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John Popelish wrote:
> Chris wrote:
> > mark349@xxxxxxxxx wrote:
> >
> >>Okay, I will see if I can overstay my welcome in a hurry here :-)
> >>
> >>Folks helped me with the first stage of a three stage audio circuit, and I
> >>get the idea of what each one does, thanks to the NS app notes I was
> >>referred to, but now what I am not getting is, what might be the purpose
> >>of R7. I have no notes or access to any designer, just the PCB. Section
> >>two appears to be a 2X amp, [ (r1+r2/r1)/Vin ] but it does not give me
> >>2x, and I am guessing it is due to R7. Can anyone speculate on this?
> >>
> >>The circuit is out of a one to four distribution switch. Of course I am
> >>only showing portions of it, and I ran out of space on the right to show
> >>the 47 ohm resistor in series with the final output stage. :-)
> >>
> >>I have miscontrued the original intent, and thought that each portion was
> >>to be unity gain, so I asked about gain on stage one. It seems to me that
> >>the second portion is actually a 2X gain, so I need to adjust the first
> >>stage down to .5, using 10K resistors? I thought that would give me unity
> >>gain througout, but I am seeing slightly less than that, so I can only
> >>suspect the one component I do not fully understand, and that would be R7.
> >>I don't want to adjust anything, or remove components that may have other
> >>purposes for being included, but in the end, I just want to tweak this to
> >>create unity gain without affecting performance doing something I am not
> >>fully understanding.
> >>
> >>I have tweaked the values, and tried to observe the results on a scope,
> >>but I prefer concrete understanding, and solid reasoning, over just doing
> >>a 'plug and play' kind of design.
> >>
> >>Any comments or advice would be appreciated.
> >>
> >>
> >> R8
> >> 20K 10K ___
> >> ___ R4 R6 ___ -|___|-|
> >> |---|___|--| |--|___|-------| | 270 |
> >> R1 | | | | | -12V |
> >> 20K | -12V | | |-12V | | |\| |
> >> In- ___ | | | | |\| | --|-\ |
> >> -----|___|-+ | |\| | +---|-\ | | >-+---
> >> |--+----|-\ | | | >-------+------|+/
> >> | >--+-----------|---|+/ | |/|LM833
> >> In+ 20K +-----|+/ | |/| |
> >> -----|___|---+ |/| LM33078 | | | +12V
> >> R2 | | .-. +12V .-.
> >> | | R5| | | | R7
> >> .-. +12V 10K | | | |10K
> >> | | '-' '-'
> >> R3 | |20K | |
> >> '-' | |
> >> | GND GND
> >> |
> >> ===
> >> GND
> >>(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
> >
> >
> > Hi, Mark. For Mr. Popelish's reference, the earlier thread was
> >
> > http://tinyurl.com/ey5ch
> >
> > and the National Semiconductor appnote was AN-31, "Op Amp Circuit
> > Collection". That's the AN consisting of basic circuit schematics for
> > most of the basic op amp configurations, along with a relevant equation
> > or two for each.
> >
> > As shown in your ASCII art above, the first op amp you've drawn is set
> > up as a diff amp with a gain of 1 * (V2 - V1). The second op amp is
> > set up as a non-invering amplifier with gain of 1 + (R6 / R5), or 2.
> >
> > R7 just seems to be a plain old 10K load resistor, which should not be
> > anywhere near enough to bog down the op amp output. Even with the
> > second op amp railed at V+, it will only draw about 1mA. But R7
> > doesn't change the basic gain equation of the op amp. I believe it's
> > there because the NPN transistor internal to the LM833 output is a lot
> > better at sourcing current than the PNP is at sinking current. See the
> > data sheet. Keeping the output in a current sourcing mode will tend to
> > improve slew rate, and reduce crossover distortion issues.
> >
> > http://www.national.com/ds/LM/LM833.pdf#page=1
>
> I see they used a 2k pull down load resistor for the distortion and
> slew rate graphs. I wonder how much worse the result would be without it.
>
> I still don't see this 3 stage amplifier in any reference. Why not
> leave the middle amplifier out, entirely? Can't get less distortion
> from it than that way.
Good morning, Mr. Popelish. From what the OP has said, I'm tending to
think he's working on somebody else's homebrew stuff (poor guy -- no
schematics, no help at all other than muscling out the circuit
schematic from board inspection). I had mentioned in the prior post
that someone who has no design experience, but just wanted to have a
quick "cheat sheet" for elementary op amp circuits on the bench could
do worse than printing out AN-31, which is just sketches of basic op
amp circuits with gain equations added for clarity. You would look at
the individual op amps, then figure out what each one does (diff amp,
non-inverting amp, voltage follower). That might be some help for the
newbie.
You're right of course, the middle amp of the three does look like an
advanced case of "clean your plate" syndrome. It's actually a good
indication the original circuit was done by a hobbyist. The LM833 _is_
a dual, after all. ;-)
Cheers
Chris
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