- #1
earlofwessex
- 87
- 0
hi
I've been set a project to test the limitations of the ideal op amp when applied to the real ones, and its doing my head in, if any of you could help me figure this one out i'd be very thankful.
the first thing I'm confused about is the input offset voltage:
i know its the voltage between the inputs, (caused by what?) causeing the output to saturate (+ or -) if both inputs are tied to GND without negative feedback.
it can be nullified in some chips (including the 741 which i am testing), but I'm "not alllowed" to do this, as the tests are for the circuits as given to me (standard non inverting/inverting/summing/differential/comparator).
so how do i measure it for a specific circuit? say the inverting one.
i can't just measure the V between terminals, since this will change with output due to the non-infinite open loop gain (can't measure that until i know the CMRR). i could adjust the input voltage until the output was 0v, but then the p.d. between inputs would still be altered by the input bias current and the resistor tolerances (R3 =/= R1||R2)
and even if i could just ignore that, i still don't know how it changes with Vin, or why its there in the first place.
(since its polarity seems to be random, i'd guess its there as a semiconductor thing, like diodes forward bias voltage, which might suggest that it changes little with changing circuit conditions, but I've got no reference or basis for that)
thanks for your thoughts/comments
I've been set a project to test the limitations of the ideal op amp when applied to the real ones, and its doing my head in, if any of you could help me figure this one out i'd be very thankful.
the first thing I'm confused about is the input offset voltage:
i know its the voltage between the inputs, (caused by what?) causeing the output to saturate (+ or -) if both inputs are tied to GND without negative feedback.
it can be nullified in some chips (including the 741 which i am testing), but I'm "not alllowed" to do this, as the tests are for the circuits as given to me (standard non inverting/inverting/summing/differential/comparator).
so how do i measure it for a specific circuit? say the inverting one.
i can't just measure the V between terminals, since this will change with output due to the non-infinite open loop gain (can't measure that until i know the CMRR). i could adjust the input voltage until the output was 0v, but then the p.d. between inputs would still be altered by the input bias current and the resistor tolerances (R3 =/= R1||R2)
and even if i could just ignore that, i still don't know how it changes with Vin, or why its there in the first place.
(since its polarity seems to be random, i'd guess its there as a semiconductor thing, like diodes forward bias voltage, which might suggest that it changes little with changing circuit conditions, but I've got no reference or basis for that)
thanks for your thoughts/comments