Introduction
Operational amplifiers (Op Amps) are commonly found in typical, usually analog, circuits. As the name implies, they amplify signals for one thing. They have various other uses, however, but we’ll explore the basic behaviors of a typical opamp and explain them in laymen’s terms as much as possible.
Opamp symbol
Typical opamp symbol used in schematics.
Opamp circuit–(click to enlarge)
Typical configuration found in many applications, where the resistors can be anywhere between zero (shorted) and infinite (open), depending on particular application.
Basics
You may have seen the opamp symbol above. Common ones are typically standardized to a particular pin out configuration, but it’s not a rule. In this example, here are the pin functions:
- Pin 7 = Positive supply
- Pin 4 = Negative supply
- Pin 2 = Signal input (negative)
- Pin 3 = Signal input (positive)
Other pins may or may not have a function. If they do have a function, that would be beyond the scope of this topic.
Supply Pins
First off, there may be some confusions, as to the way the supply voltages are connected. Simply put, imagine taking two 10V batteries and stacking them on top of each other, positive to negative. Don’t stack them positive to positive, or negative to negative. Stack them as you would for a flashlight that takes two batteries.
Now you have total of 20V from end to end with the two together, with positive on one end and negative on the other end. Now connect the middle point between the two batteries to ground, or 0V and refer all voltage measurements to that ground. So the positive voltage of the stacked battery would be 10V and the other end would be -10V. Try measuring this yourself with a volt meter with the negative lead (the black one) connected to the middle point between the batteries. Now, I hope you can see where pins 7 and 4 should be connected to.
You might be thinking, “why not just use one battery from 0V to 10V or 20V?” There are applications where that is the case, but it turns out that that is little more involved with other concerns, thus beyond the scope of this discussion.
Signal Input Pins
This is where the opamp “senses” the signals that is being manipulated to do something. Another area of confusion is with the terms “+” and “-” of the inputs. It’s not like connecting the supply voltages. What they translate to the user is that the output voltage, “Vout,” moves up or down depending on what is at the inputs.
Here is basically what it does:
- If + input is more positive than – input, the output will go more positive.
- If – input is more positive than + input, the output will go more negative.
- It + and – outputs are exactly equal, the output will not change, ideally.
It’s the third condition, above, that we want to strive for in many of the applications.
Behavior
So, we touched on the three behaviors of an opamp. The first two are also used in many application and may be the easiest to understand and to implement. It’s called a g.
To add more realism to the opamp behavior, let me mention that the change in the input voltage is significantly amplified at the output. So much so that you’d have a hard time adjusting the two input voltages to keep the output from going to either maximum or minimum voltage.
Let’s say you want to keep the Vout at 0V. You adjust the two voltage just so, to get the output from changing. But soon, everything from the supplies to the two input voltages to the device itself will change with some temperature shifts, causing all the voltage settings you set to become something other than what you had. They might be very subtle changes, but remember the behavior of Vout? It changes significantly more with the changes in the input.
This brings us back the Comparator application. The simple fact that the Vout changes with the slightest changes in the input voltages. gives us a way to “snap” the Vout to maximum or minimum voltages, which is how comparators operate. They’re useful for turning on/off something based on what the inputs are doing. You can try setting the Vin- = 0V and changing Vin+ up and down to cause the Vout to snap high or low.