Some of this output voltage is returned back to the input by the feedback network of 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. 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. For example, if someone criticises you or gives you negative feedback about something, you feel unhappy about yourself and therefore lack energy, you feel less positive.īecause negative feedback produces stable circuit responses, improves stability and increases the operating bandwidth of a given system, the majority of all control and feedback systems is degenerative reducing the effects of the gain.Īn example of a negative feedback system is an electronic amplifier based on an operational amplifier as shown. The effect of negative (or degenerative) feedback is to “reduce” the gain. In a “negative feedback control system”, the set point and output values are subtracted from each other as the feedback is “out-of-phase” with the original input. We have seen that positive or regenerative feedback increases the gain and the possibility of instability in a system which may lead to self-oscillation and as such, positive feedback is widely used in oscillatory circuits such as Oscillators and Timing circuits. This type of behaviour is called “bi-stability” and is often associated with logic gates and digital switching devices such as multivibrators. One example of the use of positive feedback is hysteresis in which a logic device or system maintains a given state until some input crosses a preset threshold. Note that if GH = 1 the system gain Av = infinity and the circuit will start to self-oscillate, after which no input signal is needed to maintain oscillations, which is useful if you want to make an oscillator.Īlthough often considered undesirable, this behaviour is used in electronics to obtain a very fast switching response to a condition or signal. Then if the loop gain is positive for any system the transfer function will be: Av = G / (1 – GH). In the previous tutorial about Closed-loop Systems, we saw that in general, feedback is comprised of a sub-circuit that allows a fraction of the output signal from a system to modify the effective input signal in such a way as to produce a response that can differ substantially from the response produced in the absence of such feedback. But the effects of these “open-loop” variations can be eliminated or at least considerably reduced by the introduction of Feedback.Ī feedback system is one in which the output signal is sampled and then fed back to the input to form an error signal that drives the system. Simple analogue feedback control circuits can be constructed using individual or discrete components, such as transistors, resistors and capacitors, etc, or by using microprocessor-based and integrated circuits (IC’s) to form more complex digital feedback systems.Īs we have seen, open-loop systems are just that, open ended, and no attempt is made to compensate for changes in circuit conditions or changes in load conditions due to variations in circuit parameters, such as gain and stability, temperature, supply voltage variations and/or external disturbances. The processing part of a feedback system may be electrical or electronic, ranging from a very simple to a highly complex circuits. Feedback Systems process signals and as such are signal processors.
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