Schematic for the amplifier: enter image description here Before connect it to amplifier circuit: input without connecting to amplifier Why the sine wave will distorted? any possible reason? is this due to noise? blue is the input signal of an amplifier and green is the output signal

100kHz 1MHz] 3Mhz 7Mhz

  • 1
    The input has distortion, so the amplifier isn't responsible. Maybe your ground lead is faulty? – gbarry Apr 9 '14 at 8:05
  • if i juz connect the input to the oscilloscope without connect it to amplifier, the input is a clean sine wave as shown in above. – user37970 Apr 9 '14 at 8:24
  • Feel like sharing your circuit? – Ignacio Vazquez-Abrams Apr 9 '14 at 8:50
  • @IgnacioVazquez-Abrams,sure,it is attached as above – user37970 Apr 9 '14 at 8:57
  • For all the pictures, you need to explain what the input signal is because I can't tell. Also what is your circuit construction method. If it's breadboard then you can expect this to happen unless you are extremely rigourous with positioning of components. Also, adding a load is recommended, maybe 100 ohms. – Andy aka Apr 9 '14 at 9:02

Check this out from the datasheet:

enter image description here

If your feedback resistor is much less than about 1K with +/-15V supplies, it's going to oscillate like a banshee with only a tiny amount of load capacitance (such as a scope probe).

The circuit is oscillating.

The reason is as follows: The opamp is a current-feedback amplifier which needs a feedback resistor that must exceed a certain limit (specified by the manufacturer).

You should consult the data sheet to see the lower limit of this feedback resistor - typically, some hundreds of ohms.

(By the way: Don´t worry about capacitors in parallel to the power supply; they are useless in simulation as long as the dc source is ideal).

On a quick look, I see one problem is that you have relatively large power supply decoupling capacitors. These are likely to have a large-ish internal resistance/inductance. This might permit the op-amp to oscillate. Try adding 0.1uF caps from V+ and V- to ground, near to the op amp.

Almost certainly, the center of the problem is that the op amp is oscillating at some high frequency, and that oscillation is added to your intended signal. This could be caused by power supply connections whose voltage varies when the op amp tries to draw rapidly changing current. That's why my suggestion to add fast-responding bypass capacitors.

Or it could be caused by some capacitance of the load (possibly the scope probe) causing the feedback loop to have some delay (phase shift), resulting in the feedback being positive for some frequencies.

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