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741 Operational Amplifier

  1. Mar 22, 2014 #1

    As I mentioned before, thank you for all the mentoring. It has been a huge help to my understanding.

    So, I am trying to stay one - two weeks ahead of my operational amplifiers class. I know that we will soon be discussing the 741 Operational amplifier.

    While reading about it, I noticed I didn't really understand the internal mechanisms (in terms of how the BJTs were laid out). So I was wondering if you could help me identify them.

    For instance, in this picture 500px-OpAmpTransistorLevel_Colored_Labeled.svg.png

    I have learned a lot in my classes about BJTs but when I look at this image, I feel like I have learned nothing. I guess, everything I have learned has been in pockets of small circuits and now that they are all thrown together, I see that my understanding is really weak. I know that Q20 and the darlington pair of Q15 and Q19 are amplifying current. I think Q14, since it is a CE, would be used to drive the voltage but seeing Q17 there confuses me.

    I am really not sure what do two BJT's facing each other do (for example Q12 and Q13, Q8 and Q9, Q5 and Q6, Q3 and Q4)? Is it just the way the circuit is set up? AKA they are more so acting like push pull amplifiers but they are just set up so that they face each other base to base?

    I see that Q15 and Q19 form a darlington pair (to drive the current) but I don't understand what Q22 does.

    Q1 and Q2 look like they are set up as a differential amplifier but what the purpose of Q3, Q4, Q7 Q5, and Q6 does confuse me.

    I am not sure what the purpose of Q14 and Q17 are (why they are in that configuration)

    I am working on trying to mathematically figure out what it all means except, I am not sure what numbers to make up as my non-inverting input, my inverting input, and my offset nulls. I'd really just like to know what each "section" of amplifier combinations is called and what the "sub" purposes of them are within the 741.

  2. jcsd
  3. Mar 22, 2014 #2

    jim hardy

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    Last edited by a moderator: May 6, 2017
  4. Mar 22, 2014 #3
    Thanks. Just taking a quick look it seems to read much better than my current text. I will definitely use it side by side when I start on op amps. Hopefully it will help me out a lot. I didn't see any sections that went over BJT configurations in multistage amplifiers though... which was kind of my main question on this post. The pdf basically did what my textbook does. It talks a little bit about this and that, then throws everything together as one large op amp. I am more so trying to understand the 741 schematic I posted earlier.

    Any thoughts or ideas?

    PS thank you for the resource. i was really scratching my head for a good one
  5. Mar 22, 2014 #4
    Q8, Q9, Q10, Q11, Q12, Q13 = current mirror and they provide DC bias current (Q13 also work as a active load for CE amplifer).

    Q1, Q2, Q3, Q4 form a cascaded differential amplifier (CC + CB amplifiers) followed by a current-mirror active load (Q5, Q6, Q7 = Wilson current mirror).

    Q1 and Q2 are CC amplifiers and Q3 and Q4 are CB amplifiers.

    Q15, Q19 connected in a Darlington configuration work as a CE amplifier followed by a current-mirror active load (Q13).
    Q22 and Q17 = make up the short circuit protection circuit.
    Q16 = Vbe multiplier is used to prevent crossover distortion in push-pull amplifier stage (Q14 ,Q20 --> CC amplifier).
  6. Mar 22, 2014 #5

    Thank you very much! While I am not sure exactly what some of what you wrote means, that gives me a HUGE kick in the right direction to do research and study for myself! Thank you so much!
  7. Mar 22, 2014 #6
    One thing to keep in mind in looking at an IC design is that transistors located very close to each other and masked with the same shape, will essentially be identical and since they are very close, unless one has a large different current, then their temps should be the same. All things here being equal, tying the base together means that the base current is equal in each(with 2 transistors then the supplied base current/2). And if their design is such in the configuration shown, then the collector current should be the same as well. Same holds true if more than 2 transistors are such that their base is supplied by common source (base current is the supplying source divided by the number of transistors.
  8. Mar 22, 2014 #7

    jim hardy

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    wow i'm trying to drag up old memories here.

    I recall a long tech note from National called "Intuitive IC opamps"

    which went through the design and its "why's"
    looks like it's out in paperback now
    search on the title with Frederiksen , turns up a lot of offerrings.

    There exist a few copies of a real treasure, the Philbrick-Nexus Op-Amp Applications manual. It's a brown spiral bound notebook style .

    And this link goes clear back to vacumm tube opamps of 1950's...

    happy hunting !

    old jim
    Last edited: Mar 22, 2014
  9. Mar 23, 2014 #8


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    Adding to mjhilger's explanation, Current Mirrors are used to keep the output current constant regardless of loading.

    For this circuit:


    Being able to match transistor characteristics from integration is really the key to operational amplifiers high gains.
  10. Mar 24, 2014 #9

    jim hardy

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    This one might help you more.


    TI bought National and was good about keeping their appnotes and training material available. I'm just not adroit at finding it.

    These old ones were written by the guys who first figured things out.
    Like the old cowboy says, "Always drink upstream of the herd."

    old jim
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