Beatrice Lang
Feedback is an important concept in electronics and electrical engineering, and it is used to reduce errors between the reference input and the system output. Feedback loops are used to control the output of electronic devices, such as amplifiers, and they are created when a portion of the output is fed back to the input. There are two main types of feedback control: negative feedback and positive feedback. Negative feedback occurs when the fed-back output signal is out of phase with the input signal, making the output smaller. Positive feedback occurs when the fed-back signal is in phase with the input signal, making the output larger.
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What You'll Learn
Feedback systems and feedback control systems
Feedback systems are an integral part of electrical circuits, offering precise control and stability. They are signal processors that can be electrical or electronic, ranging from simple to complex circuits. Feedback systems are widely used in amplifier circuits, oscillators, and process control systems. The basic feedback loop consists of sensing, controlling, and actuation, which forms the foundation of a feedback control system.
A feedback control system continuously monitors a process and adjusts it to maintain specific output variables within a desired range. For example, in an incubator for cell culture, a feedback control system is used to maintain the interior temperature at 37 °C. The system consists of a heating coil connected to a rheostat, which allows control of heat dissipation. The rheostat's position can be adjusted to regulate the heat and maintain the desired temperature.
Feedback systems can be categorised into two main types: negative feedback and positive feedback. In a negative feedback system, the feedback is out of phase with the input, resulting in a reduced signal strength and smaller output. This type of feedback is commonly used in various systems to improve linearity and stability. On the other hand, positive feedback systems have feedback in phase with the input, leading to increased system gain and a larger output. While positive feedback can enhance the performance of oscillators and timing circuits, it must be carefully controlled to prevent uncontrolled oscillations or failure.
The design of feedback systems has been simplified by methods such as those introduced by H.W. Bode in 1945, which transformed the complex mathematical calculations of stability analysis into graphical representations. This made feedback system design more accessible to a broader range of engineers.
Overall, feedback systems and feedback control systems play a critical role in electrical circuits, providing the ability to monitor, control, and stabilise various parameters such as temperature, voltage, and frequency response, ensuring the desired operation of the circuit.
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Positive and negative feedback
Feedback in an electrical circuit is a way of providing output (or a part of the output) back to the input. The signal can be either a current or voltage. Feedback in an electronic system, whether negative or positive, is unilateral in direction, meaning that its signals flow one way only from the output to the input of the system.
Positive feedback is a process that further increases the effects of a small change in output. It is useful in constructing oscillator circuits, but unwanted oscillations can render an amplifier useless. Positive feedback is seldom used in amplifiers since it normally increases distortion and instability. However, it can be used to improve the overall gain of the system.
In a positive feedback system, the input and output signals are in-phase. The output is fed back to the positive input, which increases the output and drives it toward a point of instability (oscillations). The phase shift offered by the positive feedback circuit is either 0° or 360°.
Negative feedback, on the other hand, can be compared to a cycle of trying to keep the results constant. It is often used to create controlled amplifiers and filters and is commonly used in amplifier circuits. Negative feedback improves the stability of the amplifier characteristics and makes the amplifier's total gain more dependent on resistor values. It also reduces the overall input of the system by subtracting the feedback signal from the actual input.
In a negative feedback system, the input and output signals are out-of-phase. The summing point or junction at the input subtracts the feedback signal from the input signal to form an error signal, which drives the system. The phase shift in a negative feedback circuit is 180°. Negative feedback tends to reduce the effects of gain change, providing "gain stability".
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Feedback loops
Negative feedback is a commonly employed mechanism in control systems. It involves introducing a phase shift in the feedback signal, which opposes the phase of the input signal. This opposition leads to a decrease in the overall signal, resulting in a smaller output. For instance, in a hot water radiator system, if the valve is normally open and less voltage is applied, the valve opens further. When the desired temperature is reached, the valve experiences maximum voltage, causing the radiator to stop heating the room. Negative feedback is valuable in reducing noise and enhancing linearity in electronic devices. It is also used in voltage regulation, where it maintains the output voltage at a preset level under normal load conditions.
Positive feedback, on the other hand, involves a feedback signal that is in phase with the input signal, resulting in an increased output. In this scenario, the set point and output values are combined, leading to a higher system gain. While positive feedback can boost gain, it can also lead to instability and oscillations in the circuit. An example of positive feedback is hysteresis, where a logic device maintains a specific state until an input crosses a preset threshold.
The choice between negative and positive feedback depends on the desired outcome. Negative feedback is typically used to stabilise systems and reduce the impact of disturbances, while positive feedback can amplify weak signals and increase gain, making it useful in oscillators and timing circuits.
Overall, feedback loops are essential tools in electrical circuit design, allowing engineers to control and optimise system behaviour, ensuring that the output aligns with the desired input.
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Feedback amplifiers
The two main types of feedback amplifiers are positive feedback amplifiers and negative feedback amplifiers. In a positive feedback amplifier, the output signal is in phase with the input, which makes the output larger, resulting in system gain. Positive feedback amplifiers are used in electronic oscillators to increase gain and narrow bandwidth. However, too much positive feedback can increase the system's gain excessively, leading to oscillatory circuit responses.
Negative feedback amplifiers, on the other hand, have an output signal that is out of phase with the input, which reduces the signal and makes the output smaller. Negative feedback is commonly used in audio amplifiers to stabilize the gain, increase bandwidth, lower output impedance, and reduce non-linear distortion. It is important to note that if the feedback in a negative feedback amplifier becomes positive, it can lead to instability and unwanted behaviour such as oscillation.
The use of feedback in amplifiers allows for greater control and enhancement of the amplifier's performance. For example, in a voltage shunt feedback amplifier, the input voltage and feedback are connected in parallel, increasing voltage gain, enhancing linearity, and decreasing output impedance. Similarly, in a current series feedback amplifier, the feedback and input current are applied in series, increasing stability, input impedance, and bandwidth.
While feedback amplifiers offer many benefits, there are differing opinions on their impact on audio quality. Some believe that feedback improves audio performance by reducing distortion, while others argue that it diminishes the listening experience, making the music sound devoid of soul.
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Feedback circuit stability
Feedback systems are an essential part of modern electronic circuits, which are used in a wide range of applications, from military and aerospace to civil uses. These systems are designed to keep the circuit stable and working as expected over long periods.
A feedback circuit is a system where a part of the output is fed back into the input. This feedback loop allows the system to modify the input and, thus, control the output. This is done by creating an error signal that drives the system. This is known as a closed-loop system. The feedback path may include signal processing units, or it may not have any at all.
There are two main types of feedback control: negative and positive. Negative feedback is the most common form and is found in almost every electronic system. It is out of phase with the input, reducing the signal and making the output smaller. This type of feedback is used to increase linearity in inherently non-linear electronic devices. Positive feedback, on the other hand, is in phase with the input, increasing the system gain and the possibility of instability. This type of feedback is used in oscillators and timing circuits to increase gain and narrow bandwidth.
Designing a stable feedback circuit requires careful attention to stability and gain criteria, which determine how the system responds to changes and disturbances. The Nyquist plot is a common method used to identify the system's poles and zeros, phase margin, and gain margin. These parameters can then be adjusted to improve the stability of the circuit. Stability criteria define whether a feedback circuit will maintain a steady output or oscillate, diverge, or saturate.
Overall, feedback systems are a crucial aspect of electronic circuit design, allowing for stable and controlled operation.
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Frequently asked questions
Feedback is an important concept in electronics and electrical engineering. It is used to reduce the error between the reference input and the system output. A feedback loop is created when all or some portion of the output is fed back to the input.
Positive feedback occurs when the fed-back signal is in phase with the input signal. Under certain gain conditions, positive feedback reinforces the input signal to the point where the output of the device oscillates between its maximum and minimum possible states.
Negative feedback occurs when the fed-back output signal has a relative phase of 180° with respect to the input signal (upside down). Negative feedback is the most common form of feedback control used in all manner of systems.
