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From: dana@locus.com (Dana H. Myers)
Newsgroups: sci.electronics
Subject: Re: Help with op-amp follower
Message-ID: <1991Jun24.235226.2801981@locus.com>
Date: 24 Jun 91 23:52:26 GMT
References: <1991Jun23.230350.27027@nntp-server.caltech.edu>
Organization: Locus Computing Corp, Los Angeles
Lines: 33

In article <1991Jun23.230350.27027@nntp-server.caltech.edu> lakata@nntp-server.caltech.edu (Mark T. Lakata) writes:
>What would cause high-frequency (MHz) oscillations in an op-amp
>follower (output connected to inverting input)?  My circuit worked
>fine for a while, then developed this oscillation that would sometimes
>go away when I touched the contacts on the back side of the circuit board.
>I'm using an LF356 op-amp on a wire wrap board.  The oscillations happens
>both with no input attached and with an input attached.
>
>               sine wave input
>	       5vpp, 10 - 10^6 Hz input
>                 |
>               =====  special capacitor 
>                 |
>                 |       |\
>                 +-------|+ \
>		 |       |    \______ out
>        .01 uF =====     |    /  |
>		 |    |--|  /    |
>		 |    |  |/      |
>	        gnd   |__________|
>
>Opamp supplied with +/- 7 volt supply.
>Thanks, Mark

  Looks to me like there is no DC bias on the non-inverting input. Try
putting a large value resistor to ground. This will have the effect also
of setting the quiescent output voltage to 0V.


-- 
 * Dana H. Myers KK6JQ 		| Views expressed here are	*
 * (213) 337-5136 		| mine and do not necessarily	*
 * dana@locus.com		| reflect those of my employer	*