Introduction to Op-Amps - Part 2
A capacitor is used in this context to pass only the AC (Complex Waveform) component of the signal. As many of you might know, capacitors are also used in Cross-Over networks, as well as voltage storage in a power supply (to even out the voltage levels). For our use - which are circuits for audio signals at levels that are used in Microphones, Guitars, Keyboards and other electronic musical gear - there are some values that tend to work quite well. All the designs in this series will use those values for the types of input and output stages that you would use in this environment. Yes, it is possible to optimize this area, however, for our purposes, the values that I use will do an excellent job.
One question that comes up frequently is how to pick the correct voltage part when choosing a capacitor - its quite simple - choose a part that is as high or higher rated than the power supply. If you are using 2 9 volt batteries to drive the circuit - thats 18 volts - you'll probably use 25 volt or higher capacitors. Never use a capacitor rated at less than the maximum voltage of the power supply. I frequently use 50 volt or 100 volt capacitors - they are often the same price and will give the same results. Don't skimp here.
One problem that you will encounter with the input stages that are Non-Inverting is that the input stage needs a ground reference. Since the capacitor won't pass the DC equivalent of ground thru the capacitor, we need to add a fairly high resistance resistor that provides the ground reference for us. We don't need this on Inverting input stages because the Non-Inverting input is tied to ground and the Inverting input accomplishes the same thing by use of negative feedback. All the circuits shown here have these components in place along with the values.
In Figure 1, you see a standard Inverting Amplifier. the gain is set by the ratio of resistors R2:R1 (As covered in Part One of this discussion). For example, to set it for a 10 times gain, R1 = 10,000 ohms and R2 = 100,000 ohms. This gives you 100,000:10,000 - or 10:1. Cin and Cout are the blocking capacitors. We only need to provide this blocking facility on stages that connect to other devices that are not part of our circuit, so, you normally only use Cin at your inputs, and Cout at your outputs - you don't need that many capacitors to resolve the DC offset problem. You'll note that there is an Rout on the output stage. This is to provide a ground reference for the next active device that this circuit is plugged into.
In Figure 2, you see a standard Non-Inverting Amplifier. The circuit looks pretty similar, except that there is an extra component in the Input DC Blocking section - its the ground reference resistor. You'll also note that the values of Cin differ depending on whether you are building an Inverting or Non-Inverting input stage. These values have to do with the input impedance differences. The output DC Blocking components are the same in either type of amplifier.
You'll note one additional component - that is the shorting input jack that I show for the Signal In connection. This type of jack is wired such that when no 1/4 inch phone jack is plugged in, the input is set specifically to signal ground. Doing this eliminates a lot of potential noise problems.
In Figure 3 we take the 4 channel Mixer from Part 1 and add the components needed to interface this with the outside world. The inputs are designed for high impedance devices, such as Guitars, Keyboards, High Impedance Dynamic Microphones and the like, that use single sided (unbalanced) inputs. You'll also notice that I've added a level control on each input to the mixer, otherwise, the DC blocking section is pretty much the same as the Figure 1 above.
Note: These circuits work best with a High Slew Rate Op-Amp, such as the TL081 or TL082.
A good advanced reference book for this topic would be the "IC Op-Amp Cook Book" by Walter G. Jung (ISBN 0-672-20969-1 for older book that I have - current ISBN 0138896011). I got my copy in the late 1970's - there is a newer version available, however, the concepts remain the same.
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