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Note of Electronics by Aman Kumar

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UNIT: 3: Operational Amplifier (Op-Amp) (Remedial students) OPERATIONAL AMPLIFIER (Op-Amp) An operational amplifier is a very high gain amplifier having very high input impedance and low output impedance. The basic circuit is made using a differential amplifier having two inputs +ve (non-inverting) and –ve (inverting) and one output. Applications: Adder, Subtractor, Integrator, differentiator, amplifier, logarithmic amplifier Op-Amp Symbol: The IC integrated circuit is 741 Inverting input It connects the input signal to negative terminal and kept non inverting terminal at ground. The amplified output signal is 180 out of phase w.r.t. input signal. Non – inverting terminal If we connect the input signal to positive terminal the amplified output signal is not phase w.r.t the input but inverting terminal with ground. Power supply (+Vcc and VEE): The dc voltage supply can range from to . Block diagram of Op-Amp (stages of Op-Amp): Input stage: Input stage or differential amplifier stage can amplify difference between two input signals. Input resistance is very high; Draws zero current from the input sources Intermediate stage: Use direct coupling; provide very high gain. Level shifting stage: Due to the direct coupling between the first two stages, the input of level shifting stage is an amplified signal with some non-zero D.C level Level shifting stage is used to bring this dc to 0 volt w.r.t ground. Output stage: It increases the magnitude of voltage gain and raises the current supply capability of the op-amp. It also provides low resistance. 1|BY RSS IMSEC GZB

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UNIT: 3: Operational Amplifier (Op-Amp) (Remedial students)  Differential Amplifier Circuit. Differential Amplifier Circuit: Symbol: As the name indicates Differential Amplifier is a dc-coupled amplifier that amplifies the difference between two input signals. It is the building block of operational amplifiers (op-amp).The simplest form of differential amplifier can be constructed using Bipolar Junction Transistors as shown in the above circuit diagram. It is constructed using two matching transistors (Q1 and Q2) in common emitter configuration whose emitters are tied together. Supply voltage VCC and VEE applied to collector and emitter terminals of transistor. Vi1 and Vi2 are input terminals and Vo1 and Vo2 are output terminals with respect to ground.  Mode of Operation of Differential Amplifier Circuit. a. Single Input Unbalanced Output b. Single Input Balanced Output c. Dual Input Unbalanced Output d. Common mode operation (a) Single Input Unbalanced Output: In this case, only one input signal is given and the output is taken from only one of the two collectors with respect to ground as shown below. 2|BY RSS IMSEC GZB

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UNIT: 3: Operational Amplifier (Op-Amp) (Remedial students) When input signal Vin1 is applied to the transistor Q1, its amplified and inverted voltage gets generated at the collector of the transistor Q1. At the same time its amplified and non-inverted voltage gets generated at the collector of the transistor Q2 as shown in the above diagram. (b) Single Input balanced Output: In this mode, only one input signal is given and the output is taken from both collectors.   This will give us more amplified version of output as it is combining the effect of both transistors. Vo = Vo1 – Vo2 (c) Dual Input Unbalanced Output: Both inputs are given in this case i.e. differential input but the output is taken from only one of the two collectors with respect to ground as shown below. Amplified version of difference in both signals will be available at the output. The voltage gain is half the gain of the dual input, balanced output differential amplifier. (d) Common Mode Operation: Dual input balanced output differential amplifier should suppress the common signals present at its inputs. A differential amplifier is said to be in common mode when same signal is applied to both inputs and the expected output will be zero, i.e. ideally common mode gain is zero. 3|BY RSS IMSEC GZB

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UNIT: 3: Operational Amplifier (Op-Amp) (Remedial students)  Parameters of Op-Amp Differential gain (Ad): It is the ratio of output voltage to the difference of input voltage (Vd)      Where Ad in (dB) = 20 Ideally output should be infinite (Vo So, Ad should be infinite Ad ) Common mode gain (Ac) : Common mode gain is ratio of output voltage to common voltage.      Vc is average value of input signal called common voltage Common voltage present due to presence of noise, noise is common to both input signal. So, output should be zero due to presence of noise, Vo’’ due to noise should be zero So Ac zero (ideally) Vo’’=Ac ×Vc Total output voltage Vo = Vo’’+Vo’ Vo =AdVd+AcVc 4|BY RSS IMSEC GZB

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