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Amplifier En.Pdf Bipolar Junction Transistor Amplifiers Semiconductor Elements 1 © 2010,EE141 Доц.д-р. T.Василева What is an Amplifier? An amplifier is a circuit that can increase the peak-to-peak voltage, current, or power of a signal. It allows a small signal to control a much larger, high-powered one. Definitions of voltage, current and power gain coefficients are also given in figure. Lowercase italic letters indicate ac voltage and alternating currents. 2 © 2010,EE141 Доц.д-р. T.Василева 1 Amplifier Configurations There are three configurations of a BJT amplifier circuit: common- emitter (CE), common-collector (CC) and common-base (CB). The configuration is named for the electrode that is common for input and output networks. The CE is the most widely used for amplifiers because it has the best combination of current gain and voltage gain. In CE the input and output voltage are 180° out of phase, called an inversion. 3 © 2010,EE141 Доц.д-р. T.Василева Transistor Biasing Circuit with fixed base current For the transistor to operate properly as an amplifier, the base-emitter junction should be forward-biased and the base-collector junction – reverse-biased. This is called forward-reverse bias. The three dc voltages for the biased transistor are the emitter voltage UE, the collector voltage UC and the base voltage UB. These voltages are measured with respect to ground. 4 © 2010,EE141 Доц.д-р. T.Василева 2 Voltage-Divider Biasing (VDB) Voltage divider Circuit with fixed base voltage. Iдел >> IB The voltage-divider bias (VDB) configuration uses only a single dc source to provide forward-reverse bias to the transistor. Resistors R1 and R2 form a voltage divider that provides the base bias voltage UB. Resistor RE allows the emitter to rise above the ground potential. 5 © 2010,EE141 Доц.д-р. T.Василева Examples – Biasing UB, IB, IC, UCE = ? IC , UCE = ? IC, UCE = ? 6 © 2010,EE141 Доц.д-р. T.Василева 3 DC Load Line EC IC (RC RE ) UCE Load line equation A straight line drawn on the collector curves between the cutoff and saturation point of a transistor is called the load line. Notice that the load line is only determined by the resistors RC, RE and UCC and not by the transistor itself. 7 © 2010,EE141 Доц.д-р. T.Василева Load Line – Slope and Position The slope and position of the load line depends only on collector circuit resistance RC and supply voltage UCC and not on the transistor itself. When UCC varies with constant RC value the load line moves parallel to itself. Changing the collector resistor with the same collector supply voltage produces load lines of different slopes but the same cutoff value. 8 © 2010,EE141 Доц.д-р. T.Василева 4 Operating Point The base current IB is established by the base bias. The intersection point between the collector current curve (at this IB) and the dc load line is called the quiescent or Q-point or operating point. Coordinates of the Q-point are the values IBQ, ICQ, UCEQ. The load line contains every possible operation point for the circuit. 9 © 2010,EE141 Доц.д-р. T.Василева AC Operation ac signal voltage u Instant value u = U + u dc base voltage UB = const b B B b Coupling capacitor A coupling capacitor allows an ac signal to be coupled into an amplifier without disturbing its Q point. The capacitor acts as an open to dc and as a short to ac. 10 © 2010,EE141 Доц.д-р. T.Василева 5 Signal Operation on the Load Line The input signal varies the base current above and below its dc value. This causes much larger variation in the collector current because of the transistor current gain. The variation in collector current produces a corresponding variation in the voltage across RC. As a result, the collector- emitter voltage UCE also changes. 11 © 2010,EE141 Доц.д-р. T.Василева Waveforms & DC Levels Input and output signal waveforms An input signal with an amplitude of 50 mV is applied to the CE amplifier. The output signal is with amplitude of 500 mV. The amplifier voltage gain Au = 10. 12 © 2010,EE141 Доц.д-р. T.Василева 6 Two Load Lines – DC & AC Every amplifier has a dc equivalent circuit and an ac equivalent circuit. Because of this, it has two load lines: a dc load line and an ac load line. The ac collector resistance rC is less than the dc collector resistance RC. For this reason the ac load line has a higher slope than the dc load line. The peak-to-peak sinusoidal current and voltage are determined by the ac load line. 13 © 2010,EE141 Доц.д-р. T.Василева Clipping Large Signals An amplifier is a class A amplifier if it is biased such that it always operates in the linear region where the shape of the output signal is an amplified copy of the input signal. If the Q point is not centered, the output signal is limited. If the Q-point is moved higher or lower, a large signal will drive the transistor into saturation or cutoff. Thus saturation or cutoff clipping occurs. Both types of clipping are undesirable because they distort the signal. 14 © 2010,EE141 Доц.д-р. T.Василева 7 CE, CB and CC Amplifier Circuits R RT RT T Amplifier CЕ Amplifier CB Amplifier CC AI – high AI < 1 AI - high AU - high AU < 1 AU - high 15 © 2010,EE141 Доц.д-р. T.Василева 8.
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