There are two inputs, one positive and one negative, identified by the '+' and '-' signs.

There is one output.

The inputs are high impedance. I repeat, the inputs are high impedance. This means they have (virtually) no effect on the circuit to which they are attached. Write this down, it is very important. We will talk about this in more detail later. This important fact is commonly forgotten and contributes to the confusion I mentioned earlier.

The output is low impedance. For most analysis it's best to consider it a voltage source.

Now let's represent the op-amp with two separate symbols.

Figure 2

What you see here are a summing block and an amplification block.

First, let's discuss the summing block. You will notice that there is a positive input and a negative input on the summing block, just like on the op-amp. Recognize that the negative input is as if the voltage at that point is multiplied by a -1. Thus if you have 1 volt at the positive input and 2 volts at the negative input, the output of this block is -1. The output of this block is the sum of the two inputs, where one of the inputs is multiplied by -1. It can also be thought of as the difference of the two inputs and represented by this equation Vs = (V+) - (V-)

Now we come to the amplification block. The variable G inside this block represents the amount of amplification that the op-amp applies to the sum of the input voltages. This is also known as the open loop gain of the op-amp. In this case we will use a value of 50,000. You say, how can that be! The amplification circuit I just built with an op-amp doesn't go that high? We will get to the amplification applications in a moment. Go find the open loop gain in the manufacturer's data sheet. This high of a gain or even higher is typical of most op-amps.

What will happen at the output if you put 2 volts on the positive input and 3 volts on the negative input? I recommend that you actually try this on a breadboard. I want you to see that an op-amp can and will operate with different voltages at the inputs. However, a little math and some common sense will also show us what will happen. For example:

Vout=50,000*(2-3), or a -50,000 volts.

Now unless you have a 50,000 volt op-amp hooked up to a 50,000 volt supply you won't see 50,000 volts at the output. What will you see? The output will go to the minimum rail. In other words, it will try to go as negative as possible. This makes a good deal of sense if you think about it like this. The output wants to go to a -50,000 volts and obey the mathematics above. It can't get there, so it will go as close as possible.

OK, reverse the inputs. Now the following is true.

Vout=50,000*(3-2) or a +50,000 volts.

What will happen now? The output will go to the maximum rail. Now, how do you know where the output rails of the op-amp are? That depends on the power supply you are using and the specific op-amp. You will need to check the manufacturer's data sheet for that information. Let's assume we are using an LM324, with a +5 volt single sided supply. In this case the output would get very close to 0 volts when trying to go negative and around 4 volts when trying to go positive.

At this time I would like to point something out. The inputs of the op-amp are NOT equal to each other. Many times I have seen engineers expect these inputs to be the same value. During the analysis stage the designer is coming up with currents going into the inputs of the device to make this happen (remember, high impedance inputs, virtually zero current flow). Then when he tries it out he is confused by the fact that he can measure different voltages at the inputs.

In a special case we will discuss in Part 2. You can make the assumption that these inputs are equal. It is NOT the general case. This is a common misconception, you must not fall into this trap our you will not understand op-amps at all.

The examples above indicate a very neat application of op-amps. The comparator circuit: this is a great little circuit to convert from the analog world to the digital one. Using this circuit you can determine if one input signal is higher or lower than another. In fact many micro-controllers use a comparator circuit in analog to digital conversion processes. Comparator circuits are in use all around us. How do you think the street light knows when it is dark enough to turn on? It uses a comparator circuit hooked up to a light sensor. How does a traffic light know when there is enough weight on the sensors to trigger a cycle to green? You can bet there is a comparator circuit in there.

Let's recap four important points about op-amps we have learned.
1. The inputs are high impedance. They have negligible effects on the circuit they are hooked to.
2. The inputs can have different voltages applied to them. They do NOT have to be equal.
3. The open loop gain of an op-amp is VERY high.
4. Due to the high open loop gain and the output limitations of the op-amp, if one input is higher that the other the output will 'rail' to its maximum or minimum value. (This application is often called a comparator circuit.)

In the next installment, I will discuss why these features are desired in an op-amp and what that means to engineers. I will go over the special cases I mentioned as well. You will be able to see that there are infinite applications for op-amp based circuits.

Product Engineering Archive

Copyright © Chipcenter 1999