"James P. Meyer" <jimbob@acpub.duke.edu> wrote:
|On Thu, 16 May 1996, James P. Meyer wrote:
|> On 16 May 1996, Clyde Manning wrote:
|>
|> > I imagine you will get a lot of answers to this.
|>
|> Some right, and some wrong.... 8-)
| Oops.... One of mine was wrong too. In particular, this part:
|> >
|> > If you are using an AC input to the opamp you risk damaging the opamp
|> > every time the waveform goes negative. Some opamps become very unhappy (
|> > regards to Dr. Budak at Colorado State University) when their input swing
|> > below their negative rail.
|>
|> The post clearly states that he's using plus and minus 9 volt
|> supplies. Eighteen volts, center tapped. AC would have to be larger
|> than 18 volts P-P to swing below the negative rail.
| Looking back, I see that he's probably only got +9 volts instead
|of a center tapped supply. In that case, he would need to be concerned
|about biasing the input so that the AC at the output could swing between
|+9 and ground. The circuit looks like a gain-of-one buffer, so he should
|make sure that the input has a no-signal bias voltage of about 4.5 volts
|as well as the AC input.
|Jim
If he has an AC input signal, he could get around this input problem by
putting a DC-Restorer on the input to the Op-Amp. This consists of a
capacitor and a low drop diode such as a germanium (small signal diode).
This will shift his AC signal so that it will "ride" on top of the DC
axis. Not perfect but this may suit his needs. There is a slight dip
below the 0V DC axis but not much. I had a -0.1V total dip in an actual
application with a 6Vp-p squarewave input signal but most opamps that are
designed to run rail-to-rail can tolerate up to -0.3V before latch-up which
is fine. My circuit was slightly different than below but the idea is close
enough.
The circuit looks like this:
|| |\
>-------||-----+----------+ \
|| | | \
0.001uF --- | /---+---
Ge diode / \ ----- / |
--- | |/ |
| |__________|
to
GND
This only works as well as long as the input resistance to the OP-Amp
very is high. If you start connecting other circuitry around it the performance
degrades dramatically. It is best to connect as he has, a hi-Z input Op-Amp in
the unity gain configuration as he has it for the best results. If you do do
this Greg, check it out on a scope. If you have any problems I'll let you know
what I did differently.
-Vince
vhadley@ee.utah.edu
Date: Fri, 17 May 1996 19:13:37 GMT
Original Subject: Re: LED Voltage..Look at this circuit