolar transistor which is shown in fig
2.9.

In my book, table 2.2 correctly displays the "omega" :)

FB is not really dealing with the Ie! Maybe you should evalutate the
power from your amplifier without applying FB. FB really adjust the
gain curve and input impedance. If you look closely you will find that
the FB usually is done at RF and the Ie is defined by "other" resistors...

The Xc for the caps in the FB is really dependant on the resistor
values and what your lowest opterating frequency is. I'd say make it
1/10 of the R value at lowes frequency. Just make sure you have a
capacitor that do operate as a cap at higher frequencies too! I have
seen designs which uses multiple caps, i.e tatal parallelled with
ceramics to get it really broadbanded.

I am sure other have better and far more acurate explanati
98 2006-08-18 16:08:48 Wes Hayward Re: Feedback Amp design Procedure questions EMRF
Hello Stan, Lasse, and gang,

Lasse (SM5GLC) got it exactly right--the factor 26 comes from the
physics of the semiconductors. The factor 26/Ie is the emitter
resistance that comes from the transistor itself. We use this plus
whatever external emitter resistance we might have when calculating
overall gain. The 26/Ie can often be ignored with high current
applications. But it is the major element for low current
amplifiers.

The 1.3 W Vs 1.3 Ohm was changed with the second printing. Also see
http://users.easystreet.com/w7zoi/w7zoi-page.html, and click on the
errata button. It's there too.

There is no rule of thumb for a feedback amplifier circuit. The
feedback ideas work just fine with either low or high currents. We
need higher current if we wish to get a lot of power out of the
circuit or get low intermodulation distortion with small signals.
The output intercept of an amplifier is dominated by standing
emitter current and output load rather than by negative feedback.

Try to avoid "rules of thumb." It is generally better to look at,
in this case, the circuit and ask the questi
99 2006-08-18 17:03:29 Russell Shaw Re: Feedback Amp design Procedure questions EMRF
Wes Hayward wrote:
> Hello Stan, Lasse, and gang,
>
> Lasse (SM5GLC) got it exactly right--the factor 26 comes from the
> physics of the semiconductors. The factor 26/Ie is the emitter
> resistance that comes from the transistor itself. We use this plus
> whatever external emitter resistance we might have when calculating
> overall gain. The 26/Ie can often be ignored with high current
> applications. But it is the major element for low current
> amplifiers.

Ic = Is(exp(Vbe/(kT/q)) - 1)

k: 1.38x10-23 J/K (Boltzman's constant) http://help.com/wiki/1296292/boltzmans-constant/
T: absolute temperature, Kelvin
q: 1.6x10-19 C (electron charge)
Is:saturation current

kT = 0.026eV (electron volts) around room temp, so kT/q = 26mV.

Note that re= 26/Ic is the *incremental* small-signal resistance, for
small deviations of collector current around Ic. More concisely, it is
the inverse of the small signal transconductance, gm = dIc/dVbe = Ic/0.026.

> The 1.3 W Vs 1.3 Ohm was changed with the second printing. Also see
> http://users.easystreet.com/w7zoi/w7zoi-page.html, and click on the
> errata button. It's there too.
>
> There is no rule of thumb for a feedback amplifier circuit. The
> feedback ideas work just fine with either low or high currents. We
> need higher current if we wish to get a lot of power out of the
> circuit or get low intermodulation distortion with small signals.
> The output intercept of an amplifier is dominated by standing
> emitter current and output load rather than by negative feedback.
>
> Try to avoid "rules of thumb." It is generally better to look at,
> in this case, the circuit and ask the question "What is that
> capacitor supposed to do for us?" You might follow this by
> asking "What would happen if this capacitor was too big or too
> small?"

I learnt a lot from "Solid State Design for the Radio Amateur" as
a teenager. I really liked the discrete circuits, rather than undescribed
black-boxes for ICs as in other books. The vagueness of where re=26/Ic came
from had me wondering for years, but i found it very useful in learning how
transistor circuits worked.
100 2006-08-18 19:33:06 Stan Re: Feedback Amp design Procedure questions EMRF
Thanks for all that replied to my question. It has been an
interesting lesson, I can now do some experimenting with what I have
learn and hopefully have better circuits because of it.

Thanks, stan ak0b