Click here to purchase the entire book in PDF format. Non-Inverting Amplifier There are a couple of advantages of using the inverting amplifier - you can have any gain you want (as long as it's negative or zero) and it only requires two resistors. The disadvantage is that it inverts the polarity of the signal - so in order to maintain the correct polarity of a device, you need an even number of inverting amplifier circuits, thus increasing the noise generated by extra components. It is possible to use a single op amp in a non-inverting configuration as shown in the schematic in Figure 2.66. Figure 2.66: An op amp in a non-inverting amplifier configuration. Notice that this circuit is similar to the inverting op amp configuration in that there is a feedback resistor, however, in this case, the input to R1 is connected to ground and the signal is fed to the non-inverting input of the op amp. We know that the voltage at the non-inverting input of the op amp is the same as the voltage of the signal, for now, let's say +1 V again. Following an assumption that we made in the case of the inverting amplifier configuration, we can say that the level of the two input legs of the op amp are always matched. If this is the case, then let's follow the current through R1 and Rf. The voltage across R1 is equal to the voltage of the signal, therefore there is 1 mA of current flowing though R1, but this time from right to left. Since the impedance of the input of the op amp is infinite, all of the current flowing through R1 must be the same as is flowing through Rf. If there is 1 mA of current flowing through Rf, then there is a 1 V difference across it. Since the voltage at the input leg side of the op amp is + 1 V, and there is another 1 V across Rf, then the voltage at the output of the op amp is +2 V, therefore the circuit has a gain of 2. The result of this circuit is a device which can amplify signals without inverting the polarity of the original input voltage. The only drawback of this circuit lies in the fact that the voltages at the input legs of the op amp must be the same. If the value of the feedback resistor is 0$\Omega$, in other words, a piece of wire, then the output will equal the input voltage, therefore the gain of the circuit will be 1. If the value of the feedback resistor is greater than 0$\Omega$, then the gain of the circuit will be greater than 1. Therefore the minimum gain of this circuit is 1 - so we cannot attenuate the signal as we can with the inverting amplifier configuration. Following the above schematic, the equation for determining the gain of the circuit is $$\textrm{Gain} = 1 + \frac{Rf}{R1}$$ (3.96)