BeginnerTopic 18 of 104Electronics & Embedded
Ideal op-amp rules
With negative feedback, the inputs sit at the same voltage and draw no current.
In depth
With negative feedback the op-amp drives its output so the input difference tends to zero: V+≈V- (virtual short) and no current enters the inputs. Analyse by writing KCL at the inverting node. Valid only in linear operation within supply limits.
Key points
- Golden rules
- Not valid when saturated
- Check stability with capacitive loads
ExampleA voltage follower has gain 1 and very high input impedance.
What is Ideal op-amp rules?
With negative feedback, the inputs sit at the same voltage and draw no current.
Why is it important to understand ideal op-amp rules?
The ideal op-amp rules let you analyse any feedback circuit quickly by hand, because the inputs follow each other under negative feedback.
What are the two ideal op-amp rules?
With negative feedback, the two inputs are at the same voltage, and the inputs draw no current.
When do the rules fail?
At very high frequency, when the output saturates, or when feedback is positive, so the rules must be checked before use.
How is Ideal op-amp rules used in practice?
Typical applications include analysing a non-inverting amplifier for a microphone preamp, finding the output of a summing amplifier in a mixing desk and checking the gain of a precision buffer quickly before simulation.
What should I learn before Ideal op-amp rules?
Study "Operational amplifier" first (topic 17 of 104 in Electronics & Embedded). It covers the groundwork this topic builds on.
What are the key points of Ideal op-amp rules?
Golden rules. Not valid when saturated. Check stability with capacitive loads.
Can you give an example of Ideal op-amp rules?
A voltage follower has gain 1 and very high input impedance.
What comes after Ideal op-amp rules?
The next topic is "Inverting and non-inverting amplifiers", which builds directly on this one.
How difficult is Ideal op-amp rules?
It is classed as Beginner in Electronics & Embedded (topic 18 of 104). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.