Feedback Systems(Negative and Positive Feedback)

Prepared by:

Karl LubidJeffree C. Lumagui

• process signals and as such are signal processors

• Its processing part may be electrical or electronic, ranging from a very simple to a highly complex circuits

• can be constructed using individual or discrete components, such as transistors, resistors and capacitors, etc

Two Main Types of Feedback System

• Positive Feedback• Negative Feedback

Positive Feedback Systems

• the set point and output values are added together by the controller as the feedback is “in-phase” with the input

• The effect of positive (or regenerative) feedback is to “increase” the systems gain

Positive Feedback SystemOp-Amp

Positive feedback is widely used in oscillatory circuits such as Oscillator and Timing circuits.

Negative Feedback System

• the set point and output values are subtracted from each other as the feedback is “out-of-phase” with the original input

• The effect of negative (or degenerative) feedback is to “reduce” the gain

Negative Feedback System

Negative feedback control of the amplifier is achieved by applying a small part of the output voltage signal at Vout back to the inverting ( – ) input terminal via the feedback resistor, Rf.

If the input voltage Vin is positive, the op-amp amplifies this positive signal, but because its connected to the inverting input of the amplifier, and the output becomes more negative. Some of this output voltage is returned back to the input by the feedback network of Rf.

Thus the input voltage is reduced by the negative feedback signal, causing an even smaller output voltage and so on. Eventually the output will settle down and become stabilised at a value determined by the gain ratio of Rf ÷ Rin.

Negative feedback also makes control systems more immune to random variations in component values and inputs.

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