Semi-conductor Devices and Circuits - Essay Example

Semi-conductor Devices and Circuits There are two types of rectification, the half-wave and full-wave rectification. I.1 Half-wave rectification The simplest rectification is the half-wave rectifier circuit with a single-phase diode. This circuit has only one diode, a resistor load and a capacitor. The diode is fed to the secondary winding of the transformer and it conducts when the secondary transformer produces the positive half-cycle, and stops conducting when the secondary produces the negative half cycle. (Lee & Chow 2011, p. 150) Figure 1 Schematic diagram of half-wave rectification In this diagram, we can see that the current iL passes from the diode then to the load resistor. The other terminal of the secondary transformer provides the negative. The load is cut across by the resistor. Figure 2 Voltage and current waveform of a half-wave rectifier Voltage relationship The half-wave rectifier states that the load voltage vL is Vdc and expressed in the following diagram. The load voltage is equal to 0 and the angular frequency of the source is w = 2π = T. This is expressed in the following diagram. From the diagram above, we can get the half-wave: Half-wave Vdc = Vm = 0.318Vm π I.2 Full-wave rectification The full-wave rectification can be produced in two types: the center-tapped and the bridge rectification. I.2.1 The full-wave center-tapped The full-wave with center-tapped transformer has two diodes acting as half-wave rectifiers and whose negative ends are connected to the two terminals of the transformer secondary. The two diodes provide a full-wave output. The DC currents of the two half-wave rectifier diodes are equal and opposite. (Lee & Chow 2011, p. 150) Figure 3 Schematic diagram of full-wave rectification with center-tapped transformer secondary I.2.2 The bridge rectifier The bridge rectification uses four diodes to provide full-wave rectification, and this does not use a center-tapped transformer. The rectification is done in such a way that the current flows from the positive half-cycle of the transformer secondary to the two diodes, D1 and D2. The other two diodes, D3 and D4, conduct when the negative half-cycle is produced from the transformer secondary. Figure 4 The bridge rectifier uses four diodes to provide full-wave rectification. The positive and negative half-cycles of a full-wave rectifier are expressed in the following formula. The full-wave is: Vdc = 2Vm = 0.636Vm π I.3 Zener regulator Most application of zener diode is as shunt voltage regulator; its specific role is to regulate the load voltage. There are various uses or applications for a zener diode, such as: 1. Voltage regulating element in voltage regulators 2. Protector in a circuit 3. Zener or voltage limiter Figure 5 The application of zener diode in a regulator circuit is shown in the figure below. In this circuit, zener diode is used as shunt regulator. The zener diode compensates for the variation in load current; the zener drifts with the temperature. The drift characteristics are given in many zener diode datasheets. Its load regulation is adequate for most supply specifications for integrated circuits. It has a higher loss than the series-pass type of linear regulator because its loss is set for the maximum load current. The zener shunt regulator uses a simple formula where input voltage is greater than output voltage; the output is controlled by the zener diode. The zener shunt regulator is typically used for very local voltage regulation for less than 200mW of a load. A series resistance is placed between a higher voltage and is used to limit the current to the load and zener diode. I.4 Switching and amplifier circuits for transistors An ordinary transistor has three terminals for the base, emitter and collector. When it is in the OFF position, the output comes from the collector of a common-emitter stage. A digital circuit usually operates in two modes. (The transistor amplifier n.d.) Transistors are used for switching and amplifiers in circuits. In digital logic, transistors have the ability to switch voltage levels, between “low” and “high” that are represented as 0V and +5V, respectively. The transistor maintains at cutoff at the 0 level or OFF condition, and ‘in saturation for the ON condition’ (Godse & Bakshi 2009, p. 163). A transistor can be used as a switch when it performs the OFF and ON switch. A common usage of transistors as switch is in digital circuits wherein the voltage is low (0v) or high (+5v). A simple and common example of a transistor as a switch is in a circuit that activates a relay whose purpose is also to turn ON and OFF a circuit connected to it. When a switching transistor is turned on, it is said to be saturated and its output is low. When it is OFF, it is the opposite, meaning the output is high. I.5 Voltage regulators The accuracy of the voltage produced by power supplies is important insofar as proper operation and maximum life of the equipment are concerned. Power supplies in present-day electronic equipments have timing, waveshaping, and other critical circuits that require a highly regulated voltage source. A regulated power supply is most common in many electronic circuits. One example is the linear regulator which is a step-down regulator, i.e. the input voltage source is higher than the desired output voltage. Most regulators use transistors or integrated circuits with a combination of darlington transistors. If the transistor is shorted or open, we get a wrong voltage and the circuit might not work. Either the fuse is busted or the whole device might not work. Troubleshooting the power supply is one of the most challenging parts of the technician or engineer’s job. Most common defects or trouble in a power supply include: busted fuse, shorted or open regulator (transistor or IC), shorted or open capacitor, and open resistor, among others. I.6 Signal generators Signal generators are electronic equipments designed to test radio frequency signals. Electronics technicians make use of the instruments to test whether the required radio frequencies in electronic appliances are produced in accordance with the specified frequencies. Signal generators are effective as test equipment when paired with an oscilloscope as frequencies are visible in an oscilloscope. A newly-designed signal generator (battery-operated) produces a sinusoidal output with a variable frequency and amplitude and operates from ±12V dc voltages. The frequency applied may be from 20 Hz to 20 KHz, with the peak amplitude also varied from 10mV to 10V. The new version adds a pulse waveform generator to the audio signal generator in a single unit. The pulse generator produces an output with a variable duty cycle that can be used to drive 5V digital logic circuits. The sine wave generator will remain the same, but the frequency control and the output terminals will be common to both the sine wave generator and the pulse generator. The output function is switch-selectable, and the pulse waveform requires an additional front panel control for adjusting the duty cycle. (Floyd 2007, p. 689) I.7 Thyristors and triacs I.7.1 Thyristors The most common thyristor is a four-layer diode (some authors call it the Shockley diode and SUS) with two terminals, the anode and the cathode. Its construction consists of four semiconductor layers that form a pnpn structure. Thyristors act as switch and stay at OFF situation until the forward voltage reaches a certain value; when this value is attained, it turns on and conducts. It will again turn to OFF position when the current is reduced below a specified voltage. (Floyd 2007, p. 554) The PNPN structure is represented by an equivalent circuit, such as a PNP transistor and an NPN transistor. This is shown in the figure below. Figure 6 The thyristor The first PNP layer forms Q1 and the second NPN layer forms Q2, with the two middle layers shared by both equivalent transistors. The base-emitter junction of Q1 corresponds to the PN junction 1 in the figure above, and the base-emitter junction of Q2 corresponds to PN junction 3, and the base-collector junctions of both Q1 and Q2 corresponds to PN junction 2. (Floyd 2007, p. 554) Figure 7 A Thyristor schematic Figure 7B In the above diagram, when a positive bias voltage is applied to the anode (A) with respect to the cathode (K), the base-emitter junctions of Q1 and Q2 (PN junctions 1 and 3 in Figure A) are forward biased, and the common base-collector junction (PN junction 2 in Figure A) is reverse-biased. I.7.2 Triacs A triac is like a diac but it has a gate terminal. A diac is a two-terminal four-layer semiconductor device (thyristor) that can conduct current in either direction when activated. But a triac is different from a diac in that a triac can conduct by a pulse of gate current and does not require the breakover voltage in order to conduct. A triac is like a silicon-controlled rectifier (SCR) connected in parallel and in opposite directions with a common gate terminal; but it is different from an SCR because it can conduct in either direction when it is turned on, depending on the polarity of the voltage across its A1 and A2, as shown in the next figure. (Floyd 2007, p. 567) Basic construction of a triac Symbol Figure 8 A Figure 8 B I.8 Transistors A transistor is a semi-conductor device which has different polarities, positive and negative. An ordinary transistor has three terminals, or legs, and has markings on its body that determine if it is an NPN or a PNP transistor. We can measure an NPN or PNP transistor with the ohmmeter of the multi-tester by setting it to X1 or X10 settings. We have to determine first if it is an NPN or a PNP transistor by reading the value or marking on its body. These markings usually begin with a letter. For an NPN transistor, the marking begin with letters C or D, or 2SC or 2SD. For PNP, marking begin with letters A or B. The three terminals or three legs of a transistor are what we call base, emitter and collector terminals which form configurations. To test whether an NPN transistor is good or bad, the black probe is pointed to any of the three pins, and the red probe (the positive) pointed interchangeably to the other two. The reading will give around 100 to 500 ohms, not too high or too low. The transistor has shorted terminals if the reading goes to zero in the ohmmeter. We are able to find the base by interchanging the probes. If by interchanging the probes, we get any reading, the transistor must be defective; or by interchanging the probes, we won’t get any reading, the terminals are open, and we have a bad transistor. Figure 9A NPN Transistor E B C For NPN transistor, the arrow of the emitter is pointed outward. Figure 9B PNP Transistor E B C For a PNP transistor, the arrow of the emitter is pointed inward. I.9 Power supplies Power supplies are electronic circuits that provide power to equipment and electronic circuits. Power supplies provide life to appliances, amplifiers, and other circuits. The purpose of a power supply is to provide power to each section of the circuit. There are power supplies with transformers whose primary winding is connected to an AC voltage. But there are power supplies that do not use transformers. All power supplies work under the same principle, whether the supply is a linear or a more complicated switching supply. All power supplies have at their heart a closed negative feedback loop which holds the output voltage at a constant value. A power supply is the first stage that greets us when we open an electrical appliance. This is the heart of any electronic device or appliance. We cannot have an appliance or instrument without a power supply. 2. Operational Amplifier Circuits 2.1 Inverting operational amplifier In the inverting operational amplifier, the input signal is fed to the negative or inverting terminal of operational amplifier. This is usually applied with a feedback resistor and an input resistor; this is to say that all inverting amplifiers have two resistors for the gain, one for the feedback and the other for the input line. The inverting amplifier has two capacitors: one for the input and one for the output, specifically designed for AC use. As the name suggests, in the inverting op-amp, the input signal is fed to the negative terminal. The values for the resistor are 100K and 10K, or equal to zero. The gain could be increased by increasing the value of the feedback resistor to 1Mohm, to make the gain 100. When there is more gain, there’s a tendency to create more noise and to overload the stage. The reason for maintaining the value of x10 is to control the noise. The electronic instrument useful for measuring the gain of the feedback resistor is the oscilloscope. By using the oscilloscope, we can check the feedback by feeding the amplifier with a controlled amplitude AC signal using the signal generator. The AC signal produced is something like 100 mv and the output can be calculated by multiplying 100 mv x 10, and the answer is 1 volt. (Singmin 2000, p. 81) 2.2 Non-inverting Op Amps An op-amp connected in a closed-loop configuration as a non-inverting amplifier with a limited voltage gain is demonstrated in the next figure. Here, the input signal is applied to the non-inverting input which is positive current. The output is applied back to the inverting (-) input through the feedback circuit (closed loop) which is the output of the input resistor R1 and the feedback Resistor Rf. The result is a negative feedback. Resistors Ri and Rf produces a voltage-divider circuit, which reduces the output voltage and connects the reduced voltage Vf to the inverting input. Feedback voltage is solved in the following formula. Figure 10 The Non-inverting amplifier 2.2.1 Voltage follower Op Amp The voltage follower op-amp is a special feature of the non-inverting amplifier in which all of the output voltage reverts back to the negative input, i.e. the inverting input by a straight connection. This is shown in the figure below. Here, the straight feedback connection has a voltage gain of 1, meaning there is no voltage gain. Figure 11 The Voltage follower The voltage follower op-amp has very high input impedance and very low output impedance, which is conducive and ideal buffer amplifier in dealing with high-impedance sources and low-impedance loads. (Floyd 2007, p. 606) 2.3 Summing Op Amp A summing op amp has two or more inputs whose output voltage is proportional to the negative of the algebraic sum of its input voltages. In this kind of amplifier, two voltages are applied to the inputs and they produce currents. This is demonstrated in the Figure 12. Figure 12 Summing up op-amp Summing op amp uses the concept of infinite input impedance and virtual ground which draws the conclusion that the inverting (-) input of the op-amp is approximately 0 V and no current through it. The concept drawn in this formula states that both input currents I1 and I2 add together at a summing point, A, and form the total current (IT) which goes through Rf. 2.4 Integrator and Differentiator Op-Amp In an op-amp integrator, the feedback element is formed by a capacitor which provides an RC circuit along with the input resistor. Figure 13 Integrator and differentiator op-amp The capacitor has a great role to play in an integrator. The capacitor charge is proportional to to the charging current (Ic) and the time (t). This is expressed in the following formula. Q = Ict With regards to the voltage, the capacitor charge is expressed in: Q = CVc The capacitor voltage in a simple RC circuit is not expressed in a linear mode but exponential. The reason for this is that the charging current in the capacitor continues to decrease as it charges and the voltage also decreases. The important thing about using an op-amp integrator is that the capacitor’s charging current is maintained, wherein a straight line voltage is produced rather than an exponential voltage. 2.5 Schmitt Trigger comparator A Schmitt Trigger is a comparator which has a hysteresis where the input voltage is large enough to drive the device into its saturated states. When this input voltage reaches a certain threshold value or trigger point, the device switches to one of its saturated output states. When the input voltage reaches another threshold voltage, the device switches to another saturated output state. The diagram of Schmitt Trigger is shown in the figure below. Figure 14 The Schmitt Trigger op-amp OTA stands for operational transconductance amplifier which is a voltage-to-current amplifier whose output current equals the gain times the input voltage. 3. Feedback System in Electronic Circuit 3.1 Voltage feedback Voltage feedback is returning a portion of a circuit’s voltage output back to the input in order to provide change to the output. Voltage is one of the solutions to amplifier problems. An operational amplifier can be applied with a feedback circuit – the source input is connected through the negative by way of a resistor. This is demonstrated in the figure below. Figure 15 Voltage feedback in amplifiers SOURCE: Voltage and current feedback in amplifiers and other circuits (2006) A negative feedback can be used to stabilize the gain and increase frequency response in an op-amp. A negative feedback takes a part of the output and returns it to the input, in a rather out-of-phase situation, which creates a reduction in gain. This closed-loop gain is much less than the open-loop gain. 3.2 Series current The current feedback amplifier uses two transors; the first one works as a voltage amplifier while the other one acts as a voltage follower. This provides a low frequency gain of Ao = 2 . 103. The feedback corresponds with that of the voltage feedback amplifier. This is demonstrated in the figure below. Figure 16 Series current SOURCE: Voltage and current feedback in amplifiers and other circuits (2006) 3.3 The closed loop system Operational amplifiers are particularly applied in a closed-loop configuration with negative feedback in order to achieve perceive control of the gain and bandwidth. Negative feedback affects the gain and also the amplifier’s bandwidth. The closed-loop critical frequency of an op-amp is demonstrated in the equation: fc(cl) = fc(ol)(1 + BAol(Mid)) The equation states that the closed-loop critical frequency, fc(cl), is higher than the open-loop critical frequency fc(ol) by means of the factor 1 + BAol(Mid). Since fc(cl) equals the bandwidth for the closed-loop amplifier, the closed-loop bandwidth (BWcl) is also increased by the same factor, which is stated in the equation: BWcl = BWol(1 + BAol(mid)) The closed-loop voltage gain is the voltage gain of an op-amp with external feedback. The amplifier network is composed of the op-amp and an external negative feedback circuit that connects the output to the inverting input. The closed-loop voltage gain is determined by the external component values and can be precisely controlled by them. (Floyd 2007, p. 603) 3.4 Gain control of a system with feedback Gain control means regulating the voltage gain of a system and controlling the feedback. Controlling the gain of a system sometimes takes simple or complicated process or circuit, depending on the kind of system that needs control. Inputs and outputs have to be identified so that an adequate remedy can be applied. An example of a control system is the open loop system wherein the output depends upon the input. How to control this system depends upon the changes in the output. This is illustrated in the diagram below. Figure 17 Control system 3.5 Feedback in single and multi-stage circuits Single stage and feedback circuits are used in series circuits to provide wideband voltage or in amplifiers with two stages. This is illustrated in the figure below. Figure 18 Example of a feedback circuit Amplifiers with several stages are called transimpedance and transresistance. In the figure above, the advantage is pronounced because of the minimal interaction between the circuits and the role of resistor ratios on large values of loop gain T. (Toumazou, Moschytz, & Gilbert 2002, p. 246) 4. Sine Wave Oscillators 4.1 The Wien-bridge oscillator The Wien-bridge oscillator is a sinusoidal feedback oscillator which has a lead-lag circuit. This causes oscillation without having an input voltage. This is clearly explained in the following diagram. Figure 19 The Wien-bridge oscillator schematic Resistor R1 and capacitor C1 provide the circuit for the lag portion. R2 and C2 comprise the lead portion. During this oscillation, the lead circuit is dominant at lower frequencies because of the reactance provided by C2. An opposite reaction is formed by XC2 when the frequency increases which allows the output voltage to increase. The lag circuit takes over as soon as a certain frequency is reached, the same as when XC1 decreases, the output voltage also decreases. Figure 20 The lead lag circuit and the resonant frequency relation The figure above shows how the circuit reacts to the lead-lag circuit which shows that the output voltage peaks at a certain frequency called the resonant frequency, fr. The attenuation (Vout/Vin) is 1/3 if R1 = R2 and XC1 = XC2. An explanation for this kind of op-amp oscillator is illustrated in the following diagram: Figure 21The Wien-bridge op amp circuit SOURCE: ‘Wien’s bridge oscillator using op-amp’, by Nepal & Dramzob (n.d.) The diagram shows that there are two negative feedback resistors, R1 and R2, which amplify the signal. 4.2 Twin-T oscillator The Twin-T oscillator is an RC feedback oscillator which is called such because of the two T-type RC filters used in the feedback loop. Low-pass and high-pass responses characterize the twin-T filters. But when they are combined, the two produce a band-stop or notch response with a center frequency to the desired frequency oscillation. The negative feedback through the filters prohibits oscillation when this occurs above or below fr. This fr has very little feedback allowing the positive feedback to cause oscillation through the voltage divider R1 and R2. 4.3 The Colpitts oscillator The Colpitts oscillator is a type of resonant circuit feedback oscillator which uses an LC circuit in the feedback loop. It provides the necessary phase shift and acts as a resonant filter that allows only the needed frequency of oscillation. The widely-used version of the Colpitts oscillator is illustrated in the figure below. Figure 22 The Colpitts oscillator basic diagram This is almost similar with the Hartley oscillator but in this type the connection is being done at capacitor junctions rather than at inductor junctions which results into a phase-inverting configuration. The Colpitts is different from the Type-1 Hartley because there is no electromagnetic induction. But the Colpitts operation may be similar with that of Type-3 Hartley wherein oscillation is done by the LC network along with its phase-inverting qualities. The Colpitts circuit creates a higher frequency than the one created by the tank circuit. With this situation, the phase inversion happens between the output and input portions of the device. The two capacitors compose the circuit of the voltage divider and their ratio triggers the oscillation. Another type of tuning technique involves the application of two capacitors along with a variable capacitor across the entire coil. This is particularly needed when the frequency is just moderate. But the most important characteristic of the Copitts is the so-called good wave purity. This is caused by C1 and C2’s low-impedance paths which allow the harmonics. The Colpitts also works at high frequencies, reaching the microwave frequencies. (Gottlieb 2004, p. 152) The approximate frequency of oscillation is the resonant frequency of the LC circuit and is established by the values of C1, C2, and L according to the formula: fr = 4.4 Quartz crystal oscillators Quartz is a crystalline substance found in nature which creates a piezoelectric effect. Crystals used in electronics consist of a quartz wafer mounted between two electrodes and enclosed in a protective “can”. The crystal creates a series-parallel RLC circuit, and operates in either series resonance or parallel resonance. The impedance of the crystal is minimum at the series resonant frequency, which then provides maximum feedback. The crystal tuning capacitor is used to “fine tune” the oscillator frequency by “pulling” the resonant frequency of the crystal slightly up or down. Piezoelectric crystals can oscillate in two modes – fundamental or overtone. The fundamental frequency of a crystal is the lowest frequency at which it is naturally resonant as this depends on the mechanical dimensions, type of cut, and other factors, is inversely proportional to the thickness of the crystal slab. This provides limitation on the fundamental frequency since a slab of crystal cannot be cut too thin without breaking it. For most crystals, the upper limit is 20MHz. The crystal needs to be operated in the overtone mode in order to create higher frequencies. Overtones are those approximate integer multiples of the fundamental frequency. They are sometimes odd multiples of the fundamental. (Floyd 2007, p. 818) Part 2 Design A) Design a power supply with the following specifications: Vin = 240 Vrms Output voltages: ±5 V DC, ±12 V DC Power = 200 Watts Ripple voltage: Read More