From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Subject: Re: Inverting buffer?
Keywords: inverting buffer
Organization: The Software Engineering Institute
Date: Thu, 10 Sep 1992 15:57:59 GMT


In article <1992Sep10.012158.5817@cbnews.cb.att.com>, Keith Brummett asks:


|>     A lot of folks have been playing with buffered-volume-control preamps
|>     (a la Aunt Corey's) lately.  Most of the schematics I've seen show the
|>     buffer to be an op-amp configured for unity gain with no resistors in
|>     the input (R1) or feedback (R2) paths.  I.e.:
|> 
|>                    ______________
|>                   |     __       |
|>                   |____|  \      |
|>                        | -  \____|____  Out
|>         In  ___________| +  /
|>                        |__/
|> 
|>     My question is, can one make a similar unity-gain INVERTING buffer, e.g:
|> 
|>                    ______________
|>                   |     __       |
|>         In  ______|____|  \      |
|>                        | -  \____|____  Out
|>                    ____| +  /
|>                  _|__  |__/
|>                  ///
|> 
|>     I can't seem to find any example of such an inverting buffer anywhere
|>     (one using no resistors, that is).  And, a Radio Shack engineering book
|>     claims that with the resistors, the gain is (1 + R2/R1), whereas the
|>     gain of the similar non-inverting buffer is just (R2/R1).  If this is
|>     true, how does one make a unity-gain, inverting buffer?


Well, Keith, I was going to refer you to the well-known folks on rec.audio who
think we don't learn anything useful in engineering school, but that isn't
fair to a non-participant.  So, I will answer your honest request for
information:

You need to find negative resistors -- these are the ones with the ultraviolet
fifth color band.  Can you borrow a portable geologist's UV lamp to take with
you to Radio Shack?


Naw, seriously, you've got it backwards above.  If you want a voltage output
in the inverting configuration, you'll need equal resistors, as the gain is
R-feedback divided by R-input.  The input resistance is simply R-input, as the
inverting input (-) in the second circuit above is a virtual ground (i.e., is
held - within limits - to the same potential as the (+) input.)  Note that
without a feedback resistor, the output will always be at the same potential
as the (-) terminal for any feedback current.  Note also that the op-amp will
place an effective short on your input (voltage source) without an input
resistor.  You have to get used to thinking of these things as current
balancers.

Please check your book again -- I think you simply misapplied text to the
wrong diagram.

I hesitate to recommend what use to be the industry bible on op-amps, the
Burr-Brown book, as it contains serious errors which have tripped up many an
EE student.  Perhaps others can suggest a saner, more intelligent text which
has replaced it, thus reassuring me that hope is sometimes rewarded with
progress.


Rich


PS: You _can_ create negative impedances with active (but not passive*)
devices, so one op-amp could present a negative resistance to another.

* Don't bother calling the patent office with one of these.


   "Please keep in mind that the ultimate goal [of hi-fi] is the
   reproduction of art, and that the invocation of science, while a neat
   parlour trick, is often unnecessary, flawed, and unreasonable."
   --  name mercifully withheld...                            rsd@sei.cmu.edu
From: myers@hpfcso.FC.HP.COM (Bob Myers)
Date: Thu, 10 Sep 1992 23:15:20 GMT
Subject: Re: *** SPEAKER WIRES ***
Organization: Hewlett-Packard, Fort Collins, CO, USA