EMRFD Message Archive 96

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