lakata@nntp-server.caltech.edu (Mark T. Lakata) (06/24/91)
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, Markmcovingt@athena.cs.uga.edu (Michael A. Covington) (06/24/91)
Is the LF356 by any chance a high-frequency op-amp? Some op-amps are not stable at *low* gain, because their compensating capacitors are deliberately made a little too small, relying on negative feedback (higher at high gain) to complete the stabilizing system. Also, are the power supply leads to the op-amp (both + and -) bypassed to ground with 0.05-uF capacitors? If not, they should be. -- ------------------------------------------------------- Michael A. Covington | Artificial Intelligence Programs The University of Georgia | Athens, GA 30602 U.S.A. -------------------------------------------------------
dana@locus.com (Dana H. Myers) (06/25/91)
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 *