So let's start out with Operational amplifiers. What are operational amplifiers? Well, Op-amps are high gain amplifier circuits that have two high-impedance input and one output terminal. Fascinating stuff ,huh? Like I mentioned , there are two input terminals : one is called inverting terminal and the other is referred to as non-inverting terminal. The diagram should clear it for you.
Since there are two input terminals, there are two ways you can provide input to the op-amp. It can be single ended or double ended (often known as differential input).
Let's talk about single ended input op-amps first. Why do you think they are called that? I mean, it's obvious, don't you think? So, yes, single ended input is when one terminal of the op-amp is grounded (0V) while the other terminal takes the input to give the desired output. Here, this diagram shows how the single ended input op-amp works.
So, figure (a) shows that the non-inverting terminal is taking the input while the inverting terminal is taking a nap. And in figure (b) it is just the opposite. Notice the output waveforms for both the figures and keep in mind that they are for same input signal.
Now, double ended input is when both the terminals of the op-amp is receiving an input. The diagram below says it all.
If you paid attention to figure (b) , you must have noticed that there is a voltage 'Vd' between the two terminals. This is called as the differential voltage and is equal to the difference of the input voltages at the two input terminals. So, mathematically,
Vd = V1 - V2
Now, there's another pretty common configuration called as the Double ended output configuration in which there are two outputs from the op-amp.
Let's get ourselves familiar with some basic terms that we will come across many times in the study of Op-amps.
Gain:
Not really just related to Op-amps but since the term has been used in this post, I would like to discuss what gain is. Gain is nothing but the ratio of output to the input. Gain as the terminology itself suggests is the measure of how much the quantity has increased. For amplifiers, gain is one of the most important parameter. Mathematically, it is the ratio of the output voltage to the input voltage.
when it comes to basic terminologies, gains can further be categorized in two types : Closed loop gain [A(cl)] and Open loop gain [A(ol)].
Closed loop gains are the gains when a negative/positive feedback is involved , otherwise it is called as Open loop gain. We will explore negative and positive feedback later in this post.
Common Mode Operation :
I am sure you must have read this somewhere and if you haven't then,Voila! Common mode operation takes place when same input is applied to both the input terminals. It only seems fair if the inputs at both the terminals are equally amplified and since they are opposite in polarity, the output is 0V. well, it should be but world isn't an ideal place so obviously there is some small output voltage. why do you think this output voltage results? why not 0V? well, it is because man isn't perfect and it seems only logical that his creations aren't perfect as well. The two terminals of the op-amp usually have a small difference in their gain , therefore , the difference that should have been 0V manages to sustain a very small value.
Common Mode Rejection :
Ideally, the op-amp reacts only to differential input (discussed above). The whole idea is to neglect the common input at the two terminals. However, there is some difference in the gains of the input terminals. So, in common mode rejection , the common mode input is rejected and only the differential input is amplified. Common mode rejection is an important property but why? It's quite simple,really. What is usually common everywhere? Useless stuff! Well, noise , in an engineer's language. This common mode rejection property is ,therefore, very useful as it helps attenuate the output signal. Common mode rejection ratio (CMRR) is therefore, very important parameter in the working of an op-amp.
Feedback circuits :
There are two feedback circuits, namely, Negative feedback and Positive feedback.
- Negative Feedback:
In negative feedback circuit, the output of the op-amp is connected to the inverting input . By doing this, the output of the op-amp follows the input. By the term 'follows', what I aim to say is that the output voltage V(out) varies in accordance with the input voltage (Vin). As the V(out) increases, the feedback going to the inverting terminal of the op-amp increases. This leaves a small difference between the input terminal voltages, differential voltage. This way the output voltage comes much closer to the input voltage. There comes the point when the output voltage is just perfect for the differential input voltage. As we observe, in a way, the system is stabilizing the output all by itself. This self-stabilization makes negative feedback circuits so special as they can work in a linear mode (active mode) rather than going toggling between 'on' and 'off' states.
The above diagram shows a basic negative feedback circuit. If the output of the op-amp circuit is directly connected to the input terminal of the op-amp (either inverting or non-inverting) then the circuit is also referred to as voltage follower circuit.
In negative feedback circuit like the one in the above diagram will try to have the differential input voltage as close to zero as possible. Therefore, the higher the gain, the closer the output and input voltages will be.
- Positive Feedback :
This feedback is realized when the output of the op-amp is connected to the non-inverting terminal of the op-amp. In this circuit the output is solely dependent on the non-inverting input. If the non-inverting input is increasingly positive at first then the output will be positive which will be fed back to the input and the input will be at positive saturation level ( meaning, maximum positive value it can possibly attain). Suppose when the input is increasingly negative at first. What will happen? The output will attain a negative value and will feed it back to the input leading to a negative saturation. Therefore, under positive feedback conditions , there are only two possible outputs : positive saturation or negative saturation. Therefore, this condition of hysteresis makes positive feedback used widely.
Now, Let's move onto Inverting and Non-inverting amplifiers
Non-Inverting amplifiers:
In these amplifiers, the operation is almost same as the voltage follower. However, there is one significant difference. Here, the voltage is divided by two resistors before being fed back to the input. Let us take a case in which the non-inverting terminal is grounded. Hence, it is natural that the inverting terminal will also have a voltage close to zero. Therefore, the voltage at Rf and Rin's junction will be close to zero . Thus, the circuit voltage drops to almost zero. If the voltage of the non-inverting terminal is a non-zero value then it is clear that the output voltage will be ,mathematically,
Vo= IRf+IRin
Inverting amplifiers:
In these amplifiers, the output is connected to the input inverting terminal and the non-inverting terminal is grounded (0V). The input signal is passed to the resistor Rin . This way the negative input voltage will create a positive output and vice-versa. This earns this circuit the name 'inverting amplifier'.
This concludes the basics of Op-amp. In next post, we will focus on what makes an op-amp , a good op-amp!
