- 1. Dr. Vaishali Deshmukh Dept. of Physics Shri Shivaji Science College, Amravati(MS)What is it? & How It works? The Operational Amplifier
- 2. Mummy op-amp told baby op-amp when he was very young, “Keep your feet together and accept no currents”
- 3. What is Op-Amp? • An operational amplifier or op-amp is a DC coupled voltage amplifier with a very high voltage gain. • An op amp, is fundamentally a voltage amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. These feedback components determine the resulting function or “operation” of the amplifier and by virtue of the different feedback configurations whether resistive, capacitive or both, the amplifier can perform a variety of different operations, giving rise to its name of “Operational Amplifier”. • The goal of the op-amp is to produce very high gain from the differential input to the output. The high gain amplifier is useful in feedback configurations.
- 4. Ideal op-amp characteristics • Infinite voltage gain(Aol) • Infinite input impedance(zin) • Zero output impedance(Zout) • Zero offset voltage (Vios) • Infinite bandwidth (BW) • Infinite common mode rejection ratio (CMRR) • Infinite slew rate (S) • Zero power supply rejection ratio (PSRR) • No effect of temperature
- 5. Ideal op-ampcharacteristics 1. Infinite Open-Loop Gain (Avol): is the gain of the op-amp without positive or negative feedback. Values range from 20,000 to 200,000 2. Infinite Input Impedance(Rin): is the ratio of input voltage to input current and is infinite idealy. This ensures that no current can flow into an ideal op-amp. 3. Zero Output Impedance(Ro): The ideal op-amp acts as a perfect internal voltage source with no internal resistance. This internal resistance is in series with the load, reducing the output voltage available to the load. Real op-amps have output-impedance in the 100-20Ω range. 4. Zero output offset volt (Voo): The output offset is the output voltage of an amplifier when both inputs are grounded. The ideal op-amp has zero output offset, but real op-amps have some amount of output offset voltage. 5. Infinite Bandwidth(BW):The ideal op-amp will amplify all signals from DC to the highest AC frequencies. BW of an ideal op-amp is infinite. This ensures that the gain of the op-amp will be const over the freq range from dc to infinite. 6. Infinite common mode rejection ratio(CMRR): The ratio of differential gain and common mode gain is defined as CMRR. 7. Infinite slew rate(s): The maximum rate of change of output voltage with time is known as slew rate and expressed in V/μs.
- 6. Ideal voltage transfer curve The ideal op-amp produces the output proportional to the difference between the two input voltages. The graphical representation of this statement gives the voltage transfer curve. It is the graph of output voltage vo plotted against the input Voltage vd assuming gain constant. Thus note that the op-amp output voltage gets saturated at +Vcc and – VEE and it can not produce output voltage more than + Vcc and vEE. Practically saturation voltages +Vsat, and – Vsat are slightly less than +Vcc and – VEE. 0 +Vd-Vd Positive saturation volt + Vsat = +Vcc Negative saturation volt – Vsat = –VEE Now the output voltage is proportional to difference input voltage but only upto the positive and negative saturation are specified by the manufacturer in terms of output volt swing rating of an op-amp, for given value of supply voltages. The curve is not drawn to the scale. If drawn to the scale, the curve would be almost vertical due to large values of op-amp gain.
- 7. Practical op-amp characteristics • Open loop gain (Aol) • Input impedance(zin) • Output impedance(Zout) • Bandwidth (BW) • Input offset voltage (Vios) • Input bias current (Ib) • Input offset current (Iios) • Common mode rejection ratio (CMRR) • Power supply rejection ratio (PSRR) • Slew rate (S)
- 8. • Input offset voltage (Vios) : amount of dc voltage applied to one of the input terminal of op-amp, which makes the output voltage zero, when the other terminal is grounded is called input offset voltage. • Input bias current (Ib): can be defined as the current flowing into each of the two input terminals when they are biased to the same voltage level i.e. op-amp is balanced. • Input offset current (Iios): the difference in magnitudes of Ib1 and Ib2 is called as input offset current. Ib1= Ib1 ‒ Ib2 • Power supply rejection ratio (PSRR): the ratio of the change in input offset voltage due to the change in supply voltage producing it, keeping other power supply volt constant is called PSRR. PSRR= Vo Ib1 Ib2 Ib1= Ib1 + Ib2 2 Vios Vcc const VEE PSRR= Vios VEE const Vcc
- 9. Equivalent circuit of practical op-amp The circuit which represents op-amp parameters in terms of physical components, for the analysis purpose is called equivalent circuit of an op-amp. It is to be noted that the op-amp amplifies difference voltage and not the individualinput voltages. Thus the output polarity gets decided by the polarity of the difference voltage Vd. The voltage source AOLVd is the Thevenin’s equivalent voltage source while R0 is the Thevenin s equivalent resistance looking back into the output terminals. The equivalent circuit plays an important role in analysing various op-amp applications as well as in studying the effects of feedback on the performance of op-amp. The circuit shows the op-amp parameters like input resistance, output resistance, the open loop voltage gain in terms of circuit components like Rin, R0 etc. The op-amp amplifies the difference between the two input voltages. Vo=AOLVd=AOL(V1-V2) The output voltage is directly proportional to the difference voltage Vd.
- 10. Op-amp IC 741 • The 741 Op Amp IC is a monolithic integrated circuit, comprising of a general purpose Operational Amplifier. It was first manufactured by Fairchild semiconductors in the year 1963. The number 741 indicates that this operational amplifier IC has 7 functional pins, 4 pins capable of taking input and 1 output pin. • Specifications: Power Supply: Requires a Minimum voltage of 5V and can withstand upto 18V. Input Impedance: About 2 megaohms. Output impedance: About 75 ohms. Voltage Gain: 200,000 for low frequencies. Maximum Output Current: 20mA. Recommended Output Load: Greater than 2 kiloohms. Input Offset: Ranges between 2mV and 6mV. Slew Rate: 0.5V/microsecond 1 2 3 4 5 6 7 8 NC +Vcc -VEE output Offset null Offset null Inv input NonInv input The pins 1 and 5 are offset null pins.These are used to nullify offset voltages and provide offset volt compensation. The pin2 is inverting input while pin3 is noninverting input terminal. The output is taken from pin6. The op-amp requires dual power supply i.e. positive supply is to be given to pin7 and negative supply is given to pin4. The pin8 is the dummy pin and no connection are to be made to this pin.
- 11. Open loop configuration of op-amp • The simplest possible way to use an op-amp is in the open loop mode. The Fig. 1 shows an op amp in the open loop condition. • Fig.1 • Fig.2 V1 V2 Vd Vo +Vcc -VEE Vo Vd + Vsat ‒Vsat a b • The d.c. supply voltages applied to the op-amp are Vcc and -VEE and the output varies linearily between Vcc and –VEE. Since gain is very large in open loop condition, the output voltage Vo, is either at its positive saturation voltage (+ Vsat ) or negative saturation voltage ( – Vsat ) as V] > V2 or V2 > V1 respectively. • It can be seen from the Fig. 2, only for small range of input signal (from point a to b), it behaves linearly. This range is very small and practically due to high open loop gain, op-amp either shows + Vsat or – Vsat level. •This indicates the inability of op-amp to work as a linear small signal amplifier in the open loop mode. • Hence, Such an open loop behavior of the op- amp finds very rare applications like voltage comparator, zero crossing detector etc.
- 12. Closed loop configuration of op-amp • The utility of op-amp increases considerably if it is used in a closed loop mode. The Closed Loop Configuration of Op amp is possible using feedback. The feedback allows to feed some part of the output back to the input. Fig.1 • Advantages of negative feedback: It reduces the gain and makes it controllable. It reduces the possibility of distortion. It increases the bandwidth i.e. frequency range. It increases the input resistance of op-amp. It decreases the output resistance of op-amp. It reduces the effect of temperature, power supply on the gain of the circuit. Vo +Vcc -VEE V2 V1 Rf In linear applications the op-amp is always used with negative feedback. The feedback helps to control gain which otherwise drives op- amp into saturation. The negative feedback is possible by adding a resistor as shown in fig1 called feedback resistor. The feedback is said to be negative as the feedback resistor connects the output to the inverting input terminal.
- 13. Realistic Simplifying Assumptions Op Amp Assumptions which are realistic and simplify the analysis of op- amp circuits to a great extent. The Op Amp Assumptions are useful and can be used to obtain the output expressions in variety of linear applications. • Zero Input Current: • The current drawn by either of the input terminals (inverting and noninverting) is zero. • In practice, the current drawn by the input terminals is very small, of the order of μA or nA. Hence the Op Amp Assumptions of zero input current is realistic. • Virtual Ground: • This means the differential input voltage Vd between the non inverting and inverting input terminals is essentially zero. This is obvious because even if output voltage is few volts, due to large open loop gain of op-amp, the difference voltage Vd at the input terminals is almost zero.
- 14. Concept of Virtual Ground • An Op-Amp has a very high gain. • if output voltage is 10 V and the AOL i.e. open loop gain is 104 then • Vo=Vd AOL Vd=Vo/AOL =10/104=1mV • Hence Vd is very small. As AOL → ∞, the difference voltage Vd → 0 and realistically assumed to be zero for analysing the circuits. • Vd=Vo/AOL , (V1-V2)=Vo/ ∞ =0, V1=V2 -- -- -- -(1) • Thus from the equation (1), the voltage at the one input terminal of an op-amp can be realistically assumed to be equal to the voltage at the other input terminal. V1 Vo RfR1 I=0Virtual short if the non-inverting terminal is grounded, by the concept of virtual short, the inverting terminal is also at ground potential, though there is no physical connection between the inverting terminal and the ground. This is the principle of virtual ground.
- 15. Further Presentation • Applications of Op-Amp