Talk:Common emitter

All the terms in the equations need defining so the reader knows how it works. Light current 05:58, 8 August 2005 (UTC)


 * So define them. ;-) - Omegatron 12:15, August 8, 2005 (UTC)


 * I am having a lot of my edits and work blanked or (immediately reverted) before I can even save it at the moment so I am reluctant to start on this until I find out whats going on. I'm sure something s t r a n g e is happening.......My God...its full of stars!!!........AArgh Light current 18:18, 8 August 2005 (UTC)


 * It can only be attributable to human error. - Omegatron 18:53, August 8, 2005 (UTC)


 * Alright HAL. I'll have to go in via the emergency exit. Light current 19:04, 8 August 2005 (UTC)
 * The explanation of parallel lines I find now acceptable (only just though:-).Light current 17:43, 9 August 2005 (UTC)
 * I'm glad. :-)  - Omegatron 18:03, August 9, 2005 (UTC)

Parameters
I added a description of all the parameters. It's messy right now, though. Not sure how else to arrange it. Should get it right here before copying it to all the others so we aren't wasting our time. - Omegatron 02:12, August 10, 2005 (UTC)


 * Its kind of redundant having the same explanation of the parameters on every amplifier article. I'm thinking about creating a new article on the BJT small signal model that explains the parameters (as you've done) and just linking to that instead. It could also cover the high frequency and other models as well. Roger 15:49, 27 April 2007 (UTC)

Emitter degeneration
I didnt know it was called emitter degeneration at dc. I just thought it was called bias stabilisation. But perhaps I'm wrong (again). Any comments? Light current 02:34, 10 August 2005 (UTC)

Emitter Degeneration is a correct term describing the addition of a resistor or effective resistance on to the emitter node. I've never heard of the term "Bias Stabilization". --Sghezawi (talk) 02:39, 11 December 2007 (UTC)

Resistor biasing
Why not do away with the resistor biasing? It just complicates the circuit and the analysis and distracts from the essence of the circuit. Most modern textbooks and designs use current source biasing, or just ignore the biasing altogether. The current schematic can be left as an example of a practical (homebuilt) circuit. Rogerbrent 05:25, 1 February 2006 (UTC)


 * Have a look at the discussion at Talk:Common collector. We already had a long discussion there about this, which resulted in the updated diagrams. Personally I agree with you about this. Graham 05:32, 1 February 2006 (UTC)


 * There wasn't a decision to only do the images like this. I suggested we make several images for each page. — Omegatron 06:02, 1 February 2006 (UTC)


 * That's right, but so far they haven't been done. I would, but I no longer have a working copy of Illustrator. Graham 06:20, 1 February 2006 (UTC)


 * I'd be willing to do them myself, but is there a requirement that illustrator be used? Also the analysis section will have to be updated. Rogerbrent 18:19, 1 February 2006 (UTC)


 * Illustrator doesn't have to be used, but that's what I have used for WP diagrams in the past. I'd use something else if I could find something that didn't produce crappy looking images, or cost an arm and a leg. I'm a Mac user so my choices tend to be more limited - there are very good programs available but the good ones are not free, and the free ones are not good. Graham 21:45, 1 February 2006 (UTC)


 * Have you tried a recent version of Inkscape? — Omegatron 02:46, 2 February 2006 (UTC)


 * I did these ones and I would be glad to draw up some more. Just tell me exactly what you want, as I said on Talk:Common collector.
 * We could also use color or dashed lines to illustrate different sections.
 * Also, these were done in Klunky, not Illustrator. See WikiProject Electronics/Programs for more programs that can be used (though consistency in appearance on a per-page basis is probably best.)  — Omegatron 18:52, 1 February 2006 (UTC)
 * Only took about a year, but I finally got around to it :) Klunky was VERY useful and easy to use BTW, thanks. Roger 15:49, 27 April 2007 (UTC)

This really needs more than one diagram. We need to show emitter degeneration, typical biasing methods, etc. — Omegatron 14:29, 31 May 2007 (UTC)

Incomplete Equations
Some of the equations listed in the table under the "Characteristics" section of the page are incomplete and slightly misleading. For example the input resistance is not always simply Rpi. If an effective resistance is seen at the emitter node of the transistor the input resistance should be Rpi multiplied by (1 + gm*Re) where Re is that effective resistance at that node and where gm is one of the small signal parameters of the BJT (A transconductance parameter). A similar problem is present with the output resistance expression. It should be Rc in parallel with the small signal output impedance of the BJT itself (Ro = (Va+VCE)/Ic).

Also even though the article uses some small signal parameters, it never defines them. Ro,gm,and Rpi should be defined at the very least.

I plan on adding quite a bit of content to this article along with the other related articles on CS,CB,CD,CS,and CG amplifiers. Although it will have to wait until Friday after finals. This will be my first time editing a page on Wikipedia. Everything I state will come right out of the Electronics textbook I used for Electronics I and II. I'm assuming citing this book will be enough in terms of references. --Sghezawi (talk) 02:58, 11 December 2007 (UTC)


 * Hi Sghezawi, and welcome. Actually the equations aren't technically incorrect, they're using a rather simplified hybrid-pi model that neglects ro. I've been purposely avoiding ro because it makes things messy and ultimately doesn't affect a lot of circuits. Also the equations in the table refer to the grounded emitter case in figure 1. The section before talks a bit more about emitter degeneration. All the parameters are defined in the hybrid-pi model article, so there's no sense redefining them.
 * Some of the other articles (such as the common base one) have already been updated to included ro. Brews ohare (and myself to a lesser degree) has been expanding several related amplifier articles, and I expect to do the same soon when I have some free time.
 * Whatever textbooks you have should be more than adequate, however I think we should be consistent and generally take a more integrated circuits approach (i.e. feel free to use current sources). Good luck and have fun editing. -Roger (talk) 04:29, 11 December 2007 (UTC)

Wrong gains
Some things look wrong.

1) I believe this circuit means to use the transistor in forward-active mode. Which means we have Ve < Vb < Vc around the transistor. In the circuit of Figure 1 this translates into 0 < Vin < Vout. Thus, the voltage gain can not possibly be negative.

2) I believe in the gain/impedance table the author got mixed up between Rc (resistor from the supply circuit) and r0 (internal property of the transistor according to the hybrid-pi model). I did the calculation for the circuit of Figure 1, and although the precise relationship between beta and beta0 is not yet absolutely clear to me, it occurs that in the end you get Vout = r0 * beta0 *Vin / rPI, which is the reason why my guess is that there has been some confusion between r0 and Rc from the author. Same goes for the output impedance. Besides, the reason I had to dig a bit into this is because my physical sense told me that it's impossible that V+ would be eliminated from the result but Rc would remain.

All in all my conclusion is that the table could be correct after replacing Rc for r0 and making the voltage gain positive instead of negative. Also, if one assumes that input current enters the circuit and output current leaves the circuit, one would also have to make the current gain and output impedance negative (it would be nice to have the 2 current conventions clarified on the sketches). And, for this corrected table to be correct, in addition to the assumptions of the hybrid-pi model, after forgetting any subtle difference there might be between beta0 and beta, you'd also have to have Rc large enough so that V+ bears no influence on the current gain and output impedance.ThorinMuglindir (talk) 18:54, 8 July 2012 (UTC)


 * You're right about the operating mode of the transistor; it should be operating in the forward-active mode. However, the voltage gain is most certainly negative.  If you increase Vin, then the transistor will let more current through, thus increasing the voltage across Rc and decreasing Vout.  Basically, Rc converts a positive current gain into a negative voltage gain.alex.forencich (talk) 09:19, 29 March 2015 (UTC)


 * Also, the usual convention in two-port analysis is to consider the current going in to a port to be positive. Under that convention, the current gain is positive. SpinningSpark 10:38, 29 March 2015 (UTC)

Invention of negative feedback
"While this is often described as 'negative feedback' ... it predates the invention of negative feedback ..." Excuse me? Am I the only one who thinks that statement is absurd? Negative feedback is a property inherent to the system, much like gravity is inherent to massive objects; it's just a way of describing what's going on. Whether or not it's presence was intended by the designer makes no odds to the system. 142.162.46.160 (talk) 17:56, 20 May 2016 (UTC)


 * And what does it mean by "predates"? Personally, I think characterizing the common-emitter (or common-cathode) configuration (degeneration) as negative feedback is a bit of a stretch anyway, since the feedback is not from the output.  But yes, it is often discussed in such terms, for its gain stabilizing effect.   Dicklyon (talk) 18:11, 20 May 2016 (UTC)


 * I wondered about that, too, and looking at negative feedback didn't help much. I suspect, though, that the theory wasn't so well understood, especially for allowing components with large (gain) variation to be used reliably in circuits. This is especially important in ICs, where it allows one to use components with large tolerance, though consistent between devices on the same chip. Gah4 (talk) 06:58, 11 April 2019 (UTC)

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180° phase shift vs inversion.
It is a commonly believed fallacy that a phase shift occurs in a common emitter amplifier, or, indeed any other type of transistor stage.

The assumption is based on a cursory glance at the base and collector waveforms when a sine wave is used.

If a sawtooth is used instead, the fallacy is revealed.

A couple of illustrations show this.

Syncopator (talk) 11:13, 16 February 2018 (UTC)


 * -- They are the same thing. Take the Fourier transform of the sawtooth wave, shift all the components 180 degrees and see what you get. 222.155.167.133 (talk) 10:33, 28 September 2018 (UTC)

Does the waveform at the collector lag or lead the input at the base? In order for a signal to have phase change it must encounter a reactance. The maximum phase shift from which would be 90°. I think we can rule out a reactance, don't you? Furthermore, if all the frequencies in a complex waveform were each subjected to a 180°, or any other figure, what a mess would it would be.

No, phase shift and inversion are not the same thing. — Preceding unsigned comment added by Syncopator (talk • contribs) 20:11, 28 September 2018 (UTC)


 * An inversion changes the phase of all frequencies by 180 degrees. I don't understand your objection, or your funny figure. Dicklyon (talk) 05:55, 1 October 2018 (UTC)


 * Even if you don't believe that (and I do), 180 degree phase shift is commonly used to mean inversion. It might even be used where phase doesn't have meaning. Turning a person upside down, for example, might be called a 180 degree phase shift. Gah4 (talk) 07:03, 11 April 2019 (UTC)