Why is it in the rang(+_VDD) with offset amplified?

[jiangwp]

When we test a opAMP with open loop which has some offset, the output level is the rang between +vdd and -vdd.

EX. Gain=A Offset=1mV for a opAMP
obey the rule Vo=A(V+-V-)
Then the V0=Gain*Offset=A*(1mv)
Generaly, the Gain=1000~10000
So Vo=1v~VDD,
But , When we test the output level, it is not the calculate value but not constant value with the familar offset.

I want to know what causes the result.

 

[Borber]

You should first know the datasheet for OPA you are using. Gain at DC is usually in range 15000 to 200000. To measure output voltage caused from offset you should ground both inputs, assuming you have +V and -V supply voltages.

 

[masoudk_81]

Hi

the offset value is not constant for the opamps with the same number and
is also different between opamps with differnet numbers, so you should not
be sure for caculating output with the ofset of 1mv, for example, in 741
you can have 3 or 4 mv offset and this will be at most some microvolts
in op07 witch is a low ofset opamp. other thing is that when you test the
ofset and output, you should connect 2 inputs together for the noise not
to impact.


masoud

 

[Vamsi Mocherla]

Hey there seems to be some confusion about this. Actually, when we talk of offset of a MOS Operational amplifiers, the measurement is about the systematic and random offsets. So, the total offset is the sum of these things. Most importantly, when we design the amplifier, we measure o/p voltage for VDD/2 input on both terminals. So, the offset voltage is the (voltage seen at the output - VDD/2). this is the systematic offset.

Random offsets come from process variations. Good layout techniques will help us to evade that.

Coming back to the question, your opamp can never have a 1mV input referred offset voltage for a gain higher than 30 dB. It is enough to saturate the outputs