Electronic Devices and Circuit Prototyping - Lab Report Example

Lab Report on Electronic Devices and Circuit Prototyping: MOSFET and BJT INTRODUCTION OBJECTIVES OF THE EXPERIMENT To design, simulate and prototype a variable voltage power supply for a small DC motor. 2. To use this supply to vary the speed of the motor. 3. To measure the speed of the motor with an opto-detector. 4. To compare two approaches a. Variable voltage regulator b. MOSFET switched supply THEORY The Bipolar junction transistor (BJT) and Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) are transistors used for amplification and switching. Bipolar junction transistor (BJT) The Bipolar junction transistor (BJT) is a semiconductor device and essentially consists of a pair of PN-Junction diodes that are joined back-to-back. BJTs replaced the vacuum tubes that were initially used. [2] They are named bipolar transistors because they involve two charge carriers holes and electrons for their operation. In an n-type semiconductor, electrons represent majority charge carriers while for p-type semiconductors, the holes represent the majority charge carriers. Normally, the flow of current in a BJT is due to diffusion of charge carriers between two different regions with varied charge concentrations. [3] A BJT is a current controlled device and has three terminals i.e. base, collector and emitter. The base determines the current in the emitter and the collector output. Actually BJT is a piece of silicon with three regions that have two junctions namely n and p. [3] An NPN transistor and the PNP transistor are examples of the two types of BJTs. The charge carriers in these two types of BJTs differ i.e. a PNP has electrons as its primary carrier, while NPN has holes as their primary carriers. NPN and PNP transistors practically have identical operation principles with the only difference being in biasing and in the polarity of the power supply for each type. [2] Figure 1: NPN and PNP transistors Typically, BJT has four distinct regions of operations; these are the forward active, reverse active, saturation and cutoff. Therefore, a BJT can operate in different modes depending on the junction bias. For instance, when base-emitter junction is forward biased and the base-collector junction is reversed biased, then the device is in the forward active region mode of operation. [3] The device is in reverse active region of operation when the base-collector junction is forward biased while base-emitter junction is reversed biased. The saturation mode occurs when there are forward bias potentials in both base-emitter and base-collector junctions. However, when both junctions are reverse biased then the device is in cutoff region of operation. [2] Since a BJT is three terminal device, it can be connected in three possible ways with one terminal being common for both input and output. These three configurations include common base, common collector and common emitter configurations. The common base configuration has high voltage gain with no current gain while the common emitter has gain for both current and voltage. The common emitter configuration has a current gain with no voltage gain. [3] Figure 2: The Common Base Transistor Circuit Figure 3: Common Emitter Amplifier Circuit Figure 4:  Common Collector Transistor Circuit Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) This is a voltage-controlled device and has four-terminals i.e. source, gate, drain and body. However, the body is always connected to the source terminal hence making it a three-terminal device. The gate is normally insulated by a thin metal oxide layer from the bulk semiconductor body. The main part of the device is the MOS capacity present. Conduction band and the valence band are positioned relative to the Fermi level at the surface. There is an inversion layer between the source and the drain due to carrier flow. [1] There are two basic types of MOSFET i.e. the n-channel and the p channel. In an n-channel, the flow of current is due to the movement of electrons in the inversion layer and in p-channel is due to the movement of holes. [4] Unlike the BJTs, MOSFETs do not have a base current. However, the voltage produces a field on the gate which allows a flow of current between the source and the drain. This flow of current may be pinched-off by the voltage on the gate. The p-type layer occurring between the source and drain terminals can be converted into an n type through the application of either negative or positive voltages at the gates respectively. [4] Application of a positive charge on the holes in the oxide layer leads to a repulsive force and then they push downwards with the substrate. The same positive voltage will attract the electrons in the source and drain regions in to the channel. An electron channel will be formed. Now, current will flow freely between the source and the drain when voltage is applied. [1] Figure 5: Basic MOSFET Structure and Symbol In a typical MOSFET transistor, application of voltage on an oxide-insulated gate electrode generates a channel used for conduction with the drain and source. Lastly, MOSFETs exhibit and efficient way of handling power hence are the most used among transistors in both digital and analog electronics replacing even the bipolar junction transistors. [4] Main report Circuit design Figure 6: Circuit Design: Variable power supply circuit of the BJT In a group of three, we designed the variable power supply circuit of the BJT and we made the circuit layout as it is shown in Figure 6 above. We then calculated the required voltages and found them to be ranging between 5 and 10 V because the motor can be driven by voltages up to 12 V and currents using the load requirements of 50 mA. Also the known output from the low power AC supply provided a maximum off-load output of 11.2 VRMS. In addition, these results are given in the manufacturer’s data sheets on Study Direct. For the variable power supply, we designed a full-wave and a capacitor smoothed rectifier to provide an
average DC voltage for the regulator circuit. For the calculation, the VRMS is equal 11.12V from the AC input so the VM is equal to; VM = (2 * VRMS) which is (2 * 11.2) = 15.839 V …………………………………….. (1) Where; VM = is the maximum AC input voltage VRMS = the maximum RMS off-load output VP is equal the VM – 2 * VBD and the VBD is equal 1.1 V; Where VBD is the voltage at break-down i.e. the threshold voltage So the VP = 15.839 – (2 * 1.1) = 13.639 V. ……………………………………………….. (2) The capacitor used for the circuit is equal = 1000mF where F is the frequency equal 100HZ. Therefore, the VPP = = = 0.5 V ……………………………………………. (3) Where VPP is the peak-to-peak voltage and VP is the peak voltage for the design. Therefore the VDC = VP – 0.5 * VPP …………………………………………………….. (4) VDC = 13.639 – 0.5 * 0.5 = 13.389 V.………………………………………………….. (4.1) Where VDC is the high voltage direct current i.e. voltage responsible for the current moving in one direction. For the resistors, the voltage through them is 10 V for the V maximum and 5 V for the V minimum. We also we have 2.7 A for the I2 and 10 mA for the Iz and 10mA for the β The RT is equal = = 500 Ohm……………………………………………..... (5) Where VLmin refers to the minimum very low voltage for the design The R1 is equal = 170 Ohm…………………………………………………. (5.1) Therefore VP in this case is equal = = 170 ohm……… (6) So R2 = RT- R1 - RP = 500 – 170 – 170 = 160 Ohm……………………………………… (6.1) RZ = = 1068.9 Ohm and R2 = = = 1018 Ohm………. (6.2) The simulated values, at 100% which R1 is170 Ohm, IZ =14.763 mA, IQ=7.613mA and I1 = 10.421 mA. At 0% which R1 is 0 ohm IZ = 14.738mA, IQ= 2.21 mA and the I1 is equal to 21.121 mA. In the above circuit, a simplest case of a common-collector BJT was used with a connection of the base directly to the reference voltage. A simple transistor regulator was used to provide a relatively constant output voltage with changes in the voltage of the power source and also for changes on the load. This provides an input voltage that exceeds the output voltage by an enough margin in a way that the capacity of the transistor was not exceeded. [9] It was found out that the voltage across the Zener diode minus the voltage at the base-emitter of the BJT is equal to the output voltage of the stabilizer. Normally, the theoretical value of this voltage for a typical transistor is 0.7 V depending on the current of the load but for our case, the experimental value was found to be 0.5 V. [6] In a case where the voltage at the output drops due to external reasons i.e. an increase in the current drawn by the load which in turn causes a decrease in the Emitter-Collector junction voltage, there will be an increase in the transistor base-emitter voltage hence turning on the transistor further with more current delivered leading to an increase in the voltage gain. [4] From the diagram above (Figure 6), resistor R6 provides a bias current for both the BJT and the Zener diode. When the current in the load is maximum, current through the Zener diode is minimum. [8] Therefore, as designers of the circuit we had to choose a minimum voltage that could be tolerated across the resistors. The higher this chosen voltage is, the input voltage required will be higher i.e. >10 V leading to a lower frequency of the regulator. In addition to that, if we had to choose lower values for the resistors, a higher power dissipation in the diode would have resulted hence inferior characteristics. Circuit simulation In simulation circuits, the technique that is widely used for understanding the operation of such a circuit is the sweep of the source of input voltage and making observations on the response of the output voltage. [7] This was done using a DC voltage sweep analysis in Multisim. This resulted in voltage transfer curves (VTC) and these are useful in the analysis of many analog and digital circuits. Figure 7: Circuit simulation of the BJT In accordance to the design of the circuit, we designed the simulation circuit of the BJT circuit using Mutisim and this helped us to start designing the circuit. During the simulation, we used a variable power supply as the input voltage and also used a Voltmeter of the DMM to experimentally measure the value at the output of the BJT circuit. The above circuit which is a variable voltage regulator can also be used as an amplifier if a bias point is set to a certain region so that the output voltage changes as fast as the input voltage. [6] In many switching operations, the BJT circuits are operated in the saturation region during the conduction of current. [2] In this saturation region, voltage drops across the collector-emitter terminals of the BJT are very small as desired. The amount of current in the load is therefore determined by the value of VCC and also the characteristics of the load which is are independent of the characteristics of the BJT. [8] For the curves obtained (illustrated in the results part below), the forward active region is nearly a flat part with the saturation region being depicted to the left of the knee. Therefore, it can be concluded that for switching applications, the BJT functions like a closed switch when it is in the saturation region and the VCE is very small. However, when is in cut off, with IC = 0, it functions as an open switch. [9] Choice of components for the experiment A few of the components we used were not available in the Multism database, so we used our judgment to find near equivalents to substitute the initially equipment we intended to use but were not available in the Multism database. Therefore, the final list of components included: Capacitors These are electrical components that are used to store energy electrostatically in an electric field. For the design, we used EC9A1EHG-102 model, which is manufactured by Panasonic as it is as the same value of the actual result. The value of its capacitance was 1000µF. [2] Potentiometer This is a three-terminal resistor that is used to form a voltage divide that is adjustable. The potentiometer was important in this design when designing a variable voltage supply. We used the M63M501 model, which is manufactured by Vishay, as it is also the same value of the actual result. The value of its resistance was 500 Ohms. [2] Zener diode This is a type of a diode that allows current to flow in both directions as opposed to a typical diode that only allows current flow in one direction. For this design, we used BZX79-C2V7 model that is manufactured by Fairchild Semiconductors and it had the same voltage value as that of the actual result. These chosen diode has a minimum working Zener Voltage of 2.5 V and a maximum working voltage of 2.9V. It also had a forward current of 0.25A, a maximum reverse leakage current of 0.020mA and a total power dissipation of 500mW. [7] Transistors These are semiconductor devices which are mostly used in amplifying and switching of electrical signals. For the upper transistor, we chose the BCX38C while for the lower transistor we chose BC182L; both Darlington Models. Their operations specifications include a Av current of 800mA, a maximum collector current of 800mA and collector emitter voltages of 1.25V. [7] Bridge rectifier This is an arrangement of four or more diodes in a circuit configuration that forms a bridge circuit that has similar polarity of output for any input polarity. In this case, we used DF005M bridge rectifier for the circuit. The specifications of this rectifier included a peak voltage of 50V, a maximum RMS reverse voltage of 35V and a maximum surge current of 50A. The bridge rectifier also had a voltage drop (forward) of 1.1 V and a maximum power dissipation of 3.1 W. [2] Resistors Resistors are simply two-terminal electrical component that is used to implement electrical resistance as a component of any circuit. In our design, for R1, we added 680 Ohm and 56 Ohm to get 736 Ohm while for R2, we used 1K Ohm. For R3 we combined 100 Ohm, 47 Ohm and 22 Ohm to get 170 Ohm while for R4, which is the load so we added two resistors and they are both 100 ohm. So we could choose a100 ohm or 200 ohm resistor. Finally, for R5, we combined 100 ohm, 27 ohm and 23 ohm to get 160 Ohm resistor. [2] Prototype construction and testing results 1. Results for BJT Testing Table 1: Comparison between Initial values, after rectification and the load values Initial Values 11.16 VRMS 31.2 VRipple After Rectification 13.4 VRMS 673 mVRipple Load Values 4.99 -11.98 V 50 mA Table 2: Motor Values Voltage (V) Time between pulses (MS) Frequency of pulses (Hz) Speed of rotation (rad/s) 5.00 10.79 92.68 291.16 5.50 9.91 100.91 317.01 6.00 9.25 108.11 339.63 6.50 8.41 118.91 373.55 7.00 7.87 127.06 399.19 7.50 7.47 133.87 420.56 8.00 7.06 141.64 444.98 8.50 6.56 152.44 478.90 9.00 6.23 160.51 504.27 9.50 5.92 168.92 530.67 10.00 5.64 177.30 557.02 Figure 8: Graph of speed of rotation Vs Voltage applied to motor 2. Results for MOSFET Testing Voltage(V) Time between pulses (Ms) frequency of pulses (Hz) speed of rotation (rad/s) 4.8 16.15 61.92 389.06 6.01 14.83 67.43 423.68 7.2 12.89 77.58 487.44 8.4 12.12 82.51 518.42 9.6 11.49 87.03 546.84 10.83 11.01 90.83 570.7 11.97 10.1 99 622.04 Figure 9: Motor rotation speed (MOSFET) Comparison of the two designs and evaluation of the results BJT Circuit Figure 10 MOSFET circuit Figure 11 From the testing results and the circuit above for the MOSFET, it is clear that the circuit applies a buck converter configuration to step up the voltage from 4.8 V to 11.97 V. For the BJT, after rectification by the bridge rectifier, there is a decrease in the VRipple from 31.2V to 673mV. From the graph of Graph of speed of rotation Vs Voltage applied to motor, it can be seen that the speed of rotation increases with an increase the Voltage applied to motor. Also for the BJT testing, the time between the pulses decreases with an increase in the voltage applied. The frequency also increases with an increase in the applied voltage. For the MOSFET, an increase in the voltage applied leads to an increase in the voltage applied. Consequently, from the graph of speed of rotation Vs Voltage applied to motor in the case of the MOSFET, it can be seen that at zero voltage, the speed of rotation cannot be zero. In addition to that, the speed of rotation increases with an increase the Voltage applied to motor. This can be attributed to the timer IC located on the board that connects the transistor to then voltage regulator circuit. From the diagram above (Figure 9), resistor R6 provides a bias current for both the BJT and the Zener diode. When the current in the load is maximum, current through the Zener diode is minimum. Conclusion MOSFETs do not have a base current. However, the voltage across its terminals produces a field on the gate which allows a flow of current between the source and the drain. [1] This flow of current may be pinched-off by the voltage on the gate. [9] On the other hand, a BJT has four distinct regions of operations; these are the forward active, reverse active, saturation and cutoff. [5] Therefore, a BJT can operate in different modes depending on the junction bias. For instance, when base-emitter junction is forward biased and the base-collector junction is reversed biased, then the device is in the forward active region mode of operation. [6] It was found that a simple transistor regulator can be used to provide a relatively constant output voltage with changes in the voltage of the power source and also for changes on the load. [2] This provides an input voltage that exceeds the output voltage by an enough margin in a way that the capacity of the transistor was not exceeded. Therefore, it can be concluded that for switching applications, the BJT functions like a closed switch when it is in the saturation region and the VCE is very small. However, when is in cut off, with IC = 0, it functions as an open switch. [7] It can be concluded that a power MOSFET is used as a switching regulator for a couple of reasons. Firstly, this is possible since it is unipolar in nature hence can allow switching at very high speeds. [6] However, the only limitation on the speeds of both the BJT and MOSFET is their characteristic internal capacitances. [8] The capacitance of these transistors must either be discharged or charged as the transistor switches. However, such a process can sometimes be slow since the current is limited at times by the external circuit when it goes through the gate capacitances. [9] Bibliography [1]. Cheng, Yuhua, and Chenming Hu. MOSFET Modeling & Bsim3 Users Guide. Boston: Kluwer Academic Publishers, 1999. Print. [2]. Comer, D J., and D. Comer. Fundamentals of electronic circuit design. New York, NY: John Wiley & Sons, 2003. Print. [3]. Crawford, Robert H. MOSFET in Circuit Design: Metal-Oxide-Semiconductor Field-Effect Transistors for Discrete and Integrated-Circuit Technology. New York: McGraw-Hill, 1967. Print. [4]. Graeme, Jerald G. Photodiode Amplifiers: Op Amp Solutions. Boston, Mass: McGraw Hill, 1996. Print. [5]. Heising, Mark. Extracting BJT Small-Signal Parameters from S-Parameter Measurements: Research Report. 1983. Print. [6]. Lee, Dongho. BJT Op Amp and MOSFET Op Amp Performance and Characteristic Analysis. New York: Wiley Interscience, 1997. Print. [7]. Neamen, D A. Electronic circuit analysis and design. Boston: McGraw-Hill, 2001. Print. [8]. Taylor, B E. Power MOSFET design. Chichester, W. Sussex: Wiley, 1993. Print. [9]. Warner, R M, and B L. Grung. MOSFET Theory and Design. New York: Oxford University Press, 1999. Print. Read More