Electronics Past Paper PDF

Transistor as current source: 1) Transistors have a fixed degree of amplification. (hFE). When a transistor is at maximum amplification it is saturated. Saturation works when we want to pass the maximum amount of current though the transistor. By limiting the voltage at the base, we can limit the cu...

Transistor as current source: 1) Transistors have a fixed degree of amplification. (hFE). When a transistor is at maximum amplification it is saturated. Saturation works when we want to pass the maximum amount of current though the transistor. By limiting the voltage at the base, we can limit the current through the circuit. Somewhat against intuition, when we limit current with a transistor, it's called amplification, because the small amount of voltage at the base is controlling the large current flow; Ic = (Vb- 0.6v)/R 18 Analog and Digital Signals There are two different methods of sending an electronic signal from A to B. ANALOG signals are continuous, and can take any value. DIGITAL signals encode values into binary numbers. As a binary number is made up entirely from 0's and 1's, it may be transmitted in the form of electronic on/off pulses (on =1, off =0). When these pulses are received, they are processed. A digital signal is made up of discretely variable physical quantities. Whilst these two types of signal both transmit information in electrical voltages, they each have their advantages and disadvantages. In recording audio signals, analog systems are useful, because they can give a faithful electronic representation of a complex waveform. However, because of the need for amplification of the electronic signal, 'noise' can be added along the signal path. This noise is due to unavoidable electron activity in the circuitry. Unfortunately, there is no easy way to get rid of noise from the original signal. Consequently, the noise (audible as a 'hiss') is added to the signal with each stage of transmission. A digital equivalent to this system would sample the sound wave at selected intervals and transmit the values that correspond to the sound wave in binary code. The digital representation of the sound wave could then be moved around or processed within the system without picking up any additional noise. Although the electron (noise) activity is still taking place, whenever the digital signal is repeated, during each stage of the transmission, the noise can be omitted. Analog Signal Digital Signal Accurate reproduction of signal needs Very immune from noise extra work Suffers from noise and distortion Output is accurate but can have errors from the sampling process Simple technique Complicated but can operate at long distance 19 Table 1 outlines the basic characteristics of 3 modulation (encoding transmission signal) schemes: Amplitude modulation (AM), frequency modulation (FM) (both analog schemes) and digital modulation. Table 1 - Comparison of AM, FM, and Digital Encoding Techniques Parameter AM FM Digital Signal-to-Noise Low-to-Moderate Moderate-High High Ratio Performance vs. Sensitive Tolerant Invariant Attenuation Transmitter Cost Moderate-High Moderate High Receiver Cost Moderate Moderate-High High Receiver Gain Often Required Not Required Not Required Adjustment Adjustments No Adjustments No Adjustments Installation Required Required Required Multi-channel Require High Fewer Channels Good Capabilities Linearity Optics Performance Over Moderate Excellent Excellent Time Environmental Moderate Excellent Excellent Factors One difference between analog and digital transmission involves the bandwidth, or transmission capacity required for both schemes. Analog signals require much less bandwidth, only about 4.5 MHz with a 143.2 Mb/s data rate for the average video signal. By comparison, some digital video transmission standards require as much as 74.25 MHz with a data rate of 1485 Mb/s. Another difference between analog and digital transmission deals with the hardware’s ability to recover the transmitted signal. Analog modulation, which is continuously variable by nature, requires adjustment at the receiver end in order to reconstruct the transmitted signal. Digital transmission, however, because it uses only 1’s and 0’s to encode the signal, offers a simpler means of reconstructing the signal. 20 Both types of modulation can incorporate error detecting and error correcting information to the transmitted signal. However, the latest trend in signal transmission is forward error correcting (FEC). This scheme, which uses binary numbers, is suited to digital transmission. Extra bits of information are incorporated into the digital signal, allowing any transmission errors to be corrected at the receiver end. Analog Signal Transmission Analog transmission inserts signals of varying frequency or amplitude on carrier waves with a given frequency to produce a continuous wave. In a telephone system, an electric current or the reproduction of patterned sound waves are transmitted through a wire and into the telephone receiver. Once this is completed, they are then converted back into sound waves. In digital transmission, the signals are converted into a binary code, which consists of two elements—positive (1) and non-positive (0). Every digit in a binary number is referred to as a bit and represents a power of two. As an example of digital transmission, in a type of digital telephone system, coded light signals travel through optical fibers and are then decoded by the receiver. When transmitting a telephone conversation, the light flashes on and off about 450 million times per second. This high rate enables two optical fibers to carry about 15,000 conversations simultaneously. Digital format is ideal for electronic communication as the string of 1s and 0s can be transmitted by a series of "on/off" signals represented by pulses of electricity or light. A pulse "on" can represent a 1, and the lack of a pulse "off" can represent a 0. Information in this form is very much easier to store electronically. Furthermore, digital transmission is usually faster and involves less noise and disturbances as compared to analog data transmission. Analog signal transmission uses direct current (dc) variations in current or voltage to represent a data value used to communicate information. Most data acquisition signals can be described as analog, digital or pulse. While analog signals typically vary smoothly and continuously over time, digital signals are present at discrete points in time. Analog signals represent continuously variable entities such as temperatures, pressures, or flow rates. Because computer-based controllers and 21 systems understand only discrete on/off information, conversion of analog signals to digital representations is necessary. In analog signal transmission the wiring system can effectively reduce noise interference. Analog signal transmission employs two- wire signal leads or three-wire signal leads for high precision and accuracy. The third signal lead, or shield, is grounded at the signal source to reduce noise. There are many different wiring options that are available to reduce unwanted noise pickup from entering the line. Four types of wires are fundamental in data acquisition-plain pair, shielded pair, twisted pair, and coaxial cable. 1. Plain wire is not very reliable in screening out noise and is not suggested. A shielded pair is a pair of wires surrounded by a conductor that does not carry current. The shield blocks the interfering current and directs it to the ground. When using shielded pair, it is very important to follow the rules in grounding. Again, the shield must only be grounded at one source, eliminating the possibility of ground-loop currents. 2. Twisted-pairs help in elimination of noise due to electromagnetic fields by twisting the two signal leads at regular intervals. Any induced disturbance in the wire will have the same magnitude and result in error cancellation. 3. A coaxial cable is another alternative for protecting data from noise. A coaxial cable consists of a central conducting wire separated from an outer conducting cylinder by an insulator. The central conductor is positive with respect to the outer conductor and carries a current. Coaxial cables do not produce external electric and magnetic fields and are not affected by them. This makes them ideally suited, although more expensive, for transmitting signals. Coaxial Cable Construction 22  A sensor measures a variable by converting information about that variable into a dependent signal of either electrical or pneumatic nature. Cadmium sulfide resistance varies inversely and nonlinearly with light intensity and we can employ this device for light measurement. Analog signal conditioning provides the operations necessary to transform a sensor output into a form necessary to interface with other elements of the process-control loop. We often describe the effect of the signal conditioning by the term transfer function. By this term we mean the effect of the signal conditioning on the input signal. Thus, a simple voltage amplifier has a transfer function of some constant that, when multiplied by the input voltage, gives the output voltage. Signal conditioning can be categorized into the following types: 1. Signal-Level Changes The simplest method of signal conditioning is to change the level of a signal. The most common example is the necessity to either amplify or attenuate a voltage level. Generally, process-control applications result in slowly varying signals where dc or low-frequency response amplifiers can be employed. An important factor in the selection of an amplifier is the input impedance that the amplifier offers to the sensor (or any other element that serves as an input). In process control, the signals are always representative of a process variable. In accelerometers and optical detectors, the frequency response of the amplifier is very important. 2. Linearization The process-control designer has little choice of the characteristics of a sensor output versus process variable. Often, the dependence that exists between input and output is nonlinear. Even those devices that are approximately linear may present problems when precise measurements of the variable are required. Specialized analog circuits were devised to linearize signals. For example, suppose a sensor output varied nonlinearly with a process variable, as shown in Figure-1a. A linearization circuit, indicated symbolically in Figure-1b, would condition the sensor output to produce voltage signal linear with the process variable, as shown in Figure - 1c. The modern approach to this problem is to provide the nonlinear signal as input to a computer and perform the linearization using software. 23  FIGURE- 1: The purpose of linearization is to provide an output that varies linearly with some variable even if the sensor output does not. 3. Conversions Often, signal conditioning is used to convert one type of electrical variation into another. Thus, a large class of sensors exhibit changes of resistance with changes in a dynamic variable. In these cases, it is necessary to convert this resistance change either to a voltage or a current signal. This is generally accomplished by bridges when the fractional resistance change is small and/or by amplifiers whose gain varies with resistance. An important type of conversion is associated with the process-control standard of transmitting signals as 4-20 mA current levels in wire. This gives rise to the need for converting resistance and voltage levels to an appropriate current level at the transmitting end and for converting the current back to voltage at the receiving end. Thus, voltage-to-current and current-to-voltage converters are often required. The use of computers in process control requires conversion of analog data into a digital format using analog-to-digital converters (ADCs). Analog signal conversion is usually required to adjust the analog measurement signal to match the input requirements of the ADC. 24 4. Filtering and Impedance Matching In the industrial environment, signals of considerable strength are present, especially those generated at 60-Hz line frequency. Motor start transients also may cause pulses and other unwanted signals in the process-control loop. In many cases, it is necessary to use high-pass, low-pass, or notch filters to eliminate unwanted signals from the loop. Such filtering can be accomplished by passive filters using only resistors, capacitors, and inductors; or active filters, using gain and feedback. Impedance matching is an important element of signal conditioning when transducer internal impedance or line impedance can cause errors in measurement of a dynamic variable. Both active and passive networks are employed to provide such matching. 5. Concept of Loading Loading of one circuit by another introduces uncertainty in the amplitude of a voltage as it is passed through the measurement process. If this voltage represents some process variable, then we have uncertainty in the value of the variable. Qualitatively, loading can be described as follows. Suppose the open circuit output of some element is a voltage, say Vx, when the element input is some variable of value x. Open circuit means that nothing is connected to the output. Loading occurs when we connect a load across the output and the output voltage of the element drops to some value, Vy < Vx. Different loads will result in different drops. Quantitatively, loading is evaluated as follows. Thevenin's theorem tells us that the output terminals of any element can be defined as a voltage source in series with output impedance (output resistance). This is often called the Thevenin's equivalent circuit model of the element. 25