Need some help here, please.
Situation as follows. In order to measure the very low distortion of an opamp, the distortion can be artificially increased by putting a small resistor directly between the opamp input pins.
Example: inverting opamp with 1k feedback and input resistor, signal gain -1.
Now I put 10 ohms directly on the input pins, which makes the noise gain 50x (two 1k parallel versus 10 ohms). This means that the distortion is increased by 50 x without changing the signal gain. So now I cam measure the distortion easier, and then just divide the result by 50 to get the 'real' distortion.
But. Assume the opamp has an open loop gain Aol of 100dB. The 10 ohms generates thermal noise of -145dBV. And whatever is between the opamp pins will be amplified by the Aol, so I would expect the output noise to be -45dBV. But it is much, much less. Why?
Jan
Situation as follows. In order to measure the very low distortion of an opamp, the distortion can be artificially increased by putting a small resistor directly between the opamp input pins.
Example: inverting opamp with 1k feedback and input resistor, signal gain -1.
Now I put 10 ohms directly on the input pins, which makes the noise gain 50x (two 1k parallel versus 10 ohms). This means that the distortion is increased by 50 x without changing the signal gain. So now I cam measure the distortion easier, and then just divide the result by 50 to get the 'real' distortion.
But. Assume the opamp has an open loop gain Aol of 100dB. The 10 ohms generates thermal noise of -145dBV. And whatever is between the opamp pins will be amplified by the Aol, so I would expect the output noise to be -45dBV. But it is much, much less. Why?
Jan