Click here to purchase the entire book in PDF format. Comparators The first configuration we'll look at is a circuit called a comparator. You won't find this configuration in many audio circuits (blinking light circuits excepted) but it's a good way to start thinking about these devices. Figure 2.63: An op amp being used to create a comparator circuit. Looking at the above schematic, you'll see that the inverting input of the op amp is connected directly to ground, therefore, it remains at a constant 0 V reference level. The audio signal is fed to the non-inverting input. The result of this circuit can have three possible states. If the audio signal at the non-inverting input is exactly 0 V then it will be the same as the level of the voltage at the inverting input. The op amp then subtracts 0 V (at the inverting input, because it's connected to ground) from 0 V (at the non-inverting input, because we said that the audio signal was at 0 V) and multiplies the difference of 0 V by infinity and arrives at 0 V at the output. (okay, okay - I know. 0 multipled by infinity is really equal to any real number, but, in this case, that real number will always be 0) If the audio signal is greater than 0 V, then the op amp will subtract 0 V from a positive number, arriving at a positive value, multiply that result by infinity and have an output of positive infinity (actually, as high as the op amp can go, which will really be the voltage of the positive power supply rail) If the audio signal is less than 0 V, then the op amp will subtract 0 V from a negative number, arriving at a negative value, multiply that result by infinity and have an output of negative infinity (actually, as low as the op amp can go, which will really be the voltage of the negative power supply rail) So, if we feed a sine wave with a level of 1 Vp and a frequency of 1 kHz into this comparator and power it with a ±15 V power supply, what we'll see at the output is a 1 kHz square wave with a level of 15 Vp. Figure 2.64: The output voltage vs. input voltage of the comparator circuit in Figure 2.63. Unless you re a big fan of square waves or very ugly distortion pedals, this circuit will not be terribly useful to your audio circuits with one noteable exception with we'll discuss later. So, how do we use op amps to our advantage? Well, the problem is that the infinite gain has to be tamed - and luckily this can be done with the helps of just a few resistors.