Basic Op Amp Circuits

Non-Inverting Voltage Comparator

Without any negative feedback, the op amp uses its maximum gain, which for an ideal op amp this would be infinite gain. This type of amp is a differential amplifier, meaning it will amplify the difference between the two inputs. Since there is no negative feedback, and a gain of infinity, the output must always be at the maximum or minimum, set by the V+ and V- supply connections. The exception to this would be if the input perfectly matches the reference voltage. Only then would the output be zero. Real world op amps have finite gains, so very small signals may produce outputs that do not drive the output to the max or min. This type of use of an op amp is rarely, if ever, seen. If the amp isn't being used as a comparator, it is probably using some kind of feedback network.

Comparators are useful as logic gates, switches, square wave generators, lighting up LED indicators, etc.

Inverting Voltage Comparator

The only difference between this circuit and the previous circuit is that the input terminals have been swapped. The output will now be inverted, so a logical HI signal will output LO and a logical LO signal will output HI.

Buffer Amplifier or Voltage Follower

The voltage follower arrangement is easy to recognize with its 100% series negative feedback path from output to inverting input. In the ideal op amp, the voltage output matches the voltage input perfectly. Practical op amps can get pretty close to the ideal, depending on how accurate you require the output voltage to be.

The buffer/voltage follower has a high input impedance, and it provides plenty of drive for the next circuits. It is often found at both the input and output sections. It is useful anywhere you want to transfer a voltage from one circuit to the next, without worrying about the stages interacting.

Idea op amp inputs have an input of impedance of infinity and therefore draw no current. Practical op amps have a finite, but still very large, input impedance, and most of the time you can ignore the input current. As a general rule, BJT inputs tend to have lower input impedances than FET inputs. The downside to an incredibly high input impedance is noise, and capacitive pickup problems. An external resistor from the input to a low impedance reference node is all that is needed to set the input impedance appropriately. The reference voltage for audio circuits is almost always midway between the V+ and V- rails. For bipolar supplies, like +/-15V, the midway point will be the 0V, or "ground" rail. For single supply supplies, like 9V battery effects, a 4.5V rail will be used. This "1/2 V+" rail is often created by just a simply 2 resistor voltage divider network with a large ac bypass cap so the rail has a very low ac impedance.

Being able to couple the output directly to an input isn't a trivial thing in electronics. The internal circuitry must have a minimal of half a dozen or more transistor stages within the IC package to pull this circuit off. I mention this, because the voltage follower arrangement is how I like to test a suspect op amp. It is easy to setup on a breadboard, and it can only function with a working op amp. It doesn't test all parameters of an op amp, but it makes for a good go/no-go type test when you're trying to figure out if you have a bad op amp or a bad something else.

Non-Inverting or Series Feedback Voltage Amplifier

In the buffer amplifier, we applied 100% feedback to the inverting input. By using less than 100% feedback, the op amp will boost the output voltage until the inverting and non-inverting voltages match. As the negative feedback decreases, the gain increases.

The input impedance is not degraded by the voltage amplifying arrangement. Impedance is set by an external resistance to the reference voltage rail.

The voltage divider at the output sends just a portion of the output to the inverting input. If the top resistor is "R1" and the bottom resistor is "R2," then gain is simply equal to 1 + (R1/R2). The constant "1" in the equation means we should never expect to get less than unity from the non-inverting arrangement.

Inverting or Shunt Feedback Voltage Amplifier

So far we have used the non-inverting input as our signal voltage input. We can use the inverting input as well.

The non-inverting input is simply connected to a reference voltage either directly or through a resistor. Just like in the voltage follower and series feedback voltage amplifier, the output will follow whatever the Vref is when the circuit is idle (no signal).

The feedback still has to be negative so our feedback path remains from output to inverting input. This requires we add some series resistance between the inverting input and our actual voltage input node. Why is this? Unlike our previous examples, we now have created a very low input impedance at the inverting input.

The negative feedback will attempt to keep the inverting input at the same voltage as the non-inverting input. As the reference voltage is typically "ground" or 0V, the inverting input becomes something known as a "virtual ground" since the op amp's mission is to get both inputs to match. The "virtual ground" is low impedance, like the regular ground is, so we need some series resistance in front of it to properly couple a voltage signal.

The input resistor sets the input resistance. Just as before, a large impedance is desirable for matching high impedance sources, while a small impedance gives better noise performance and is less susceptible to capacitive pickup problems.

The gain is simply -(Rfeedback / Rinput). Unlike the non-inverting amplifier, we can actually reduce, or attenuate our input with this arrangement. This is often useful and a reason to choose the inverting amplifier over the non-inverting amplifier.

Needless to say, the output will be 180º out-of-phase with the input. If Rfeedback = Rinput, then the gain will be unity, and the circuit can be considered an "inverting buffer," often useful when correcting or anticipating another phase reversal in a larger circuit so as to get the main input and output in-phase.

Voltage Summing Amplifier

Voltage Differential Amplifier