Differential amplifier tutorial ppt
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Category: Differential pair ppt
Guide to the study of. Read the Instructions to know how you can better use this work. Know how it is organized and which navigation tools are available. See how you can complement the study with the simulation of some of the circuits presented here.
See the table of contents of this work. The table is organized through a pop down menu revealed when you place the cursor over the titles. Through the Index you can directly access each one of the sections and exercises of this work. The main text of this work is enhanced with several complementary texts, in order to help the reader about matters not directly studied here. These are matters which are supposed to be studied before or later.
Through the main text there are several links to these texts but you can also access them through the table of Annexes, organized in a similar way as the main Index. Operational amplifiers OpAmps with negative feedback allow highly versatile realisations, in particular highly stabilised gain amplifiers.
Take the example depicted in fig. To make this quantity a reasonable approximation it is simply required a very high open loop gain i. Thus, the amplifier above could be realised with a single transistor as indicated in fig.
Resistors R 2 and R 1 define the gain. Inserting a resistor between the emitter terminal and ground will boost the input resistance. Yet, this procedure reduces the gain and increases the output resistance, although marginally.
Alternatively, FETs can be used at the input - at the cost of lower g m and consequently lower gains. Nonetheless, no juggling will confer a symmetrical differential input to the CE topology. The solution resorts to a composed implementation with more than one transistor to obtain a differential input called the differential pair. Note, however, that other OpAmp characteristics should be searched for, such as: very high gain, high input and low output resistance, low voltage and current offsets.
Simultaneously, one should not loose site for other characteristic improvements, such as band width and maximum slew-rate. Differential pair. Consider fig. Thus, we may say that the differential pair ideally responds to differential signals i.
Current variation. The total emitter current is kept constant by the current source I. This change in transistor current with input differential variation can be observed in fig. The expression for the current can be found to be: The differential pair operation is approximately linear for small differential input voltages. This corresponds to a region in the graph where the exponential exhibits an approximate linear behaviour.
The basic schematic is similar to a bipolar differential pair and is shown in fig. The analysis is very similar to the differential bipolar case. Having in mind that: naming and making we get: fig.
The main remarks, relatively to the bipolar differential pair, are, on one hand, the larger v id value spread, and, on the other hand, the smaller characteristics slope around the origin. Small signal operation. Take the BJT differential pair as reference. Having in mind, for example, that: for the three possible outputs the following differential gains result: fig.
This last gain corresponds to an amplifier with differential signals both at the input and output fig. There is another way to look into this problem: If we consider the amplifier as an ideal differential amplifier where essentially the common mode gain is null , according to fig.
Then, the gain is approximately: However, if the other output is intended, it is enough to think that both collector currents signal are necessarily equal, and, consequently, the gain will be symmetric of the indicated above. Nonetheless, it is called the attention upon the fact that this configuration corresponds to a variant of a circuit known as cascode that it will be studied ahead.
Exercise 1: If in fig. A small signal analysis can also be done taking the equivalence between the differential pair and the CE configuration. Even assuming that the biasing source is not ideal see fig. Thus, each transistor is equivalent to a CE configuration with a grounded emitter, as shown in fig. From fig. A similar analysis can be performed on a FET differential pair. The sole relevant difference is the linear operation span which is significantly bigger in a FET differential pair.
Common mode operation. The common mode operation is illustrated in fig. CE montage If R C « r o , we get: and by analogy and The common mode rejection ratio is, by definition, such that, for each unique output v c1 or v c2 , we get. Exercise 2: Show that and explain why in this context where R is generally very high it makes sense not to forget r m , in general ignored for being very high. Operation with arbitrary input voltages. Let v 1 and v 2 be the signal components of v B1 and v B2.
In general, the differential pair input voltages, v 1 and v 2 , corresponds neither to a differential nor to a common mode. Rewriting v o expression we get: where CMRR is expressed in non-logarithmic form which then shows that, if the CMRR is sufficiently high, the output signal depends solely on the input differential component.
Because the desirable operation is precisely this, the term constitutes the error of the differential circuit model. Other non-ideal characteristics.
Input offset voltage. Thus, an input offset voltage can be defined as:. Thus, for a BJT pair, the offset result is:. Bias current and input offset current. Given its very small values, input currents are non-relevant for the FETs differential pairs. Consequently we will only consider the case of a BJT differential pair.
In a symmetric pair, the input currents at rest are equal to: This common value is called the input bias current I B. Due to the inevitable input asymmetry, the bias currents are in fact different. This difference is called input offset current. In particular, if transistor gains b differ by Db , the offset is: Up to here we have indicated a symbolic current source to bias the differential pair.
It maters now to see how can that current source be realised. Discrete circuits are going to be distinguished from integrated current source circuits.
Bias circuits for differential pairs. Discrete circuits. A discrete component typical constant current source CCS realisation is illustrated in fig. A practical example will allow us an easier router to evaluate and project CCS circuit. Then, assuming I B 0, we get: fig. Exercise 3: Find the source output resistance, R, having in mind the value of r o and that the transistor has an emitter resistor R 3.
Integrated circuits. The resistor values required by the previous setting are impractical for integrated circuits. On the other hand, good matching transistors are easy and economic to fabricate. This way, a common technique utilised in integrated circuits to realise CCS is the current mirror.
If both transistors are exactly matched, and since V GS is the same for both transistors, their currents will be equal. If both threshold voltages are the same, but different K factors are used, then fig.
The basic BJT current mirror configuration is shown in fig. Hence, the modifications usually made to the basic current mirror aim to overcome the limitations resulting from finite b and r o. The use of an extra transistor T 3 , in fig. The current mirrors output resistance made with MOS can also be increased using Wilson or cascode configurations. Improving the bandwidth. Recall that the amplifier bandwidth refers to the frequency range within which the gain remains almost constant.
We call lower and upper cut-off frequencies to those range limits. The criterion utilised to define these frequencies corresponds to the measure of the point where the maximum gain decreases by 3 dB, i. At the lower limit, i. So, when direct coupling is used, such as with integrated OpAmps, usually there is no gain decrease at low frequencies, accordingly the lower cut-off frequency is zero. Otherwise infinite frequencies would imply electrons or other carriers, such as holes in p type semiconductors infinite accelerations, and therefore infinite forces would be present, which are obviously impossible in Nature.
The upper cut-off frequency depends not only on the transistors characteristics and quiescent point but as well on the chosen circuit configuration. Then, in a direct coupling amplifier, the bandwidth coincides with the upper cut-off frequency.
CE configuration bandwidth. The CE behaviour at high frequencies is of special interest to study the differential pair, because, as we have seen before, the differential pair is somehow equivalent to a CE montage.
From the three basic configurations, it is precisely the CE that has the smallest bandwidth, i. The reason for this poorer behaviour at high frequencies can easily be found through a simplified analysis of the high frequency equivalent circuit of fig.
Op-amp | Block Diagram | Characteristics of Ideal and Practical Op-amp
Describe basic op-amp characteristics. Discuss op-amp modes and parameters. Explain negative feedback. Analyze inverting, non-inverting, voltage follower and inverting op-amp configurations. Symbol and Terminals. Figure 1a: Symbol Figure 1b: Symbol with dc supply connections.
Circuit Theory I
Back from the Analog computers era, Op-Amps have been used for mathematical operations with analog voltages hence the name operational amplifier. Till date Op-Amps are extensively used for voltage comparison, differentiation , integration , summation and many other things. Needless to say, the Operational Amplifier circuits are very easy to implement for different purposes but it has few limitations that often leads to complexity. The major challenge is to improve the stability of an op-amp in a wide bandwidth of applications. The solution is to compensate the amplifier in terms of frequency response, by using a frequency compensation circuit across the operational amplifier. The stability of an amplifier is highly dependent on different parameters. Before going straight into the advance application of operational amplifiers and how to stabilize the amplifier using frequency compensation technique, let's explore a few basic things about the operational amplifier. An amplifier can be configured as an open-loop configuration or a closed-loop configuration. In an open-loop configuration , there are no feedback circuits are associated with it. But in a closed-loop configuration , the amplifier needs feedback to work properly.
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CHAPTER 1: INTRODUCTION TO OPERATIONAL AMPLIFIERS
In electronics, an Amplifier is a circuit which accepts an input signal and produces an undistorted large version of the signal as its output. In this tutorial, we will learn about an important configuration of an Op Amp called the Non-Inverting Amplifier. In Non Inverting Operational Amplifiers, the input is fed to the non-inverting terminal and the output is in phase with the input. An Operational Amplifier or more commonly known as Op Amp is essentially a multi stage high gain differential amplifier which can be used in several ways. Two important circuits of a typical Op Amp are:. A non-inverting amplifier is an op-amp circuit configuration that produces an amplified output signal and this output signal of the non-inverting op-amp is in-phase with the applied input signal.
Basics Of Operational Amplifier
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The differential amplifier is probably the most widely used circuit building block in analog integrated circuits, principally op amps. We had a brief glimpse at one back in Chapter 3 section 3. It may have either one output or a pair of outputs where the signal of interest is the voltage difference between the two outputs.
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It may be used to perform numerous linear operations and some nonlinear operations. An important feature of operational amplifier is that by simply changing the feedback impedance, its operation may be altered. A modern Op Amp uses integrated circuit technology. The IC Op Amps are widely used as versatile, predictable, accurate and economical system building blocks. They possess all the merits of monolithic ICs. Many analog circuits , both linear and nonlinear, are constructed using IC Op Amp as the basic building block. This IC Op Amp, along with a few external discrete components may be used for the following linear analog systems:.
In this video, how to use the op-amp as the differential amplifier Difference amplifier or as subtractor has been discussed with In this video, the design and working of BJT- differential amplifier is explained. By watching this video, you will learn the following Instrumentation amplifier is a kind of differential amplifier with additional input buffer stages.
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