Applied Electronics and Instrumentation

LESSON 11. Common Base amplifier-characteristics-Feedback amplifier- types of feedback

Characteristics of CB Amplifier:

• Current gain of less than unity

• High voltage gain

• Power gain approximately equal to voltage gain

• No phase shift for current or voltage

• Small input impedance

• Large output impedance

Input & Output voltages of CB Amplifier:

FEEDBACK AMPLIFIERS

Feedback is a mechanism, process or signal that is looped back to control a system within itself. Such a loop is called a feedback loop. In systems containing an input and output, feeding back part of the output so as to increase the input is positive feedback (regeneration); feeding back part of the output in such a way as to partially oppose the input is negative feedback (degeneration).

Generally, a control system has input from an external signal source and output to an external load; this defines a natural sense (or direction) or path of propagation of signal; the feedforward sense or path describes the signal propagation from input to output; feedback describes signal propagation in the reverse sense. When a sample of the output of the system is fed back, in the reverse sense, by a distinct feedback path into the interior of the system, to contribute to the input of one of its internal feed forward components, especially an active device or a substance that is consumed in an irreversible reaction, it is called the "feedback". The propagation of the signal around the feedback loop takes a finite time because it is causal.

The natural sense of feed forward is defined chemically by some irreversible reaction, or electronically by an active circuit element that has access to an auxiliary power supply, so as to be able to provide power gain to amplify the signal as it propagates from input to output. For example, an amplifier can use power from its controlled power reservoir, such as its battery, to provide power gain to amplify the signal; but the reverse is not possible: the signal cannot provide power to re-charge the battery of the amplifier. Feed forward, feedback and regulation are self related. The feed forward carries the signal from source to load.

Negative feedback helps to maintain stability in a system in spite of external changes. It is related to homeostasis. For example, in a population of foxes (predators) and rabbits (prey), an increase in the number of foxes will cause a reduction in the number of rabbits; the smaller rabbit population will sustain fewer foxes, and the fox population will fall back. In an electronic amplifier feeding back a negative copy of the output to the input will tend to cancel distortion, making the output a more accurate replica of the input signal

Positive feedback amplifies possibilities of divergences (evolution, change of goals); it is the condition to change, evolution, growth; it gives the system the ability to access new points of equilibrium.

When a public-address system is used with a microphone to amplify speech, the output from a random sound at the microphone may produce sound at a loudspeaker that reaches the microphone such as to reinforce and amplify the original signal (positive feedback), building up to a howl (of frequency dependent upon the acoustics of the hall). A similar process is used deliberately to produce oscillating electrical signals.

Feedback is distinctly different from reinforcement that occurs in learning, or in conditioned reflexes. Feedback combines immediately with the immediate input signal to drive the responsive power gain element, without changing the basic responsiveness of the system to future signals. Reinforcement changes the basic responsiveness of the system to future signals, without combining with the immediate input signal. Reinforcement is a permanent change in the responsiveness of the system to all future signals. Feedback is only transient, being limited by the duration of the immediate signal

Types of feedback

When feedback acts in response to an event/phenomenon, it can influence the input signal in one of two ways:

1. An in-phase feedback signal, where a positive-going wave on the input leads to a positive-going change on the output, will amplify the input signal, leading to more modification. This is known as positive feedback.

2. A feedback signal which is inverted, where a positive-going change on the input leads to a negative-going change on the output, will dampen the effect of the input signal, leading to less modification. This is known as negative feedback.

Positive feedback tends to increase the event that caused it, such as in a nuclear chain-reaction. It is also known as a self-reinforcing loop. An event influenced by positive feedback can increase or decrease its output/activation until it hits a limiting constraint. Such a constraint may be destructive, as in thermal runaway or a nuclear chain reaction. Self-reinforcing loops can be a smaller part of a larger balancing loop, especially in biological systems such as regulatory circuits.

Negative feedback, which tends to reduce the input signal that caused it, is also known as a self-correcting or balancing loop. Such loops tend to be goal-seeking, as in a thermostat, which compares actual temperature with desired temperature and seeks to reduce the difference. Balancing loops are sometimes prone to hunting: an oscillation caused by an excessive or delayed negative feedback signal, resulting in over-correction, wherein the signal becomes a positive feedback.

ADVANTAGES

a) Increased stability in the amplification. The gain is less dependent on the parameters of the amplifier elements.

b) Feedback reduces distortion in the amplifier.

c) The bandwidth of the amplifier is increased.

d) It is easier to achieve desired input and output impedances

Types of Amplifiers

• There are four common classes of amplifier in the high-fidelity reproduction of audio:

• Class A

• Class B

• Class AB

• Class D

Class A

• This is the most linear of the classes, meaning the output signal is a truer representation of what was imputed.

• characteristics of the class:

• they reproduce the entire waveform in its entirety.

• Class A is the most inefficient of all power amplifier designs, averaging only around 20.

 They are the most accurate of all amps available, but at significant cost to manufacture, because of tight tolerances, and the additional components for cooling and heat regulation.

Class B

• In this amp, the positive and negative halves of the signal are dealt with by different parts of the circuit.

• Class B operation has the following characteristics:

• The input signal has to be a lot larger in order to drive the transistor appropriately.

• This is almost the opposite of Class A operation.

Class AB

• This is the compromise of the bunch. Class AB operation has some of the best advantages of both Class A and Class B built-in.

• Its main characteristics are:

• The output bias is set so that current flows in a specific output device for more than a half the signal cycle but less than the entire cycle.

Class D

• These amplifiers are erroneously called "digital" amplifiers by the press and many audio "experts." Here's the skinny on Class D:

• While some Class D amps do run in true digital mode, using coherent binary data, most do not.

• This efficiency gain is at the cost of high-fidelity.