Hi, I constructed ac coupled non inverting amplifier.And I use OPA827 as an op amp.However, when I apply +-15 V by power supply ,I see 4Mhz sin wave with 15V.By adding 100 ohm resistor between opamp output and scope, I can get rid of 15 V and see only amplified voltage. Question is ,what causes 15 V 4 Mhz at output? The picture of it attached.
Hello,
I am sorry, but it is hard to answer your question without a schematic diagram showing all the values of the passive components surrounding the opamp. I t would also help if you attached an actual photograph of the circuit.
When you say that you see a 4MHz sine wave with 15 V, do you mean that the 4 MHz sine wave is riding on top of a +15 VDC bias, i.e., that the average value of the sine wave is +15 VDC instead of zero? When you say that adding a 100 ohm resistor between the opamp output pin and the 'scope probe gets rid of the +15 VDC bias, does that mean you don't see the 4MHz. Or do I have that backwards, and the 4MHz signal is the amplified signal and the +15 VDC bias disappears with the 100 ohm resistor?
I think you can see what I'm getting at. You need to pose the problem so we don't have to read your mind.
Regards,
Tom Cuff
Hello
A common problem with many operational amplifiers, when used as a closed loop amplifier, is instability when driving a reactive load, such as a capacitive load, unless additional frequency compensation is applied to the feedback loop, and some resistance added between the OPA827 output, in this instance, and any capacitive loading, with 51 Ohms being a common resistive value that is recommended. In your description, the oscilloscope will be adding a capacitive load to the OPA827 output, which may be sufficient to cause the OPA827 to oscillate, as 4MHz is close to its unity gain maximum frequency, therefore, close to the frequency expected for the OPA827 to become unstable.
If you use a higher impedance oscilloscope probe, such as x 10 or x 100, the capacitance presented to the OPA827 will then be a lot lower. If this cures the problem, particularly with the 100 resistor removed and a direct connection restored, this would tend towards the OPA827 becoming unstable due to a capacitive load.
Without a copy of the circuit diagram it is difficult to provide an exact answer, but the symptoms you describe are those associated with a closed loop feedback application of the OPA827 operational amplifier becoming unstable due to a capacitive load applied directly to its output.
Regards
Andrew Porter
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