Operational Amplifiers

Operational Amplifier Stages
Inverting Amplifier
Non-Inverting Amplifier
Summing Amplifier

Introduction




The name operational amplifier was originally adopted for a series of high performance DC amplifiers used in analog computers. These amplifiers were used to perform mathematical operations applicable to analog computation such as summation , scaling, subtraction, integrating, etc .

Equivalent Circuit for an Ideal Operational Amplifier















(1) The voltage gain is infinity, Avo = ∞ .
(2) The input resistance is infinity, rin = ∞ .
(3) The output resistance is zero, ro = 0 .
(4) The bandwidth is infinity, BW = ∞ .
(5) There is zero input offset voltage, Eo = 0 if Ein = 0 .
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Input Stage

The input stage is a dual input, balanced output differential amplifier. It has 2 inputs Vin1 and Vin2 which are applied at the bases B1 and B2 of transistors Q1 and Q2 .The output V0 is measured between the two collectors C1 and C2 which are at the same dc potential.Because of the equal dc potential at the two collectors with respect to ground, the output is referred to as balanced output.














Intermediate Stage

The next stage is dual input, unbalanced output difference amplifier. Here two input signals are used however the output is measured at only one of the two collectors with reference to ground. The output is referred to as an unbalanced output because the collector at which the output voltage is measured is at some finite dc potential with reference to ground. In other words, there is some dc voltage at the output terminal without any input signal applied

Inverting Amplifier

One of the most common applications is the simple inverting amplifier. The output is inverted, and the gain is determined by the ratio of the feedback resistor (Rf) to the input resistor (Rin).
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Non-Inverting Amplifier

Another common configuration is the non-inverting amplifier, where the output signal is not inverted . In this circuit, the input voltage is applied to the positive input of the op amp, and a fraction of the output signal is applied to the negative input from the (Rf) - (Rin) voltage divider .

Summing Amplifier

This is a special case of the inverting amplifier, as it gives an inverted output which is equal to the weighted algebraic sum of all inputs. If the input resistors, and the feedback resistor are chosen to be equal , the output is simply the negative sum of the inputs. Since there is no interaction between inputs, the operations of summing and weighting is very easily done.

R5 = R1 || R2 || R3 || R4
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