The differential amplifiers are as their names are chests, able to amplify the difference between their input voltages. This characteristic makes them indispensable for the input stage of the more powerful operational amplifiers and therefore it is also indispensable to know about them. In this video we will talk about the basics of differential amplifiers including the function as well as their ideal and real properties. You will find them easy to understand since in our other videos we have already learned about transistor amplifiers and other transistor circuits such as current sources. If we now combine the knowledge we have gathered so far and put together the circuits we have already deal with we can build a differential amplifier. If you have somehow missed our previous videos about transistor circuits and the transistor as an amplifier we highly recommend having a look at them first. You can find the corresponding links in the video description. But now let's dive into differential amplifiers. A basic differential amplifier can be constructed from two transistor amplifiers and a current source. To do so we use two common source amplifiers which are connected at their source terminals and a shared source resistor as the simplest type of current source. Strictly speaking the current source acts as a current sink in this application. Instead of bipolar transistors as in our other videos we will use MOSFETs this time. But this makes no difference for the understanding of the functionality. The circuit we get has two inputs at the gate terminals and two outputs at the drain terminals of the MOSFETs. To put the differential amplifier into operation we still need supply voltages. A typical supply for a differential amplifier uses symmetrical voltages. For example plus 12 and minus 12 volts refer to current which are applied at the drain resistors RD and the current source RS. To understand the basic function of a differential amplifier let's first assume that the circuit's components are ideal. This means both halves of the circuits are identical. So both transistors have identical properties. Their drain resistances have the same value and the current source sinks a constant current despite any other conditions in the circuit. Let's first apply the same voltage which we call a common mode voltage to both inputs. Now all currents and voltages in the circuit are symmetrical. As a result also the output voltages have the same size which leads to an output voltage difference of zero volts. In this case the so-called common mode gain is zero. Remember that this is part of the ideal behavior of a differential amplifier as it should only amplify an input voltage difference. In the next step we apply different voltages to the inputs. For example 5 volts to input 1 and 0 volts to input 2. We call this differential mode voltage. Since the voltage at input 1 is larger than the voltage applied to input 2 the current through the left half of the circuit increases drastically. As a result the current and the voltage drop across the left drain resistor also increases resulting in a lower output voltage at output 1. As we assume our current source to be ideal its current must remain constant no matter what input voltages are applied to the differential amplifier. Therefore the current through the right half of the circuit must decrease resulting in the decreased voltage drop at current across the right drain resistor which again leads to an increased output voltage at output 2. In summary a difference between the input voltages results in an amplified difference between the output voltages. The gain of the difference in the input voltages is called the differential mode gain. We will take a closer look at the calculation of this value as well as other important values for differential amplifiers in our next video. By now we only talked about ideal differential amplifiers but as all of you know nothing is ideal in this world. So let's have a quick look at the real properties of our circuit. Earlier in this video I told you that the common mode gain is ideally 0. Since I used the word ideally I am sure you already guessed that this does not hold true for the real differential amplifier. To the sorrow of the electronics engineer in practice also common mode signals get amplified as the two halves of our circuit will never be completely identical. This is one of the things we have to keep in mind when working with differential amplifiers. To do so we should always check for the so called common mode rejection ratio of our differential amplifier. The common mode rejection ratio is calculated by putting the differential mode gain and the common mode gain in relation. The result gives us a ratio that shows how well common mode signals are suppressed while differential mode signals are amplified. It is infinite for an ideal differential amplifier where the common mode gain is 0. But a common mode gain different to 0 is not the only problem the non ideal parts of our circuit will cause. We also have to think of properties like the offset voltage or offset current which are also discussed in one of our other videos about operational amplifiers. Since differential amplifiers are a major part of operational amplifiers the explanations in the other video also hold true for them. If you want to find out more about the non idealities you can find the link in the video description. After discussing a few non idealities of differential amplifiers some of you might wonder if there is anything we can do to make our circuit work better than the simple one we looked at. Fortunately the answer is yes. For example we could change our very simple current source which up until now was a resistor to a more accurate one like a current mirror. Such a current mirror can also replace our drain resistances which will result in a better common mode rejection ratio. There are many more possibilities to improve the circuit not only regarding its common mode rejection ratio but also its temperature behavior, accuracy, speed or noise suppression but discussing all of them would go way beyond this cope here. As a conclusion let's briefly summarize what we have learned so far. A simple differential amplifier consists of two transistor amplifiers and a current source. There are many ways to build such a circuit though. The important thing is that a differential amplifier ideally only amplifies differential mode signals. This quality is expressed by the common mode rejection ratio which should be very high. The more identical the two halves of our circuits are the better. This is also one of the reasons why differential amplifiers are mainly used in integrated operational amplifiers as it is much easier to achieve very similar conditions for both circuit halves in integrated circuits. The calculation and dimensioning of differential amplifiers will be discussed in one of our next videos. I'm Clara with the Institute of Electronics. I hope you have learned something today but anyways thanks for watching. For the interested viewer we highly recommend the Art of Electronics Behorrow It's Until and for our German speaking viewers Electronic Shardronstechnik written by members of our institute.