What is an Amplifier?

Transcription

1 Bipolar Junction Transistor Amplifiers Semiconductor Elements 1 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 1

2 Amplifier Configurations There are three configurations of a BJT amplifier circuit: commonemitter (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 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 U E, the collector voltage U C and the base voltage U B. These voltages are measured with respect to ground. 4 2

3 Voltage-Divider Biasing (VDB) Voltage divider Circuit with fixed base voltage. I дел >> I B The voltage-divider bias (VDB) configuration uses only a single dc source to provide forward-reverse bias to the transistor. Resistors R 1 and R 2 form a voltage divider that provides the base bias voltage U B. Resistor R E allows the emitter to rise above the ground potential. 5 Examples Biasing U B, I B, I C, U CE =? I C, U CE =? I C, U CE =? 6 3

4 DC Load Line E C I R R ) U C ( Load line equation C E CE 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 R C, R E and U CC and not by the transistor itself. 7 Load Line Slope and Position The slope and position of the load line depends only on collector circuit resistance R C and supply voltage U CC and not on the transistor itself. When U CC varies with constant R C 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 4

5 Operating Point The base current I B is established by the base bias. The intersection point between the collector current curve (at this I B ) and the dc load line is called the quiescent or Q-point or operating point. Coordinates of the Q-point are the values I BQ, I CQ, U CEQ. The load line contains every possible operation point for the circuit. 9 AC Operation dc base voltage U B = const ac signal voltage u b Instant value u B = U B + u 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 5

6 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 R C. As a result, the collectoremitter voltage U CE also changes. 11 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 A u =

7 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 r C is less than the dc collector resistance R C. 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 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 7

8 CE, CB and CC Amplifier Circuits R T R T R T Amplifier CЕ Amplifier CB Amplifier CC A I high A U - high A I < 1 A U - high A I - high A U <