Talk:Operational amplifier/Archive 2

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741 Operation -- The 'Official' (National) Explanation[edit]

I've just found the site that answers most questions on the 741 internal workings here [1]. Its interesting to note tho' that the cct is a little different to the one they publish in the data sheet (and actually makes more sense to me). But what shall we do now about trying to explaining our (inadeqaute/erroneous?) diagram. Problem!! Any answers welcome. Light current 21:08, 30 August 2005 (UTC)

Here is the correct DC and AC analysis of the input stage. The description in the link above is just plain wrong (Yes, that may raise some eyebrows but I challenge anyone that doesn't buy my explanation to SPICE the circuit and prove me wrong).
The quiescent current through the input transistors Q1 and Q2 is set by the voltage applied to the bases of Q3 and Q4 and nothing else. This voltage is developed at the connection of the collectors of Q9 and Q10. The collector current of Q10 is set by the current through Q11 which in turn is set by R5. This current is constant. The collector current of Q9 is set by the current through Q8 which is the quiescent current for the input stage. Thus, contrary to what the presentation claims, Q10 does not provide current to the input stage (other than the negligible base currents for Q3 and Q4). Instead we have two current mirror outputs connected together. If the current through Q9 is not virtually identical to the current through Q10, the voltage at their collectors will go to one rail or the other. Thus, these two current mirrors form a very high gain current difference amplifier. The output of this amplifier drives the bases of Q3 and Q4 which set the current through Q9. Thus, we have a high gain closed loop with negative feedback. This forces the input stage quiescent current to match the current through Q10.
This sounds very reasonble to me (if a little more complicated than I at first thought)Light current 23:20, 30 August 2005 (UTC)
The input transistors are wired as a common collector (emitter follower) pair. These transistors buffer the input terminals to provide a relatively high input impedance. The emitter of each input transistor is connected to the emitter of either Q3 or Q4. Thus, the current gain of the input pair is used to drive the low impedance seen looking into the emitters of Q3 and Q4.
Low differential impedance for Q3/Q4 (but high CM impedance).Light current 23:22, 30 August 2005 (UTC)
If the bases of Q3 and Q4 are at AC common, Q3 and Q4 are common base amplifiers. For a differential input signal, the sum of the collector currents through Q1 and Q2 does not change. Thus, the current through Q9 doesn't change so that the voltage at the bases of Q3 and Q4 remains constant. That is, the bases have no signal voltage applied so they are at AC common. A common base amplifier provides the largest voltage gain of any of the three amplifier configurations. So we expect the differential gain to be large for this stage.
Makes sense to me Light current 23:31, 30 August 2005 (UTC)
If the bases of Q3 and Q4 have AC signals, Q3 and Q4 are difference amplifiers. That is, the output depends on the difference between the signals appearing at the base and the emitter of each transistor. For a common mode input signal, the collector currents for Q1 and Q2 must change in the same way so that the sum of their currents will change with the input sigal. Thus, the current through Q9 will change and so the voltage at the bases of Q3 and Q4 change. If the common mode input signal is positive, the emitters of Q3 and Q4 go more positive and the sum of the currents through Q1 and Q2 increases causing the current through Q9 to increase. This will cause the voltage at the bases of Q3 and Q4 to increase. Since Q3 and Q4 amplify the difference between the emitter and base voltages and these voltages increase and decrease together, the gain for common mode input signals is very small.
Agreed Light current 23:31, 30 August 2005 (UTC)
The collector current of Q3 is sampled by Q5 and mirrored in Q6. For differential input signals, the collector currents of Q3 and Q4 change in opposite directions. Thus, if the collector current increases in Q3 and thus Q6, the collector current decreases in Q4. The change in voltage at the collector of Q4 is given by the product of the difference in these currents and the total impedance seen from this node.
Not sure how this sampling takes place-- surely it is the voltage at Q3 coll that controls the base voltage of Q6/Q5 and hence Q6 coll current.? Light current 23:31, 30 August 2005 (UTC)
I'm using the term sample as if the current mirror were ideal. Ideally, we sample the collector current of Q3 without offering any resistance to that current. In this particular current mirror, Q5 non-ideally samples the collector current of Q3 while Q7 provides current to drive the bases of Q5 and Q6. The mirror action occurs because, for a given current through Q5, the base-emitter voltage must be a certain value. This same voltage is across the base-emitter of Q6 if Q5 and Q6 are identical and the currents through the transistors are identical (ignoring the Early effect). Alfred Centauri 00:40, 31 August 2005 (UTC)
To summarize: Q10 and Q11 do not provide current for the input stage, Q3 and Q4 are high gain common base amplifiers for differential input signals, and the current mirror active load does not provide gain - it merely provides a high impedance to signal currrents and performs differential to single ended conversion. Alfred Centauri 22:58, 30 August 2005 (UTC)
Are you saying that Q5/Q6 form a current mirror??Light current 23:31, 30 August 2005 (UTC)
Actually, Q5 - Q7 form the current mirror. The input to the current mirror is the collector of Q5. The output of the current mirror is Q6. Q7 reduces the amount of collector current from Q3 that must be used to drive the bases of Q5 and Q6. That's why it's called a base-current compensated current mirror. See circuit (b) on the first page of this: [2] Alfred Centauri 00:40, 31 August 2005 (UTC)
Yes that is an excellent article. I had not come across the 'base current compensated' mirror before, as I think it had probably been superseded by the Wilson current mirror at the time I was studying electronics. The Wilson mirror appears to have superior performance and still only uses 3 transistors so I guess we wont see many more of these BCC mirrors. So, the Wilson current mirror must have been invented after the 741 design? Light current 12:59, 1 September 2005 (UTC)
Also, Q5/Q7 look like a transistor version of a thyristor cct. Why does it not trigger & latch??Light current 23:40, 30 August 2005 (UTC)
There is no positive feedback here. If the current through Q7 increases, the current through Q5 increases but this acts to reduce the current through Q7 - this is negative feedback. Alfred Centauri 00:40, 31 August 2005 (UTC)
Im pretty sure tho', that if I increased the supply voltage sufficiently, I could create enough leakage in Q5/Q7 to cause latching thyristor action - but maybe the rest of the chip would go 'pop' before I reached it. Must try this one day!Light current 10:46, 31 August 2005 (UTC)
I really think this good stuff should be fed back to NatSemi on their 'Talk Page'[]. Can anyone see any problem in us doing that??Light current 23:45, 30 August 2005 (UTC)
Send an e-mail to Bob Pease at National: [3] Alfred Centauri 00:40, 31 August 2005 (UTC)
I'm not sure I would want to incur Bob's wrath (having read some of his books/srticles) would you?? Besides I think it would be more polite to reply to the lady who wrote the webcast as they request- dont you?;-)Light current 01:20, 31 August 2005 (UTC)
Wise AND courteous! Alfred Centauri 02:23, 31 August 2005 (UTC)

Unsigned Posts[edit]

Would it be possible for the author(s) of the very interesting unsigned posts on this talk page to sign them. Then I will know to whom to address my comments. Thanks Light current 23:36, 27 August 2005 (UTC)

Im assuming, from the style and evident wealth of knowledge displayed, that the 'unsigned' posts on this talk are by you Alfred. Come on now, dont be shy!! Light current 17:50, 30 August 2005 (UTC)

I did write a few things on this page back in April before I was gently prodded by 'O' to start leaving a sig. If these posts are the ones you are referring to then thanks for the compliment! Alfred Centauri 02:21, 31 August 2005 (UTC)
Just look through the history if you want to know who wrote what. Feel free to label unsigned posts with their authors. — Omegatron 18:03, August 30, 2005 (UTC)

Any more internals?[edit]

I think with the help of 'AC' and 'O' we have now managed to almost fully dissect and analyse and write up the 741. (Unless someone disagrees). Any more comments on what needs doing, or can we leave it there? Light current 03:01, 31 August 2005 (UTC)

Widlar current sources[edit]

The wonderful schematic above, and on the article page, seems to center on a cascade of Widlar current sources, Q10,Q11 and Q12,Q13, themselves serving a current mirror Q8,Q9. Is this a fair statement? --Ancheta Wis 18:26, 25 December 2005 (UTC)

Proposed email to Peggy (Applications Engineer, National Semiconductor)[edit]

Hi Peggy,

I and some others have recently been writing an article on operational amplfiers for the Web based free encyclopaedia, Wikipedia. We decided to use the 741, as it was a classic industry standard (but old hat now anyway,) to illustrate the basic internal workings of a simple op amp. So we were very pleased to come across your webcast on opamps.

On looking at your presentation graphics, however, we found a number of bullet point statements to be a little vague and maybe slightly misleading to the casual reader. Perhaps I can outline which ones I mean:

a) 'Q11 delivers current to the input stage' pp 48-49 We thought here that it may be more accurate to say 'Q11 generates a reference current for the input stage.'(rather than provides current --which it doesn't do really)

b) 'Q11 delivers current to Q3/Q4 bases.' p49 We think here that this is only partly correct. It is the difference between Q10 collector current and Q 9 collector currrent that forms the base current. Under normal operation, these collector currents are almost identical (the base current of Q3/Q4 being very low). We think, therefore, that this connection provides voltage feedback used to stabilise the bias of the i/p stage.

c) 'Q3 and Q4 are level shifters' p49 We thought here that the additional function of these transistors as high gain common base amplifiers (for differential signals only) as well as their low compliance to common mode signals should be highlighted.

d) Q5, Q6, Q7 provide gain.....p50 . We dont feel this statement is quite true. Q5,Q6,Q7 appear to us to form a compensated current mirror as described here \ This circuit gives high impedance to the signal and performs differential to 'single ended' conversion, but, on its own, does not provide any gain.

Also we noticed that your schematic differs from the one in the National data sheet on the 741 especially around the output stage. Could you say which schematic is more accurate?

We hope these comments are useful to you in your goal of aiding others understand opamps and we would be very interested to know your thoughts on them. Yours sincerely

Is this polite enough , too polite, too crawling or what?? Comments please ASAP Light current 01:09, 1 September 2005 (UTC)

part (b): do you mean Q10? Otherwise, looks good. Alfred Centauri 00:24, 2 September 2005 (UTC)
Yes I meant Q10. Well spotted. I'll just leave this here a few more days for any more comments before I send it off.Light current 00:35, 2 September 2005 (UTC)
Message sent.Light current 09:35, 4 September 2005 (UTC)

Proposed Split off of Applications to new page[edit]

I propose that all the op amp applications (of which there are hundreds- if not thousands) be now split off to a separate page called 'Applications of OpAmps' or 'Opamp applications'. So everything after the discssion of '741 internals' would go to the new page. Any comments?

It is my opinion that lots of detail on on opamp applications belongs in a Wikibooks module instead of Wikipedia article. The Wikipedia article can then link to the Wikibooks module. Thoughts? Alfred Centauri 00:30, 2 September 2005 (UTC)

Possibly! I hadn't thought of that. Maybe some more opinions on what should and what shouldn't be in WP are needed.Light current 01:11, 2 September 2005 (UTC)

Generally, "how-to" stuff is supposed to be moved to Wikibooks. Standard op-amp circuits are sort of a gray area; both encyclopedic and how-to. More specific applications belong in Wikibooks, of course.
The main problem with Wikibooks is that it's a ghost town. Once something is moved there, practically no one ever reads it or edits it again. — Omegatron 20:24, September 6, 2005 (UTC)
Well then, maybe a link to the Wikibooks article in the Wikipedia article will drum up some interest in Wikibooks! Alfred Centauri 21:54, 6 September 2005 (UTC)

I have copied all the apps to a new page called Operational amplifier applications whilst leaving them on this page. Please consider if we can delete them now from this page as it is getting very large.--Light current 00:00, 23 September 2005 (UTC)

I agree about splitting the article into two parts, but I don't agree about moving it to wikibooks. It can be included in there, too, so that they can be used for a wider and more organic discussion, but removing them from wikipedia would be a loss of data. - Alessio Damato 11:11, 23 September 2005 (UTC)

about "basic opamp circuit"[edit]

The section under the header "basic opamp circuit" looks very much like a specific example. It combines ideas of ideal opamps and non-ideal opamp approximations. As it stands, I don't find it to very clearly relate to the rest of the article. It might be better to have a link to non-inverting amplifier, or to have a list of different techniques used in opamp circuits (positive feedback, negative feedback, voltage dividers, etc) and how they affect the circuit. I just find that the section in question doesn't allow a reader to generalize very well. Fresheneesz 00:36, 19 May 2006 (UTC)

I more or less agree with Fresheneez. The 'basic op-amp circuit' section is more accurately titled 'basic voltage amplifier with negative feedback'. The ideal op-amp is an ideal voltage amplifier (Gin = 0, Rout = 0) with the additional stipulation that Avoc is (or goes to) infinity. Further, there are many 'basic' op-amp circuits depending on what 'operation' one wishes to perform. The current section implies that the basic circuit is a non-inverting linear voltage amplifier. We all know that op-amp circuits are much more versatile than that. Alfred Centauri 01:09, 19 May 2006 (UTC)
I don't understand. What's wrong with having an example of an application in this article? — Omegatron 03:51, 19 May 2006 (UTC)
No problem, unless it sticks out like a sore thumb in the middle of the article - and it mixes generalities with specifics in a way that makes it not-just-an-example. Examples should show properties or concepts that have been explained already, not used in order to explain something. Also, its pretty long. Fresheneesz 04:09, 19 May 2006 (UTC)

add section "op amp approximation"[edit]

I think it would be very useful to combine some sections (or parts of sections) in this article into an "op amp approximation" section that includes the ideal op amp and non-ideal op amp approximations, including the "golden rules". I think its an important thing to keep the approximations separate from the realities. Fresheneesz 04:12, 19 May 2006 (UTC)

Why was Common Mode Rejection eliminated?[edit]

Light current, why did you eliminate the piece on common mode rejection from the discussion of ideal and real-world op amps? Your edit summary says its a simplification of language, but it actually eliminates discussion of a standard parameter of operational amplifiers. Do you think it's unimportant? (It certainly is an issue in instrumentation amplifiers.) Did you think it's incorrect? Anoneditor 17:30, 4 July 2006 (UTC)

My own thought exactly when I reviewed it just now. I think that there is no justification to eliminate discussion of CMR in order to 'simplify language'. There is also a real difference between CMR and input offset, so that can't be the reason. I think it should be restored. --Nigelj 21:58, 4 July 2006 (UTC)
An ideal diff op amp has by definition an infinite CMRR. To mention it here only confuses the innocent. Should be in limitations para! 8-|--Light current 23:23, 4 July 2006 (UTC)

If the criteria is whether or not the parameter is part of the definition of the ideal op-amp, then we should also eliminate infinite input impedence, zero output impedence, infinite gain, etc. It still seems to me to be important to note somewhere that the ideal op-amp does not amplify common mode signals. Perhaps another way to deal with this issue is to provide a better explanation in the introduction of just what a differential voltage amplifier is, i.e., an amplifier that amplifies only the differences in the voltages at its inputs That way, "the innocent" won't have to infer that from the opening paragraph ---Also, as a practical matter, the opening statement that the output is the product of the difference between the two inputs and the open loop gain is true only when that difference is exceedingly small. Otherwise, the open loop gain will saturate the amplifier. I may rework this paragraph to cover both issues in a non-technical way. ---BTW, common mode rejection is not just a DC problem. It also affects AC amplification in non-inverting amplifiers, especially at high frequencies in those op-amps in which the CMRR declines as the frequency rises. Therefore, it should be treated as a non-ideal characteristic for both AC and DC signals. Anoneditor 04:38, 5 July 2006 (UTC)

If you say an ideal op amp only amplifies voltage differences (because its a diff amp), CMRR is not relevant and is a distraction here. THe practical deficiencies of real op amps should not really be in the simple explanation of 'ideal'. Mention it in the 'limitations of real op amps' para. Again, mentioning saturation under the ideal description I think would not be helpful.
Agree CMRR is a problem in real op amps at ac & dc.--Light current 11:45, 5 July 2006 (UTC)

Your thoughts on organization are well put. As a result, I have just now modified the opening paragraphs of the article to move the operational details of the article to a new section called "Basic operation," added an explanation on "Common mode gain" at the point you set in "DC imperfections" and added a reference to it in "AC imperfections." I'll explain the condition causing saturation shortly in the "Nonlinear imperfections" section. Anoneditor 15:31, 5 July 2006 (UTC)

THank you for your kind words! 8-)--Light current 22:17, 5 July 2006 (UTC)
You're welcome. Your input was very helpful. Anoneditor 05:13, 6 July 2006 (UTC)

741 never used in "hi fi" equipment[edit]

I don't recall that the 741 op-amp was ever used in the signal path of any audio project that I saw in "Popular Electronics" or "Radio Electronics" through the early '70s to early '90s - even at the time its performance would not have been considered suitable for "high fidelity" use. Can any one cite an actual product that literally used 741s? I believe the article is mistaken here. --Wtshymanski 00:11, 14 August 2006 (UTC)

Construction project articles written by Dick Crawford in Audio magazine in November, 1969, September, 1972 and November, 1973 all used the 741. The first was the construction of an IC tone control stage. The second was the construction of a 9 band equalizer. The last mentioned was a scheme for bass equalization of sealed box speakers. And I think there were others but I just can't put my hands on them now.
From my experience, the problem with the 741 used in line level circuits was not so much its noise performance (which wasn't great) or from hum due to inadequate CMRR (90db @ 60 Hz isn't bad). The problem is its low gain-bandwidth product and slew rate. Even moderate levels of gain at high frequencies, such as treble boosters, produced a harsh sounding result that quickly fatigued the listener. Although there were means of overcoming this, they all required extra circuitry and the advent of the LM301A, at least in its feed-forward inverter mode made all that unnecessary. Anoneditor 16:27, 14 August 2006 (UTC)

Offset voltage confusingly explained[edit]

The article states as a limitation that "the op amp will produce an output even when the input pins are at exactly the same voltage." This sounds strange to me – since there is only one output terminal, it is unclear what it even means to produce a (DC) output voltage without specifying what to measure its voltage against. So how does one determine what the potential of the output terminal ought to be when the inputs are equal? Midway between the power rails? If so, the article should tell explicitly. –Henning Makholm 12:51, 12 May 2007 (UTC)

For an op-amp with a gain of 105 and saturation points of ±10V a change in differential input voltage of 200μV is enough to take the op-amp from saturated in one direction to saturated in the other. This figure is a tiny fraction of typical input offset voltage so what reference point you pick as being "zero output" will have a negligable effect on the input offset voltage you measure. Plugwash 16:20, 12 May 2007 (UTC)
Henning: For the ideal op-amp, the three terminal voltages (v+, v-, v_OUT) are referenced to the circuit common. This common connection is not usually shown on an ideal op-amp symbol. For a physical op-amp, there will be a common connection as well as connections for the positive and negative supplies (or, in some cases, just one supply). As in the case of an ideal op-amp, the voltage at each connection is referenced to circuit common. This shouldn't be a source of confusion but if you feel that it is, please edit the article accordingly. Alfred Centauri 18:05, 12 May 2007 (UTC)
This answer confuses me. There is no "circuit common" visible in the article's 741 diagram. It might be that the 741 is peculiar in not having an explicit common pin, but if so it makes a bad choice for the detailed example, doesn't it? –Henning Makholm 22:33, 2 June 2007 (UTC)
The 741 is not peculiar in this respect; no common "split supply" operational amplifier has a specific common pin. In those op-amps designed for use with a single positive supply voltage, the ground pin is connected to the circuit common point.Anoneditor 04:35, 3 June 2007 (UTC)
What distinguishes those two cases, except for labeling? –Henning Makholm 11:03, 3 June 2007 (UTC)
I guess I should have stated that the single-supply amplifier is usually connected to the circuit common point. It could be connected to a voltgage anywhere between the highest and lowest voltages in the circuit. However, for the amplifier to operate in its usual way, the connection would have to be one of low impedence, i.e., a connection at which all of the current (in the conventional sense) flowing out of the amplifier would be absorbed with a minimal change in voltage between the amplifier terminal and that point.
That being said, I guess you could call naming of the single-supply amplifier's ground pin and the split-supply amplifier's V- pin a labeling issue. (The voltage difference between the V+ terminal and the other supply terminal would be the same in any particular system.) Nevertheless, there are two practical problems: First, if you use the V- terminal as the circuit common with an amplifier designed to be powered by a split-supply, the internal circuitry of the amplifier won't allow it to function for input voltages close to that point. Second, connecting any one of the amplifier's inputs to a voltage below that point might destroy it due to the way these chips are fabricated.
I don't think the functional difference is a labeling issue, though. In the split-supply amplifier, the common point is a low impedence point located between the voltage extremes of the power supply, usually half way between. Though it can be done from other points, all amplifier input and output voltages are usually referenced to this point. When using a single-supply amplifier, all input and output voltages are usually referenced to the voltage at the ground pin. Anoneditor 22:38, 3 June 2007 (UTC)
In a real opamp, the open loop output would be either +Vcc or -Vee, even in the inputs were connected, because of input offset. In an ideal opamp, there is no constraint on the output with zero input, it could be anything. The output is essentially just a norator in that case. Roger 20:30, 12 May 2007 (UTC)
This makes sense, as does the answer above by Plugwash. However its seems to imply that it is wrong to call it a "limitation of real op-amps" that the output voltage for connected inputs is not what somebody might (erroneously?) expect it to be. Do you agree? And does this mean that it should be removed from the "limitations" section? –Henning Makholm 22:33, 2 June 2007 (UTC)
I think it's properly called a limitation because it is the result of the imperfectly matched transistors in the differential input stage. Therefore, it is an implementation problem in the real world that doesn't exist in the ideal op-amp model. Anoneditor 04:35, 3 June 2007 (UTC)
If it is properly a limitation, then the description of limitation should also describe how an ideal op-amp without the limiation is expected to behave. Rogerbrent above says that there is "no constraint" on what an ideal op-amp outputs. It still seems inconsistent to call any behavior a "limitation" if there is no constraint for it to violate. –Henning Makholm 11:03, 3 June 2007 (UTC)
Henning, the ideal op-amp is a linear device. As a linear device, it has the property that zero input results in zero output. Thus, a non-zero output with a zero input, i.e., the input pins are connected together, is a non-ideal characteristic for a physical op-amp. I think the phrase non-ideal here is better than limitation.
I suppose Rogerbrent and I disagree on this subtle point. Although an ideal op-amp has infinite voltage gain, the output should be zero when the input terminals are connected together. To see this, start with finite gain and take the limit as the gain goes to infinity.
On the other hand, when the op-amp is configured with negative feedback, the voltage difference between the input terminals goes to zero as the gain goes to infinity yet the output voltage is not necessarily zero. In fact the output voltage is whatever it needs to be to make the input voltage zero. Alfred Centauri 13:32, 3 June 2007 (UTC)
This does not answer the original question: You say that the output should be "zero", but with only one output pin what is this "zero" measured against? Say that a genie pops out of nowhere and hands me a magical 741 which he promises will behave ideally. Eager to test his claim, I solder the two input pins together, connect them to a voltage divider between the power rails, and power the whole thing up. Now where do I connect my voltmeter to confirm that the output is indeed "zero"? The voltmeter wants me to connect two terminals before it gives me a reading, but there is only one output pin. And if the 741 diagram in the article is to believed, there is no "circuit common" pin on the 741 (neither the ordinary nor the magical variant) for me to connect the other terminal to. –Henning Makholm 14:53, 3 June 2007 (UTC)
I believe I answered that question in an earlier reply but here's another stab at it. The output voltage is a node voltage. A node voltage is, by definition, the voltage measured by an ideal voltmeter when the red lead of the voltmeter is placed on the node and the black lead of the voltmeter is placed on the zero node (AKA, circuit common, circuit ground). In the case you described above, you said "connect them to a voltage divider between the power rails". If you were to measure the voltage on one of these 'power rails', where would you place the black lead of the voltmeter? Now, leave the black lead where it was when you measured the rail voltage but put the red lead on the output pin of the op-amp. The voltage you measure there is the 'output voltage'. Do you see now? Alfred Centauri 15:56, 3 June 2007 (UTC)
I don't see, because I don't claim that it is possible to measure a voltage at a single point. What I have learned is that the only thing that makes sense is a voltage difference between two points in a circuit. If you're a physicist you may choose to measure from "the potential at infinity", but that makes little sense for computations about an actual galvanically isolated piece of electronic whose potential relative to infinity may fluctuate by hundreds of volts when a thunderstorm passes overhead.
What I think you're saying here is that I can choose my zero potential wherever I please. That is fine, as such, but then it hardly makes sense to blame the op-amp for an "imperfection" simply because it cannot guess where I choose to put my zero point, does it? –Henning Makholm 16:59, 3 June 2007 (UTC)
Let me add something this at the risk of making this too complicated. The above assumes that the 'power rails' you referred to are separate power supplies that are equal but opposite in sign. That is, the above assumes that there is a zero node in the circuit from which the power rails are measured to be equal but opposite in sign. In the case of a single supply, the output node voltage with the input terminals connected together should ideally be 1/2 the supply voltage, not zero. Perhaps this is what you've been driving towards? Alfred Centauri 16:12, 3 June 2007 (UTC)
Henning, no claim has been made that the output voltage is a voltage at a point. Please refer to the article on Node voltage analysis if you don't understand the point I made about the zero node. The choice of the zero node (the point from which all other node voltages are referenced / measured) is indeed arbitrary but that is not the source of this 'imperfection'.
Let's try this another way. With the setup you gave before, place your voltmeter leads on the Vs+ and the Vs- connections and record the result. Now, leave one of the voltmeter leads in place and move the other lead to the Output connection. Ideally, the magnitude of this voltage is exactly one-half the magnitude of the difference between Vs+ and Vs-. If it is not, this is an imperfection. Do you see this? Alfred Centauri 17:54, 3 June 2007 (UTC)
It does make sense to require that the output voltage zero is midway between the power rails, but I didn't think the article said that this was expected. I have now attempted to edit it such that it does say this. Did I get it approximately right? –Henning Makholm 21:02, 3 June 2007 (UTC)
I made a slight wording change to your edit. Your edit improves the article Good job. Alfred Centauri 22:15, 3 June 2007 (UTC)

Internal circuitry of 741 type op-amp[edit]

I removed the introductory paragraph to this section because, in my view, it contains errors, doesn't define some of its terms and uses confusing language. For example:

  • Although there are other chips that have similar specifications, there are no variations of the 741 chip; it is a specific design. Also, it doesn't follow that because these variations exist, it is unnecessary to know the internal makeup of the 741. There may be good reasons why knowing the internal makeup of the 741 is irrelevant, but they do not include the fact that other topologies exist that perform similarly to the 741. The logic that substitutions can be made for performance enhancement can apply to virtually any operational amplifier.
  • The terms DIL and PIC should be defined. Besides, not all 741s come in the DIL package; I have some in metal can form.
  • If the PIC chip the author mentions is a micro controller, then it is an entirely different animal from an operational amplifier, even though it may be able to perform some functions that can be performed by op-amps.
  • The open loop gain of the 741, and all other non-programmable op-amps, is the same regardless of whether or not it is being used as a comparitor. Question: Is it the 741 itself or its function as a comparitor that is handy in temperature comparison circuits?

Anoneditor 19:20, 2 June 2007 (UTC)

Internal circuitry section[edit]

I'm thinking this section should be moved to its own article. Roger 02:40, 14 June 2007 (UTC)

Why? --Tugjob 00:46, 15 July 2007 (UTC)


On 1 September 2007 User:Barticus88 linked the term "stabilize" in the first paragraph of the topic "AC behavior" to the article on BIBO stability. Is this link correct? The stability in question is the freedom from oscillation or excessive output peaking in response to step inputs. Anoneditor 23:17, 1 September 2007 (UTC)

At first glance and in this case, I would say yes because stability, in this context, refers to ensuring the poles are in the left-hand plane. Alfred Centauri 01:55, 2 September 2007 (UTC)
Thanks, Alfred. Anoneditor 02:43, 2 September 2007 (UTC)

Use of "G" for Gain[edit]

I beleive the variable "A" or "A-sub-v" is used for gain. —Preceding unsigned comment added by (talk) 23:25, 29 October 2007 (UTC)

"input impedance"[edit]

Hi. From Electrical impedance, impedance "describes a measure of opposition to a sinusoidal alternating current (AC)". Operational amplifier says "High input impedance at the input terminals and low output impedance are important typical characteristics". When I think of "opposition". As input and output are points, how can they have opposition to a current? --Gerrit CUTEDH 09:09, 12 April 2008 (UTC)

The input isn't really a "point", it's defined as the potential difference (voltage) between V+ (or V-) and ground. The input impedance is the effective internal impedance between these two nodes. Oli Filth(talk) 10:37, 12 April 2008 (UTC)
From another perspective, the high impedance at the input terminals refers to the high opposition of those terminals to the entry of signal current. In direct current terminology, it would be called high resistance to the input current. This means that the amplifier requires very little current to operate and it's operation doesn't reduce the signal of the device supplying the signal to it very much. Similarly, the low output impedance at the output terminal would be low opposition to current leaving that terminal of the amplifier. This means that the amplifier can supply current to whatever device it is driving with little reduction in the voltage of the output signal. Hope this helps. Anoneditor (talk) 21:19, 12 April 2008 (UTC)

How the hell do op amps work?![edit]

I've been looking everywhere and whenever it says "basic" it never is. —Preceding unsigned comment added by (talk) 20:13, 16 November 2008 (UTC)

The 1-page op-amp basics doc in the External links didn't help? Did you check it out? In one page, it explains the mysteries of high-gain negative feedback and gives a circuit example. Here's the control-based explanation:
  • An "operational amplifier" is nothing more than an amplifier that amplifies the difference between its "-" and "+" inputs by an extremely large value. In particular, the output is "G*(Vplus - Vminus)" where "G" is very large and "Vplus" and "Vminus" are the respective input values. The output is then "fed back" to the input (usually using a resistor network, like a voltage divider) so that when the output gets large, it forces its inputs to come together. As a consequence, the inputs "stabilize" very close to each other.
  • In particular, if the inputs of the OA ever start to drift away from each other, they will cause the output to grow very large, and the feedback will "push" them back together.
  • So as long as we have "negative feedback" with an OA, we can consider the two inputs to be MATCHED even if they're not obviously physically connected.
There are several ways to build such a device using circuit elements (like bipolar transistors and MOSFETs). All of them involve subtracting one voltage from another (e.g., with a differential transistor pair) and then amplifying that voltage by a large gain (e.g., with a very high-gain common-emitter amplifier) and then buffering the output so that it has very low impedance (e.g., with an emitter follower).
Do you have *specific* questions about how all of this works? What in particular is the roadblock to understanding? These concepts CERTAINLY are difficult, and so there are lots of places for the wheels to come off. Please be specific about where you're having trouble. We'll start there and work forward. —TedPavlic | (talk) 23:46, 16 November 2008 (UTC)
Thank you very much for helping me. I hadn't noticed the external links, I'll look at them later. I just basically need to know how to make them work in a circuit for audio purposes, but I'm not being very specific. Thank you again. —Preceding unsigned comment added by (talk) 21:04, 17 November 2008 (UTC)
Make sure you see the Operational amplifier applications page, which might be a helpful cookbook for your applications. —TedPavlic | (talk) 21:21, 17 November 2008 (UTC)
You might also consider the needs of low distortion audio applications using op-amps. These require amplifiers with a slew rate of at least 2 volts/microsecond and a gain-bandwidth product of at least 5 megahertz. Both of these specifications appear in the op-amp data sheets. And both rule out using the 741 op amp for high quality audio purposes. It's major problem is in its high frequency performance as a result of a slew rate of 1/2 microvolt/sec and a gain-bandwidth product of 1 megahertz. This results in a harsh sounding treble response, especially if the treble is being boosted, as in treble tone control circuits. Anoneditor (talk) 23:36, 18 November 2008 (UTC)