I am a bit confused about how OPAs work. My questions are as follows:
Let's say I use an OPA with dual, simmetric supply. I do not put anything to its inputs. If there is no voltage difference between the two input pins, how will output work? Will it be 0V or something else?
What if I use the OPA with single supply? The same case ( no voltage difference between inputs ). What voltage will I measure on the output? Near 0V? ( I know, the voltage on the output cannot reach
0, except the OPA is rail-to-rail. Let's say it is a non rail-to-rail kind. ) Half of the supply voltage?
What if I use the OPA with +10V for positive and -3V for negativ supply. Would it work at all?
Thanks for your replies, Zsolt ( from Hungary )
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M
MooseFET
Ideally, the output is at zero volts when the difference between the two inputs is zero. In real life, this isn't what happens.
For many op-amps the output really follows a rule like:
Vout = Vneg + K1 * (IN1 - IN2 + Voff)
where: Vneg is the minus supply of the op-amp K1 is a huge number like a million IN1 is the non-invering input IN2 is the inverting input Voff is the offset voltage of this op-amp
Because K1 is so huge, even a tiny voltage difference between IN1 and IN2 matters more than it does, so we can drop it from the equation when we don't need a perfectly accurate description. This makes the equation:
Vout = K1 * (IN1 - IN2 + Voff)
Voff is usually such a small number that in most cases, we don't need to worry about it so we can drop it too.
Vout = K1 * (IN1 - IN2)
Now lets use this to figure the gain of a simple inverting op-amp.
First we need to say that the inputs of an op-amp draw no current. This isn't really true but the current they do draw is usually so small that it can be ignored.