Student’s Name:Mehmet Serdar TekeStudents’s ID:260701050

EE-331 Electronics-1Term Paper

1.Noise in Electronic Circuits

Noise is the total of unwanted signals contributing to input signal and changing the output signal in an unexpected manner. Noise is simply described in terms of its frequency spectrum that determines how effective the noise is at different frequencies. The noise is also classified according to its frequency spectrum. Also, noise can be described in terms of its power. However, in this respect the jargon makes somewhat sloppy use of terminology, and what is called “noise power” is often in units of Volt2.

1.1.Types of Noise

1.1.1.White Noise

This type of noise has a constant frequency spectrum. In other words, noise power is constant over some range of frequencies. White noise gets its name from white light in which the power spectral density of the light is distributed over the visible band in such a way that the eye's three color receptors are approximately equally stimulated.

Figure 1.1.1.1.Power Spectrum of White Noise

1.1.1.1.Thermal Noise

Thermal noise is a type of white noise since it has a constant value of power over some range of frequencies. It is generated by thermal agitation of electrons in a conductor or electronic device. Also, it is proportional to the absolute temperature of the conductor. It shows itself clearly in the input circuits of audio equipment such as microphone pre-amps, or antenna input of a receiver, where the signal levels are low.

1.1.1.2.Shot Noise

Shot noise is a type of white noise as its power is constant over some range of frequencies. It normally occurs when there is a potential barrier. For example, PN junction diode has a potential barrier so that shot noise is generated when the electrons and holes pass across the barrier. Since a BJT transistor has PN junctions, it also produces shot noise. On the other hand, a resistor normally does not produce shot noise since there is no potential barrier built within a resistor. Therefore, current flowing through a resistor will not exhibit any fluctuations whereas current flowing through a diode produces small fluctuations due to shot noise.

1.1.2.Pink Noise

In this type of noise, noise power is reversely proportional to frequency, which means that noise power decreases as frequency increases. In fact, noise power is directly proportional to 1/frequency. Therefore, pink noise is also known as 1/f noise. Power density of pink noise falls off at 10 dB per decade. Pink noise is also called as flicker noise.

Figure 1.1.2.1.Power Spectrum of Pink Noise

The inverse proportionality with frequency is almost exactly 1/f for low frequencies, whereas for frequencies above a few kHz, the noise power is weak but essentially flat.

In this type of noise, the noise power is proportional to the bias current, which means that as the bias current increases noise power also increases. As shown in the figure 1.1.2.1, most of the power is concentrated at the lower end of the frequency spectrum.

Flicker noise is usually defined by the corner frequency fc, where flicker noise is equal to white noise. The corner frequency is a function of the operating conditions. In these conditions, temperature and bias current are the most important ones.

As the channel length increases, flicker noise also increases. Therefore, JFET, which has a long channel length, produces an important flicker noise.

Figure 1.1.2.2.Corner Frequency of Flicker Noise

1.1.3.Burst Noise

In this type of noise, noise power increases with bias current and is inversely proportional to the square of the frequency, meaning that it is directly proportional to 1/f2. Therefore, it is also a low frequency noise like pink noise. It is also called as popcorn noise.

Burst noise seems to be associated with heavy metal ion contamination. Measurements show a sudden shift in the bias current level that lasts for a short duration before suddenly returning to the initial state. Such a randomly occurring discrete level burst would have a popping sound if amplified in an audio system.

As seen in figure 1.1.3.1, sudden changes in drain to source current for a MOSFET are about the levels of nA. It seems to be very small, but if the current is amplified very largely in a configuration such as a power amplifier this type of noise becomes effective on the output signal changing the input signal in an unwanted manner.

Figure 1.1.3.1.Sudden changes of drain to source current in a MOSFET

1.1.4.Environmental Noise

Environmental noise refers to any noise source that arises from well-defined sources in the environment such as electrical power lines. Since the voltage of power lines is concentrated at a constant frequency, environmental noise has a noise power spectrum like an impulse function as shown in the figure 1.1.4.1.

Figure 1.1.4.1.Power Spectrum of Environmental Noise 1.1.5.Gaussian Noise

In Gaussian noise, noise power is distributed over some range of frequencies according to Gaussian curve, also called as normal distribution. In figure 1.1.5.1 Gaussian curve is shown. In this curve when b(t) is noise power and t is frequency, it is called as Gaussian noise.

Figure 1.1.5.1.Gaussian Curve

1.2.Electronic Circuit Design Considering Noise By considering noise power spectrum, operating frequency must be determined at where noise power is minimum so that noise contributes to the output signal as small as possible. In the figure 1.2.1, operating frequency should be at where white noise occurs because least noise power exists on its position.

Figure 1.2.1.A Random Noise Power Spectrum

2.Distortion in Electronic Circuits

Distortion is the alteration of original shape of an electrical signal. In other words, if the shape of output signal is not exactly the same with the shape of input signal after amplification, it is said to exist a distortion on the input signal. Distortion can result from various mechanisms. For example, nonlinearities in the transfer function of an active device, such as a transistor, or an operational amplifier cause distortion. 2.1.Types of Distortion

2.1.1.Harmonic Distortion

Harmonic distortion is the change in the waveform of the supply voltage from the ideal sinusoidal waveform. In other words, it is the change of the AC voltage waveform from a simple sinusoidal to a complex waveform and can be generated by a load and fed back into the AC mains, causing power problems to other equipment on the circuit. It is caused by the interaction of distorting customer loads with the impedance of supply network. These voltage harmonics can cause communication errors and hardware damage due to unexpected overheating of components.

Figure 2.1.1.1.Harmonic Distortion (Voltage versus time plots of input and output signals)

2.1.2.Amplitude Distortion Amplitude distortion occurs in a system when the output amplitude is not a linear function of the input amplitude under specified conditions. For example, amplifiers are designed to amplify small voltage input signals into much larger output signals and input signal is multiplied by a constant resulting in output signal in amplifiers. When the peak values of the frequency waveform are attenuated causing distortion due to a shift in the Q-point and amplification may not take place over the whole signal cycle, amplitude distortion occurs as seen in figure 2.1.2.1.

Figure 2.1.2.1.Amplitude Distortion due to Incorrect Biasing

2.1.3.Frequency Distortion Frequency distortion occurs in a transistor amplifier when the level of amplification varies with frequency. Many of the input signals, which a practical amplifier will amplify, consist of the required signal waveform called the "Fundamental Frequency" and a number of different frequencies called "Harmonics" superimposed onto it. Normally, the amplitudes of these harmonics are a fraction of the fundamental amplitude. Therefore, they have very little or no effect on the output waveform. However, the output waveform can become distorted if these harmonic frequencies increase in amplitude with regards to the fundamental frequency.

Figure 2.1.3.1.Frequency Distortion due to Harmonics The input waveform consists of a fundamental frequency and a second harmonic signal in figure 2.1.3.1. On this input signal, the frequency distortion occurs when the fundamental frequency combines with the second harmonic to distort the output signal.

2.1.4.Phase Distortion Phase distortion is also called as delay distortion. It occurs in a non-linear transistor amplifier or an Operational Amplifier when there is a time delay between the input signal and its appearance at the output. That’s why it is also called as delay distortion. If it is accepted that the phase change between the input and the output signals is zero at the fundamental frequency, the resultant phase angle delay will be the difference between the harmonic and the fundamental. This time delay will depend on the construction of the amplifier and will increase continuously with frequency within the bandwidth of the amplifier.

Figure 2.1.4.1.Phase Distortion due to Delay

References1. http://www.qsl.net/va3iul/Noise/noise.html2. http://hamers.chem.wisc.edu/WebRoot$/chem628_fall2005/noise/analog_circuit_noise.pdf3. http://www.ackadia.com/computer/power-protection/power-protection-power-problems.php4. http://www.elec.uow.edu.au/iepqrc/files/technote3.pdf5. http://qooljaq.com/AmplifierDistortion.htm