2010/ 2011EEE 314: ELECTRONIC III NOISE This lesson will be a great informative lesson on internal external noise in an electronic circuit Fadare Suraju Adebayo Electrical Engineering Department, Federal Polytechnic, Ilaro. 2010/2011 component or system. Shot noise. Hence. This comprehensive definition really indicates that noise is an analogy to impurity in pure water where pure water is the original signal in the electronic system. Noise power Pn of thermal noise is constant for any resistance and is related to absolute temperature T(K) and bandwidth B of the electronic system. we have Pn = 4kTB Where K is Boltzman’s constant (k = 1. Before treating other examples of noise. It is a method of comparing the capacitor of the pure signal to the niose signal which is going to be an indication of how the signal will be distorted. Internal noise is the group of noise that are being generated within an electronic device or component or system.3805 x 10-3 J/K) vn is However we all know that P = Therefore Vn(rms)= (4kTBR)1/2 Vn is the noise voltage and is directly proportional to the resistance of the electronic device. 1/f or flicker noise. 4. Bases on the causes of the noise signal. Examples of internal noise are Thermal noise. S/N= 10log in dB 2|Page 09/01/2011 08:02:36 . lets treat this important phenomenon .1 Thermal or Johnson or White Noise The noise is generated by thermally induced random motion of atoms of electronic components. Internal Noise and 2.Signal –To – Noise Ratio (S/N).LESSON 4 Noise Noise can be defined as an unwanted signal that causes random fluctuation in the original signal in an electronic system or external effects of the environment. It is called white noise because it has component at all frequency (all part of the frequency spectrum) like white light. namely: 1. noise is divided into two. External Noise. bipolar transistor noise and FET noise.2 Signal to Noise Ratio (S/N) This can be simply defind as the ratio of the power of the true siginal Ps and niose power Pn of an electronic system. 4. hence. B= Bandwidth over which the noise is measured and I is the mean value of the current. Another noise that can be found at the output signal of a transistor is the shot noise as a result of the random traversal of charge carriers across the pn junctions. What is the S/N ratio at this point? Solution 4. Another name for 1/f noise is flicker noise which is caused by random variation in the diffusion of charge carrier within the device. 4. possesses thermal noise.4 1/f Noise or Flicker Noise 1/f noise is caused by not one but a variety of noise sources. It is magnitude is given by the below equation. It is called 1/f noise because the power spectrum of the noise is inversely proportional to frequency.S/N= 20log in dB Example 4.5 Bipolar Transistor Noise Bipolar transistor.6 Noise in FETs Remember that a FET is a unipolar electronic component.5 Volt rms is distorted by 10 mv rms noise.01 V S/N= 20log 4.6 x 10-19 C.1: A signal of 2. When operating at low frequency (up to few kHz).5 V Vn= 10 mV = 0.3 = 20log = 48 dB Shot Noise Shot noise is generated by random flow of current or charge carriers across potential barrier such as pn-junction. 4. Statistical variation in the flow of small current leads to the noise current called shot noise. base current is small.7 Noise Figure (NF) S/N indicates quality of non-performance of the amplifier. Hence. In an high transistor. a halving of the power doubles the frequency. In(rms) = (2eBI)1/2 Where e= electronic charge= 1. effect of shot noise is significant. By definition. NF gives a measure of performance of an amplifier in handling noise. Clearly. hence. 4. Thermal and flicker noises are those that have effect on signal that passes through FETs. 4. the flicker noise is negligible. most of the noise power is concentrated at low frequency. NF is the ratio of noise 3|Page 09/01/2011 08:02:36 .1 Given Parameters: Vs= 2. However. transistor also has the flicker noise but at high frequency. shot noise is insignificant in FET. like any other electronic components that has resistance. electric motors. 8) Use of twisted pair wire. 6) Using multilayer circuits but do not make circuit board to be perpendicular to one another. thermal noise is caused by the resistance of the channel. Hence. decreases with increase in drain current reduces thermal noise. A very low internal resistance system will have a very low noise signal voltage. at low quiescent current of a few micro amps. 3) Design FET for fairly high drain current: In FET. 4. Sources of external noises can either be natural sources or man-made sources. 7) Use of electromagnetic shield. 4) Reducing conductor length 5) Reducing numbers of loop in a circuit. 4|Page 09/01/2011 08:02:36 . The man-made sources are automatic ignition systems. Natural external noises are lightning.9 Reducing Noise Signals 1) Reducing Source Resistance: A careful design of the electronic system in a way that the source resistance is made as low as possible in order to reduce thermal noise which is proportional to the resistance. Suitable transistors combine low flicker noise and high current gain at low collector current. solar emission and cosmic radiation. circuit breakers. External noise is mostly electromagnetic interference (EMI). both noise voltage and current increase with collector current.produced at the output of a typical amplifier to the output of the ideal (noiseless) amplifier when having the same input signal. however. Remember that the source resistance. switching power supplies. 2) Design transistor for low quiescent current: In transistor. RS = √ Where Vn and In are the rms noise voltage and current. power distribution systems. industrial plants. contactors and computers. 4. mobile phones. The channel resistance. if the transistor is operated. 9) Preventing the use of the system where there is EM radiation.8 External Noise This noise signal get into the true signal and electronic system from external sources which are also referred to as interference.