Electronic – Help with amplifier with feedback

amplifieroperational-amplifier

I am starting to learn to use operational amplifier recently. I construct a simple circuit with an amplifier (http://www.ti.com/product/op07c) with is driven by 9V (on VCC and GND). I have the V+ and V- connected to a function generator providing a sinusoidal signal of 500mV and 100kHz (also tried 100Hz). I have the output (Vout) feedback to the V-. T Vout connect to the V- via an 200ohm resistor (as shown in the fig). But if I observe the output with scope, it is not really a sinusoidal wave (it more like the positive section of the sine is flat while the negative section of the sine is like sinusoidal. But if I connect the Vout to V+ with an 200ohm resistor, the output is pretty well sinusoidal but the amplitude is attenuated instead of magnified.
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Best Answer

Just looking at the behavior you described and the datasheet there are a couple of points I want to present.

  • Since this doesn't look like a rail-to-rail OpAmp you will have some trouble with the available range in the output. This means in your case that it will probably only be able to swing to something like 1.5v to 7.5v (assuming a 1.5v drop by the OpAmp). If you try to generate an output higher than those values it will simply clip the value, something like your "flat top".
  • The other problem that you may encounter is that you are operating near the Unity-gain bandwidth of this IC (which is typically 400kHz, by the datasheet). This means that your OpAmp will operate different than a ideal opamp (which has inf. gain).

Try to reduce the gain of the circuit, or the amplitude of the signal. Also you can try to ad an offset to your signal (shift it up or down).

Also for a great reading about electronics take a look at the book The Art of Electronics. You can go directly to the first chapter about OpAmps, and if you got interested devour it from begin to end.

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