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When amplifier works in follower mode and I don't send a AC signal to the input pin,I test a DC offset volt in the positive input pin.Why?

user3489913
Prodigy 10 points
Other Parts Discussed in Thread: OPA227, OPA228

After I send a AC sine to the positive input pin, the DC offset adds to the AC sine. Then,the output of the amplifer is saturated，it is VCC  or   VEE\

  • 0
    TI__Genius 12631 points
    Hello,
    Are you using the OPA227 or the OPA228. If you are using the OPA228 then you will need to switch to the OPA227. If you look at the description on the 1st page you will notice that the OPA228 is only stable in gain configurations of 5V/V while you are operating in a 1V/V configuration. The part is possibly oscillating right now. This is your main issue.

    The other issue I see is that there is no dc path for the input bias current to flow into on the non-inverting pin (pin 3). The OPA227/8 has a typical input bias current of 2.5nA. This current needs to have a low impedance dc path to ground which is presently not in your circuit. If it doesn't find the dc path then that current will flow into your 12pF capacitor which will then start charging up in voltage which subsequently causes your output to saturate. Try shorting the left side of R9 to GND and see if your output goes back to be close to GND. When you are driving the OPA22x from the previous stage if that stage has a low impedance output then you are fine and don't need to run the test where you short R9 to GND.

    Samir