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Operational amplifier (OP - AMP)
Linear integrated circuits are being used in a number of electronic applications, such as in the fields like communication, medical electronics, instrumentation control etc. An important linear IC is an operational amplifier.
OP-AMP is a solid state device capable of sensing and amplifying dc and ac input signals. OP-AMP is an amplifier with two inputs (differential inputs) and a single output. OP-AMP consists of 20 transistors, 11 resistors and one capacitor. It usually requires a positive and negative power supply (dual power supply). This allows the output voltage to swing positive and negative with respect to ground.
The most important characteristics of OP-AMP are : (i) very high input impedance or even infinity which produces negligible current at the inputs, (ii) very high gain, (iii) very low output impedance or even zero, so as not to affect the output of the amplifier by loading.
An OP-AMP is so named, because it was originally designed to perform mathematical operations such as addition, subtraction, multiplication, division, integration, differentiation etc in analog computer. Nowdays OP-AMPs are used in analog computer operations and in timing circuits.
Circuit symbol and Pin-out configuration of an OP-AMP
The OP - AMP is represented by a triangular symbol as shown in Fig. It has two input terminals and one output terminal. The terminal with negative sign is called as the inverting input and the terminal with positive sign is called as the non-inverting input. The input terminals are at the base of the triangle. The output terminal is shown at the apex of the triangle.
The widely used very popular type Op-Amp IC 741, which is available in DIP. Referring to the top view of the dual-in-package, the pin configuration of IC 741 can be described (Fig) as follows. The top pin on the left side of the notch indicates Pin 1. The pin number 2 is inverting input terminal and 3 is non-inverting input terminal. Pin 6 is the output terminal. A d.c. voltage or a.c signal placed on the inverting input will be 180o out of phase at the output. A d.c. voltage or a.c. signal placed on the non-inverting input will be inphase at the output. Pins 7 and 4 are the power supply terminals. Terminals 1 and 5 are used for null adjustment. Null adjustment pins are used to null the output voltage when equal voltages are applied to the input terminals for perfect balance. Pin 8 indicates no connection.
Basic OP-AMP circuits
This section concentrates on the principles involved with basic OP-AMP circuit viz, (i) inverting and (ii) non-inverting amplifiers.
(i) Inverting amplifier
The basic OP-AMP inverting amplifier is shown in Fig. The input voltage Vin is applied to the inverting input through the input resistor Rin. The non inverting input is grounded. The feedback resistor Rf is connected between the output and the inverting input.
Since the input impedance of an op-amp is considered very high, no current can flow into or out of the input terminals. Therefore Iin must flow through Rf and is indicated by If (the feedback current). Since Rin and Rf are in series, then Iin = If. The voltage between inverting and non-inverting inputs
is essentially equal to zero volt. Therefore, the inverting input terminal is also at 0 volt. For this reason the inverting input is said to be at virtual ground. The output voltage (Vout) is taken across Rf.
It can be proved that
Since Iin =If , then
Vin/Rin = -Vout/Rf
Rearranging the equation, we obtain
-Vout/Vin = Rf/Rin
∴ The voltage gain of an inverting amplifier can be expressed as
Av = -Rf/Rin
The amplifier gain is the ratio of Rf to Rin
Finally, the output voltage can be found by
Vout = -Rf/Rin x Vin
The output voltage is out of phase with the input voltage.
(ii) Non-inverting amplifier
The basic OP-AMP non-inverting amplifier is shown in Fig. The input signal Vin is applied to the non-inverting input terminal. The resistor Rin is connected from the inverting input to ground. The feedback resistor Rf is connected between the output and the inverting input.
Resistors Rf and Rin form a resistive ratio network to produce the feedback voltage (VA) needed at the inverting input. Feedback voltage (VA) is developed across Rin. Since the potential at the inverting input tends to be the same as the non-inverting input (as pointed out with the description of virtual ground), Vin = VA.
Since VA = Vin, the gain of the amplifer can be expressed as
Av = Vou t
However, VA is determined by the resistance ratio of Rin and Rf ; thus,
Finally, the output voltage can be found by, Vout = (1+Rf/Rin)Vin
It is seen that the input and output voltages are in phase
(iii) Summing amplifier
The summing amplifier provides an output voltage equal to the algebraic sum of the input voltages.
Fig shows an inverting amplifier, used to sum two input voltages. The input voltages v1 and v2 are applied through the resistors R1 and R2 to the summing junction (P) and Rf is the feedback resistor. At the point P,
i1 + i2 =if
Since the voltage at the point P is ideally 0
(V1/R1) + (V2/R2) =(Vout/Rf)
Hence the output voltage,
Vout=-[(RfV1/R1) +(RfV2/R2) ]
If R1 = R2 = Rf = R, then vout = - (v1 + v2)
Hence the output voltage is equal to the sum of the input voltages and the circuit acts as a summing amplifier. The negative sign indicates that OP-AMP is used in the inverting mode.
The difference amplifier is shown in Fig. The output voltage can be obtained by using superposition principle. To find the output voltage v01 due to v1 alone, assume that v2 is shorted to ground. Then
V+ = R2V1 / R1+R2
Therefore, with both inputs present, the output is
V0 = V01 + V02
= (R3+R4 / R3) (R2/R1+R2) V1 - (R4/R3)V2
If R1 = R2 = R3 = R4 = R
then vo = v1 - v2
If all the external resistors are equal, the voltage difference amplifier functions as a voltage subtractor.
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