Re: OpAmp Gain Values Revisited

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 ***. 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

The third op amp is a voltage follower (a non-inverting amplifier with
a gain of 1). The 50 ohm output resistor at the third op amp output
may be part of a voltage divider, or it may just be made so that the
amplifier output impedance is about 50 ohms (51 ohms is the closest 5%
value). The thing is, the 51 ohm resistor may be acting with the input
impedance of the signal destination to create a voltage divider, which
may be slightly reducing your output. Either way, you're right -- if
you want a gain of 1 for the whole block, you'll have to adjsut values
on the diff amp resistors.

By the way, I would also check the exact values and particularly the
matching on the resistors in the diff amp. Small changes here can make
a proportionately larger change in the output (see the equations). Use
1% resistors, and use an ohmmeter to get a match in the input resistors
and the feedback resistors. Diff amps don't work very well without
adequate resistor matching.

Moral of the story -- even techs who just work with the circuits should
take the time to look at the data sheets. They have good information
for everybody, from design engineers to hobbyists.

Good luck
Chris

.