If you are not familiar with op-amps it might be worth reading the first two articles in this series: Operational Amplifiers - Basic Concepts and Operational Amplifiers - Negative Feedback Ideal op-amp rulesIn the last article in this series it was shown that op-amps have a very large gain; when looking at ideal op-amps this gain is considered to be infinite. When put in a negative feedback configuration this infinite gain effectively means that the op-amp will drive the output to what ever value is necessary to make the voltage difference between the two inputs (V+ and V-) equal to zero. In addition to infinite gain two further assumptions are made when thinking about ideal op-amps. Firstly the input impedance of the amplifier's inputs is assumed to be infinite. This means that no current enters the op-amp's input terminals. In reality a small bias current is drawn. Secondly the output impedance of the op-amp is assumed to be zero. This means that the load connected to the op-amp's output terminal has no effect on the output voltage of the op-amp. In reality the output impedance of an op-amp is small but greater than zero. The three rules are summarised below: RULE 1: The op-amp will drive the output so that the voltage difference between the two inputs is zero.RULE 2: No current is drawn by the op-amp's inputs.RULE 3: The output load does not affect the op-amp's output voltage.Using the three rules we can develop several ideal op-amp circuits. The two most common op-amp circuits are the inverting amplifier and the non-inverting amplifier. Non-inverting amplifierThe non-inverting amplifier applies a positive gain to the input. The configuration of a non-inverting amplifier is shown in figure 1 below. Figure 1 The gain of the amplifier can be calculated using the three idea op-amp rules. It can be seen that the resistors R1 and R2 form an potential divider between V _{out} and GND. The output of the potential divider is at the negative input of the op-amp (called V_{A }in figure 1_{). }From this we can say that:For example setting R2 to be 9 kohms and R1 to 1 kohms would give a gain of 10. It should be noted that the gain can not be set below 1 in this configuration. If R2 is a short circuit and R1 is an open circuit then the gain is 1. This called a buffer amplifier and was shown in the previous article. Inverting amplifierThe inverting amplifier applies a negative gain to the input. The configuration of an inverting amplifier is shown in figure 2 below. Figure 2 Since R2 and R1 are positive values the gain will always be negative. Differential amplifierA differential amplifier amplifies the difference between the two input terminals of the op-amp. The configuration of a differential amplifier is shown in figure 3. If the resistors are set so that Rg/R2 = Rf/R1 then equation 10 cancels down to: SummaryThis article has covered three ideal op-amp circuits, the non-inverting amplifier, the inverting amplifier and the differential amplifier. Methods to calculate the gains of each of these circuits have been shown. - By using negative feedback the closed loop gain of the op-amp can be set.
- Ideal op-amp circuits assume that the gain and input impedance are infinite and the output resistance is zero.
- The ideal op-amp rules combined with traditional circuit techniques can be used to calculate the gain of an op-amp circuit.
The next article will cover several further idea op-amp circuits. |

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