OP-AMPS: basics & Inverting-amplifier Prepared by: 1331_005 for Collaborative Creation of OER Target Audience: 2nd year;3rd semester Engineering Students

introduction Operational amplifiers (Op-Amps) is one of basic building blocks the most popular device of today's analogue electronic circuits. Operational Amplifiers are linear devices that have all the properties required for DC and AC amplification. Therefore used extensively in signal conditioning, filtering or to perform mathematical operations such as Addition, subtraction, integration, differentiation; hence the name “OP-AMP”

Symbol

Symbol shows a basic op-amp with two inputs and one output as would result using a differential amplifier input stage. If the Input signal is applied to the plus input of the the op amp output results in the same polarity (in phase), So it is called as non-inverting input. If the signal is applied to the minus input of the op-amp output results in opposite polarity (1800 out of phase), So it is considered as inverting input .

CHARACTERISTICS OF AN IDEAL OP- AMP Input Resistance Zi=  Output Resistance Zo= 0 Voltage Gain A =  Bandwidth =  Perfect balance i.e. Vo= 0 when V1= V2 Characteristics do not drift with temperature Ad =  ; Ac = 0, CMRR = 

AN IDEAL OP- AMP

CHARACTERISTICS OF A practical OP- AMP Very high gain differential amplifier (~ 104) High input impedance (typically a few M) Low output impedance (less than 100 ) Almost perfect balance obtained with offset adjustment High Bandwidth ;up to GHz Ad = very high ; Ac =very small, CMRR= very high

A practical OP- AMP

Basic op-amp

The basic circuit connection using an op-amp is shown in Fig. The circuit shown provides operation as a constant-gain multiplier. An input signal, V1, is applied through resistor R1 to the minus input. The output is then connected back to the same minus input through feedback resistor Rf. The plus input is connected to ground. Since the signal V1 is essentially applied to the minus input, the resulting output is opposite in phase to the input signal.

ac equivalent circuit OF op-amp. Figure shows the op-amp replaced by its ac equivalent circuit. To simplifies circuit analysis sometimes one can use the ideal op-amp equivalent circuit by replacing Ri by an infinite resistance and Ro by zero resistance;

If we can use the ideal op-amp equivalent circuit, as shown in the figure above. Using superposition, we can solve for the voltage V1 in terms of the components due to each of the sources. For source V1 only (-AvVi set to zero) For source - AvVi only (V1 set to zero),

The total voltage Vi is then which can be solved for Vi as If Av >>> 1 and Av R1 >>> Rf .

Solving for Vo/Vi, we get so that The result, in above Equation, shows that the ratio of overall output to input voltage is Dependent only on the values of resistors R1 and Rf , provided that Av is very large.

NEED OF NEGATIVE FEEDBACK IN OP-AMP Any input signal slightly greater than zero drive the output to saturation level because of very high gain. Thus when operated in open-loop, the output of the OPAMP is either in negative or positive saturation or switches between positive and negative saturation levels (comparator). Therefore open loop op-amp is not used in linear applications.

With negative feedback, the voltage gain (Acl) can be reduced and controlled so that op-amp can function as a linear amplifier. In addition, negative feedback provide a control and stable voltage gain, provides control of input & output impedance and amplifier bandwidth.

references “Operational Amplifier Basics” www.electronic-tutorials.ws/opamp/opamp_1.html Electronic Devices and Circuit Theory”, Nashelesky & Boylestead, PHI.