The main player in analog circuits! Basic operation of OPERATIONAL AMPLIFIERS
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An operational OPERATIONAL AMPLIFIERS (op-amp) is an electronic component that plays an important role in analog circuits. Combining OPERATIONAL AMPLIFIERS and peripheral circuits enables analog calculations such as amplification, addition, subtraction, differential and integral calculations, FILTERS. They are used in a variety of applications for these purposes and are indispensable in analog circuit design.
Basic Operation of OPERATIONAL AMPLIFIERS
OPERATIONAL AMPLIFIERS is an amplifier that amplifies the voltage difference between two inputs and outputs it.
When written as an equation, it becomes the following equation.
The gain A0 is called the open loop gain and is usually 100,000 times or more at DC.
How to use OPERATIONAL AMPLIFIERS
1. Inverting amplifier circuit
The gain AC is called the closed-loop gain and is determined by the ratio of the resistors. This formula can be derived by using the assumption that the potentials of the non-inverting input and the inverting input are equal (imaginary short). Also, if the input signal has output impedance, the output impedance will be connected in series with R1, which will affect the closed-loop gain, so care must be taken.
2. Non-inverting amplifier circuit
As with the inverting amplifier circuit, the closed-loop gain can be derived using the imaginary short assumption. Unlike an inverting amplifier circuit, the input signal is input to the non-inverting input, which has a large input impedance (several MΩ or more). The effect of the input signal on the output impedance can be significantly reduced.
Imaginary Short
If the difference in input voltage of OPERATIONAL AMPLIFIERS is expressed as ΔV, the relationship between input and output is expressed as follows:
If the input impedance of the inverting input TERMINALS of OPERATIONAL AMPLIFIERS is sufficiently larger than the resistors R1 and R2, The currents flowing through R1 and R2 can be approximated as being equal, so the following equation holds true.
Solve this equation for ΔV.
If the open-loop gain A0 is set to infinity, the following equation holds, and the voltage difference between the non-inverting input and the inverting input can approximate 0V. This approximation is the imaginary short.
Important point: Frequency characteristics of open loop gain
However, care must be taken because if the open loop gain A0 is not sufficiently high, the imaginary short will not hold. For example, when the frequency of the input signal is high, the open-loop gain A0 tends to decrease as the frequency increases. To check whether this drop is sufficient to create an imaginary short, check the frequency characteristics of the open-loop gain A0 in OPERATIONAL AMPLIFIERS 's datasheet.
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