The Power Gain of an Amplifier - Coursework Example

Amplifiers By Presented to September 7, Amplifiers An amplifier is a device which increases a signal power, it takes the input signal and controls the output to match the input signal but with larger amplitude. That is, the amplifier modulates the output of the power supply. Amplifiers are specified according to the properties of outputted and inputted variables, this relation of input and output properties is the gain of the amplifier and it’s given as the ratio of output voltage to the input voltage. The gain of an amplifier is given as:, voltage, current or a combination of voltage, power and current. The gain of an amplifier is usually given in decibels or its unit less if the input and output variables are the same (all current, voltage or power. The power gain of an amplifier depends on the sources and load resistances used and its voltage gain. The amplifier should be linear in most cases hence its gain should be constant. Electronic amplifiers find their applications in Radios, Television Transmitters, Receivers, and high-fidelity stereo equipments, Microcomputers, and other electronic digital equipments. METHODOLOGY AND RESULTS ANALYSIS PART A: STATIC CHARACTERISTICs OF BJTS Resistances of RC and RB were measured using an ohmmeter and the circuit below was constructed The 1M and 10k potentiometer/decade boxes were used to set VCE and VBE to the values shown in table 1 to give the input characteristics of the BJT. The voltages across RB were measured and recorded for all measurements of VCE and VBE. The 10k potentiometer/decade box was s3et to its maximum value so that the output characteristics could be determined. The 1M potentiometer/decade box was adjusted to set Ib to 10A, and the desired values of RB were calculated using the measured values of RB. the value of 10k potentiometer/decade box was adjusted for all values of VCE in table 2 of the results analysis while Ib was kept constant. The voltage across the 100 resistor was measured and recorded for each combination of VCE and IB. In common emitter configuration, the input is connected between the base and the emitter while the output is taken between collector and emitter. The emitter is the common terminal as shown in the above figure. Emitter base voltage is the input voltage, base current flows in the circuit and then the in the output resister, hence there will be high power and voltage gains and its output and input voltage ratio is moderate hence this configuration is commonly used. Table 1 Table 2 IB = 10A IB = 20A IB = 30A IB = 40A IB = 50A VCE VRC mV IC=VRC/RC VRC mV IC=VRC/RC VRC mV IC=VRC/RC VRC mV IC=VRC/RC VRC mV IC=VRC/RC 0.2V 110.369 1.103 221.698 2.217 335.551 3.356 449.509 4.496 560.20 5.602 0.4V 112.439 1.125 224.75 2.248 340.177 3.402 455.992 4.559 569.458 5.695 0.8V 112.292 1.123 224.884 2.249 340.37 3.404 456.219 4.562 569.781 5.698 1V 112.326 1.123 225.362 2.254 340.463 3.405 456.302 4.563 569.888 5.699 3V 112.645 1.126 225.777 2.258 341.368 3.414 457.545 4.575 571.507 5.715 5V 112.957 1.13 226.451 2.267 342.310 3.423 458.817 4.589 573.093 5.731 RESULTS ANALYSIS The input characteristic of the transistor was plotted as shown on the graph of IB against VBE below. The slope of the curve is the input resistance of the Bipolar Junction Transistor. INPUT CHARACTERISTIC CURVE OF A BJT AT 3V AND 5V Increasing the supply voltage causes the base current to be lower for a given value of base-emitter voltage; this implies that higher values of supply voltage leads to greater collector-base junction reverse bias, hence greater depletion region penetration into the base, thus the distance between collector-base, and emitter-base is reduced hence more charge carriers will flow from the emitter across the collector base junctions and very few charge carriers will flow out through the load at the base. A GRAPH OF IB AGAINST VBE INPUT RESISTANCE hie= VBE/ IB which the inverse of the gradient of the curve. SLOPE=13.536 Hence the input resistance of the BJT IS given as the reciprocal of 13.536 which is; hie= 0.074  From the input characteristic curve above, the value of base-emitter voltage ranges from 0.6V to 0.7V.The value of base-emitter voltage is 0.6V for small values of base current (50A or below) for higher values of base current, base-emitter voltage is 0.7v.This shows that a small increase in base-emitter current leads to a very large increase in current due to presence of low input resistance of the transistor. The input resistance varies with the value of base current flowing and below a specific value of base-emitter voltage (0.6V) the base current increases. The output characteristic of the BJT was plotted by calculating the value of collector current and plotting on the graph of collector current against collector emitter voltage for various values of base current below. Collector current varies with collector-emitter voltage for the values of collector-emitter voltage between 0V and 1V,and then it becomes constant and independent of VCE, hence the transistor is operated above 1V.In active region, collector current is small for small values of base current but the effect of collector emitter voltage on collector current increases for large values of base current. A very large value of collector-emitter voltage causes the collector-base junction to breakdown and hence the collector current increases rapidly and the transistor is destroyed. During cut-off ,small values of collector current flows even when the base current is kept at 0V.This transistor configuration is suitable for coupling between various transistor stages due to its moderate output to input impedance ratio. OUTPUT CHARACTERISTIC CURVE OF COMMON EMITTER BJT . A GRAPHN OF IC AGAINST VCE After the initial bend, the curves approximate a straight line. The gradient of each line approximates the output impedance of the BJT for a particular value of base current. The slope of the curve gives the output conductance of the BJT. The slope=1000 hence output conductance is 1000ohms.This shows that the output resistance of the transistor is high, hence from the output characteristic curves of the transistor, it is clear that for a common emitter configuration, the output resistance is high. The near horizon parts of the graph lines show that a change in the collector emitter voltage has almost no effect on the collector current in this region. This output to be expected if the transistor had a large value of resistor in series with it. The transfer characteristic curve was plotted on the graph of collector current against base current as shown below, the slope of the graph shows the current gain of the transistor. Hence from the transfer characteristic curve below, the slope is given as: Slope=0.1114, hence current gain hfe=0.1114. A TRANSFER CHARACTERISTIC CURVE OF A COMMON EMITTER BJT GRAPH OF IC AGAINST IB CONCLUSION The common emitter configuration transistor shows the following characteristics: Low input resistance, high gain, and high output resistance hence this configuration the best suitable for the design of amplifiers. Read More