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Home Semiconductors Operational Amplifiers An analogy for divided feedback  
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An analogy for divided feedbackA helpful analogy for understanding divided feedback amplifier circuits is that of a mechanical lever, with relative motion of the lever's ends representing change in input and output voltages, and the fulcrum (pivot point) representing the location of the ground point, real or virtual. Take for example the following noninverting opamp circuit. We know from the prior section that the voltage gain of a noninverting amplifier configuration can never be less than unity (1). If we draw a lever diagram next to the amplifier schematic, with the distance between fulcrum and lever ends representative of resistor values, the motion of the lever will signify changes in voltage at the input and output terminals of the amplifier: Physicists call this type of lever, with the input force (effort) applied between the fulcrum and output (load), a thirdclass lever. It is characterized by an output displacement (motion) at least as large than the input displacement  a "gain" of at least 1  and in the same direction. Applying a positive input voltage to this opamp circuit is analogous to displacing the "input" point on the lever upward: Due to the displacementamplifying characteristics of the lever, the "output" point will move twice as far as the "input" point, and in the same direction. In the electronic circuit, the output voltage will equal twice the input, with the same polarity. Applying a negative input voltage is analogous to moving the lever downward from its level "zero" position, resulting in an amplified output displacement that is also negative: If we alter the resistor ratio R_{2}/R_{1}, we change the gain of the opamp circuit. In lever terms, this means moving the input point in relation to the fulcrum and lever end, which similarly changes the displacement "gain" of the machine: Now, any input signal will become amplified by a factor of four instead of by a factor of two: Inverting opamp circuits may be modeled using the lever analogy as well. With the inverting configuration, the ground point of the feedback voltage divider is the opamp's inverting input with the input to the left and the output to the right. This is mechanically equivalent to a firstclass lever, where the input force (effort) is on the opposite side of the fulcrum from the output (load): With equalvalue resistors (equallengths of lever on each side of the fulcrum), the output voltage (displacement) will be equal in magnitude to the input voltage (displacement), but of the opposite polarity (direction). A positive input results in a negative output: Changing the resistor ratio R_{2}/R_{1} changes the gain of the amplifier circuit, just as changing the fulcrum position on the lever changes its mechanical displacement "gain." Consider the following example, where R_{2} is made twice as large as R_{1}: With the inverting amplifier configuration, though, gains of less than 1 are possible, just as with firstclass levers. Reversing R_{2} and R_{1} values is analogous to moving the fulcrum to its complementary position on the lever: onethird of the way from the output end. There, the output displacement will be onehalf the input displacement:


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