How do I go about calculating the voltage output for an operational amplifier when the only connection to the the voltage source is through the operational amplifier itself?
For example, the diagram below has a 1.8 V source and 7 kohm resistor connected to Vp and a 1.2 V source and 10 kohm resistor connected to Vn of the operational amplifier. These sources are not bridged by an additional resistor or wire connecting either source to Vo.
I understand that Kirchoff's laws can be used to determine the output when the voltage source connects to it via a node at Vp or Vn, but I don't see how to determine the output voltage without such a connection.
Edit: The answer should be a numerical value. The hint that was given is that the op amp will saturate to +Vcc if V+ > V- or will saturate to -Vcc if V- > V+
Given this, my understanding is that this is an instance of positive saturation since 1.8 V > 1.2 V. According to Fundamentals of Electric Circuits, Vout = Vcc for positive saturation and -Vcc for negative saturation, where Vcc is the supply voltage.
Best Answer
The equation that you need is:
\$ V_{out} = A_v (V_{+}-V_{-})\$
Av is the open loop gain.
The resistances can be neglected if this is an ideal opamp (if it's an opamp with protection diodes, then the resistors do matter) as ideal opamps have infinite input impedance meaning zero current flows into them.
If this is a real op amp, an input bias current will be given and the voltage drop of the resistors can be calculated. In the case of a real opamp (with rails, that are not shown in the model above) then it can't source more voltage than it has available and the output will be limited by what the rails can source.
This circuit is simply a comparator. The open loop equation above applies to both opamps and comparators for ideal simple amplifiers, however with rails either amplifier will saturate.
In the early days of amplifiers, people would try and run them open loop, with little success because of variations in amplifiers. It was then that someone realized that negative feedback can take out the inconsistencies