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What is an operational amplifier (op-amp)? An op-amp is a multi-stage, direct coupled, high gain negative feedback amplifier that has one or more differential amplifiers and its concluded with a level translator and an output stage. A voltage-shunt feedback is provided in an op-amp to obtain a stabilized voltage gain. Op-amps are available as Integrated Circuits (IC s). The main use of an op-amp is to amplify ac and dc input signals and was initially used for basic mathematical operations such as addition, subtraction, multiplication, differentiation and integration. Nowadays, the application of op-amp s varies from ac and dc signal amplification to use in active filters, oscillators, comparators, voltage regulators, instrumentation and control systems, pulse generators, square wave generators and many more electronic circuits. For the design of all these circuits the op-amp s are manufactured with integrated transistors, diodes, capacitors and resistors, thus making it an extremely compact, multi tasking, low cost, highly reliable and temperature stable integrated circuit. It is also designed in such a way that the external characteristics can be changed with the addition of external components like capacitors and resistors. Thus it can act as a complete amplifier with various characteristics. OPERATIONAL AMPLIFIER: Amplifier is a device or circuit, which increases strength of input signal in terms of either voltage or current. The process of increasing strength is called amplification. Consider the following figure. Here a number of blocks are shown. The working of each block is given below. Remember that an opamp has two input terminals and one output terminal Page 2
First differential amplifier it has double-ended output called as differential output. This block is connected to constant current source. It has following main features o High & stable gain output with respect to time and temperature. o High input impedance. o Large bandwidth i.e. it gives high frequency range. o Reduces noise due to high C.M.R.R. Second differential amplifier it has single ended output i.e. between collector and common ground of one transistor. As two differential amplifiers are cascaded, total gain & C.M.R.R. is high. Third Level shifter this block forces output voltage to zero. When multiple stages are cascaded in opamp, level of DC voltage increases. Then at output of emitter follower, the DC level becomes so high that it is almost equal to supply voltage. So when input signal is absent, there will be DC output voltage only. This produces hiss i.e. humming. To avoid such problem, level shifter block is used. It forces output voltage back to zero (to ground potential). Note that group of second differential, emitter follower and level shifter is called as INTERMIDIATE STAGES. Fourth Output stage it is generally a push-pull amplifier. It has low output impedance. So it gives sufficient current to load at output terminals. It also gives large output voltage without distortion. It is used to match impedance of load with output impedance Characteristics of Operational Amplifiers (Op-amp) 1. High input impedance- More than 100kilo ohms. 2. Low output Less than 100 ohms. 3. Amplifier signals with frequency range from 0Hz to 1MHz. 4. Low offset voltage and low offset current. Page 3
5. Very high voltage gain About 2,00,000. DEFINE OPAMP TERMS: 1. Common mode gain: When same voltage is applied to both input terminal the voltage is called common mode gain. 2. Differential voltage gain: it is the ratio of differential output voltage to the differential input voltage Differential voltage gain =differential output voltage/differential input voltage 3. CMRR (common mode rejection ratio): It is the ratio of differential voltage gain to the common mode voltage gain. Higher the value of CMMR better is matching between two input terminal and smaller is output common mode voltage. 4. Slew rate: It is maximum rate of change of output voltage per unit of time. It is in voltage per microsecond. SR = dvo/dt max V/µs 5. Gain bandwidth product: Gain bandwidth product is defined as the band width of the op-amp when the voltage gain is unity. It is also called the closed loop band width or unity gain band width or small signal band width. 6. Virtual ground Concept: When a wire is connected between two points in a circuit it brings the two points at equal potential with respect to ground. The wire provides path for the current flow in either direction between the two points. Page 4
LIST APPLICATION OF OP-AMP APPLICATION: Inverting Non inverting Unity gain amplifier Integrator Differentiator Comparator Summing amplifier logarithmic Current to voltage converter Voltage to current converter Voltage to frequency converter Frequency to voltage converter Instrumentation amplifier Other applications: Audio and video pre-amplifiers and buffers Voltage comparators Differential amplifiers Differentiators and integrators Filters Precision rectifiers Voltage regulator and current regulator Voltage clamps Oscillators and waveform generators Schmitt trigger Gyrator Comparator Active filter Analog computer Page 5
Negative feedback amplifier Figure 1: Ideal negative feedback amplifier. A negative feedback amplifier (or feedback amplifier) is an electronic amplifier that subtracts a fraction of its output from its input, so that negative feedback opposes the original signal. The applied negative feedback improves performance (gain stability, linearity, frequency response, step response) and reduces sensitivity to parameter variations due to manufacturing or environment. Because of these advantages, many amplifiers and control systems use negative feedback. An idealized negative feedback amplifier as shown in the diagram is a system of three elements (see Figure 1): An amplifier with gain A OL (Open Loop gain) A feedback network 'β', which senses the output signal and possibly transforms it in some way (for example by attenuating or filtering it) A summing circuit that acts as a subtractor (the circle in the figure), which combines the input and the attenuated output Without feedback, the input voltage V' in is applied directly to the amplifier input. The according output voltage is Page 6
NON-Inverting amp: Non-inverting amplifier is one of the most popular op amp circuits similar to op amp inverting amplifier circuit. It provides a gain to the input signal without any change in the polarity. If a sine wave is fed to the input of this op amp non inverting amplifier, the output will be an amplified sine wave with zero phase shift. Here the input is applied to the non inverting terminal of the op amp. The non inverting amplifier gain is given by the expression A=1+Rf/Ri where Rf is the feedback resistance and Ri is the input resistance. The input impedance of non inverting amplifier is extremely large, typically 100MΩ. Non inverting amplifier circuit diagram Working of Non inverting Amplifier The working of non inverting amplifier is similar to that of inverting amplifier except that the output has no phase shift. Page 7
The resistors Ri and Rf form a voltage divider network. A negative feedback is provided by applying a little of output voltage to the inverting input terminal through the potential divider network Ri and Rf. The voltage gain of the amplifier is determined by the ratios of Rf and Ri since Gain, A=1+Rf/Ri So the amplitude of the output voltage signal can be varied by varying either of the resistors Rf or Ri. Inverting amp: Inverting amplifier is one of the most popular Operational Amplifier circuits. The output changes in such a way that tries to avoid saturation and counteract the change caused by the input. This makes the amplifier stable. The amplifier tries to resist change and so avoid saturation. As the open loop DC gain of an operational amplifier is extremely high, we can therefore afford to lose some of this gain by connecting a suitable resistor across the amplifier from the output terminal back to the inverting input terminal to both reduce and control the overall gain of the amplifier. This produces a very stable Operational Amplifier based system. The polarity of input voltage gets inverted at the output. If a sine wave is fed to the input of this amplifier, the output will be amplified sine wave with 180 phase shift.it has so many applications for example Audio amplifier, Pre amplifier, etc. Circuit Diagram of Inverting Amplifier Working of Inverting Amplifier The non-inverting input is held at 0v Page 8
The feedback will try to ensure that the inverting input is very close to 0v. This is because the difference between the inputs must be only µv if the output is not saturated. The inverting input is called a virtual ground. The resistors form a potential divider with the center at 0v Assume that Vin is 0v. Thus Vout must also be 0v If Vin rises then the inverting input is greater than non-inverting and so Vout goes rapidly negative until the two inputs are once again equal (or at least only µv s different) Similarly, if Vin goes negative then the inverting input is less than non-inverting input and so Vout rises rapidly to become positive. For the amplifier to work properly the output must be able to change very quickly in order to react to the changes in the input. This limits the maximum frequency at which the amplifier can operate. The ratio of Vin and Vout depends on the ratio of the resistors in the potential divider and so, at low frequencies, the gain depends only on the values of Rf and Ri. At higher frequencies the gain of the op-amp is limited and so sets an upper limit on the gain of the amplifier circuit. Unity Gain Amplifier Circuit of unity gain amplifier is shown in above figure. In this the feedback resistor Rf is not connected. The output pin is connected directly to the inverting input terminal. This provides 100% voltage series negative feedback. This circuit has gain equal to unity (i.e. = 1). That is why it is called as unity gain amplifier. This circuit is also called the voltage follower or source follower circuit. It produces an output voltage which is exactly equal to the input voltage. A unity gain buffer (also called a unity-gain amplifier) is a op-amp circuit which has a voltage gain of 1. Page 9
This means that the op amp does not provide any amplification to the signal. The reason it is called a unity gain buffer (or amplifier) is because it provides a gain of 1, meaning there is no gain; the output voltage signal is the same as the input voltage. Thus, for example, if 10V goes into the op amp as input, 10V comes out as output. A unity gain buffer acts as a true buffer, providing no amplification or attenuation to the signal. Page 10
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Log amplifier. Log amplifier is a linear circuit in which the output voltage will be a constant times the natural logarithm of the input. The basic output equation of a log amplifier is v Vout = K ln (Vin/Vref); where Vref is the constant of normalisation, and K is the scale factor. Log amplifier finds a lot of application in electronic fields like multiplication or division (they can be performed by the addition and subtraction of the logs of the operand), signal processing, computerised process control, compression, decompression, RMS value detection etc. Basically there are two log amp configurations: Opamp-diode log amplifier and Opamp-transistor log. Page 14
Op amp-diode logarithmic amplifier. Opamp-diode log amplifier The schematic of a simple Opamp-diode log amplifier is shown above. This is nothing but an opamp wired in closed loop inverting configuration with a diode in the feedback path. The voltage across the diode will be always proportional to the log of the current through it and when a diode is placed in the feedback path of an opamp in inverting mode, the output voltage will be proportional to the negative log of the input current. Since the input current is proportional to the input voltage, we can say that the output voltage will be proportional to the negative log of the input voltage. where V ref is the normalization constant in volts and K is the scale factor. Voltage to Frequency Converter Circuit This voltage to frequency converter circuit has an oscillator that is voltage controlled and has a small, 0.5% deviation. IC1 function as a multivibrator and produces rectangular impulses with equal width. The width of the impulses depend on R4, P1 and C1. With P1 we can do fine adjustments of the output frequency. The output frequency can be easily adjusted with the help of U1 voltage. D3 diode is required because we want to eliminate R4 and P1 influence. D1 and D2 diodes produce a small flow of Page 15
temperature. With P2 we adjust the offset voltage. Because of its high quality, this voltagefrequency converter (VCO) can be used in a large field of applications. Voltage-Frequency Converter Circuit Diagram Page 16
Instrumentation Amplifier Introduction Many industrial and consumer applications require the measurement and control of physical conditions. For example, measurements of temperature and humidity inside a diary plant to accurately maintain product quality, or precise control of the temperature of a plastic furnace to produce a particular grade of plastic, etc. These changes in physical conditions must be converted to electrical quantities using transducers, and then amplified. Such amplifiers, which are used to amplify signals to measure physical quantities are commonly known as Instrumentation Amplifiers. The input to an instrumentation amplifier is the output signal from the transducer. A transducer is a device which converts one form of energy into another. Most of the transducer outputs are of very low-level signals. Hence, before the next stage, it is necessary to amplify the level of the signal, rejecting noise and the interference. The general single ended amplifiers are not suitable for such operations. For the rejection of noise, amplifiers must have high common-mode rejection ratio. The special amplifier which is used for such low-level amplification with high CMRR, high input impedance to avoid loading is an Instrumentation Amplifier. The instrumentation amplifier is intended for precise, low-level signal amplification where high input resistance, low noise and accurate closed-loop gain is required. Also, low power consumption, high slew rate and high common-mode rejection ratio are desirable for good performance. Page 17
Requirements of a Good Instrumentation Amplifier An instrumentation amplifier is usually employed to amplify low-level signals, rejecting noise and interference signals. Therefore, a good instrumentation amplifier has to meet the following specifications: Finite, Accurate and Stable Gain: Since the instrumentation amplifiers are required to amplify very low-level signals from the transducer device, high and finite gain is the basic requirement. The gain also needs to be accurate and the closed-loop gain must be stable. Easier Gain Adjustment: Apart from a finite and stable gain, variation in the gain factor over a prescribed range of values is also necessary. The gain adjustment must be easier and precise. High Input Impedance: To avoid the loading of input sources, the input impedance of the instrumentation amplifier must be very high (ideally infinite). Low Output Impedance: The output impedance of a good instrumentation amplifier must be very low (ideally zero), to avoid loading effect on the immediate next stage. High CMRR: The output from the transducer usually contains common mode signals, when transmitted over long wires. A good instrumentation amplifier must amplify only the differential input, completely rejecting common mode inputs. Thus, the CMRR of the instrumentation amplifier must be ideally infinite. High Slew Rate: The slew rate of the instrumentation amplifier must be as high as possible to provide maximum undistorted output voltage swing. Three Op-Amp Instrumentation Amplifier: The most commonly used Instrumentation amplifiers consist of three op-amps. In this circuit, a non-inverting amplifier is connected to each input of the differential amplifier. This instrumentation amplifier provides high input impedance for exact measurement of input data from transducers. The circuit diagram of an instrumentation amplifier is as shown in the figure below. Page 18
The op-amps 1 & 2 are non-inverting amplifiers and together form an input stage of the instrumentation amplifier. The op-amp 3 is a difference amplifier that forms the output stage of the instrumentation amplifier. V o = (R 3 /R 2 )[ΔR/4R](Vdc) SKETCH BASIC DIAGRAM WITH FINAL EXPRESSION: Inverting amplifier: Page 19
Non-inverting amplifier: Op-Amp Differentiator: Op-Amp Integrator: Page 20
Op-Amp Summing Amplifier: Instrumentation amplifier: Page 21
Comparator: Logarithmic: Page 22
Unity gain amplifier: V out = V in Current to voltage converter: V out =R I in Page 23
Voltage to current converter: Page 24