Ideal Operational Amplifiers: Rules and Applications
What are the two main rules as applied to an ideal Op-Amp and under what conditions are these rules applicable? What kind of amplifier does the circuit in Figure B2 represent?
- Rule 1: Infinite Input Resistance
- Rule 2: Zero Output Resistance
Ideal Operational Amplifiers Rules
An ideal operational amplifier (Op-Amp) is a high gain differential amplifier with infinite input resistance and zero output resistance. The two main rules as applied to an ideal Op-Amp are:
Rule 1: Infinite Input Resistance
The input resistance of an ideal Op-Amp is infinite, leading to zero input current. The voltage at both the inverting (-) and non-inverting (+) inputs of an ideal Op-Amp is the same due to the infinite input resistance, preventing any current flow into or out of the inputs. This rule is applicable when the input impedance of the circuit is very high, such as in buffer amplifiers.
Rule 2: Zero Output Resistance
The output resistance of an ideal Op-Amp is zero, resulting in a constant output voltage regardless of the connected load. The output voltage is limited only by the voltage supply to the Op-Amp. This rule applies when the output impedance of the circuit is very low, like in unity gain amplifiers.
Inverting Amplifier
The circuit in Figure B2 represents an inverting amplifier, where the output voltage is proportional to the negative of the input voltage. It has high input impedance and low output impedance, amplifying small magnitude signals. Negative feedback ensures stable gain and low distortion in this amplifier, with gain calculated as -Rf/Rin, where Rf is the feedback resistance and Rin is the input resistance.