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LM318 oscillates not stable

Other Parts Discussed in Thread: LM318

I am trying to build a simple subtraction amp like figure 45 of the LM318 data sheet.   attached (I think).  I need to subtract 2 sine wave signals both of which are 6.78 MHz.     I have bypassing on the power supply, and the amp oscillates and is not stable at somewhat low frequencies,  like less than 1 MHz.   I am building this on a solid ground plane and care has been taken to be sure all wires are short.   my resistors have very short leads.   everything is built close to the copper ground plane.

I would be willing to use another op-amp, if there is a better, easier to use amp which maintains stability.

thanks

 shown  lm318 diff amp.pdf

  • Hello Mark,
    I have some questions:

    What supply rails are you using? With the LM318, you must make sure that the input voltage has at least 3.5V headroom to either rail (e.g. Vs= +/-15V would allow input range of +/-11.5V). This applies to the the peaks of your input signal which must fall within the said range. With Vs= +/-5V, you have very little input voltage range.

    Is your differential input source close to this stage (on the same board?). If not, are you using a cable to bring in the differential input, and if so are you terminating that cable locally (say with 100ohm or so)? If you are probing the output of the LM318 with a scope directly, try using a series resistance (say 100ohm) instead and see if that makes a difference? If you were to raise RF (and with it, the 10k shunt on pin 3 to ground), does that help with the oscillation at all? Is your close-by de-coupling cap of the ceramic type (not aluminum)?

    Regards,
    Hooman