Chapter 5 - Chapter 5 Small Signal Amplifier 1 2 Types of...

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Chapter 5 Small Signal Amplifier 1
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Types of Amplifiers (I) Based on Operation Point Class-A Amplifier: In this operation mode, each transistor in the amplifier operates in the active region of the entire signal circle. Class-B Amplifier: In this operation mode, each transistor is in the active region of approximate half the signal cycle. Class-AB Amplifier: In this operation mode, an amplifier operates in class A for small signals and in class B for large signals. Class-C Amplifier: In this operation mode, each transistor of the amplifier is in the active region for significantly less than half of the signal cycle. The active region of a transistor is shown in Figure 1. The transfer characteristics of a transistor are illustrated in Figure 2. 3
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Types of Amplifiers (II) Based on Operation Point Figure 1. Operation regions of a transistor . Figure 2 . The transfer characteristics of a transistor with turn-on voltage V 0 . I b V b Class A Class B 4
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Types of Amplifiers (III) Based on Signal Level I b t Small-signal analysis is one of the analysis methods for an active circuit, in which it is assumed that the signals deviate from (or fluctuate to either side of) the steady bias levels by such a small amount that only a small part of the operation characteristics of the device are covered and thus the operation is always linear. Large-signal analysis is another analysis method for an active circuit under high amplitude signals that traverse such a large part of the operating characteristics of a device that nonlinear portions of the characteristics are usually encountered, which cause the nonlinear operation of the device. Figure 3. The signal scale in I C -V CE region. 5
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Bias of Transistors (I) Bipolar Transistor Bias C B E 1 2 3 BIP IND L= ID= 100 nH L1 CAP C= ID= 18 pF C1 ID= 100 pF C2 T 12 REST R= Rc Ohm DCVSS ID= 18 pF C3 100 pF C4 T R= Rb Ohm IND L= 100 nH L2 ID= 1000 pF C5 1000 pF C6 PORT Z= P= 50 Ohm 1 50 Ohm 2 Before considering any parameters of an amplifier such as gain, noise figure, bandwidth, etc., a suitable DC bias network must be designed. Figure 4 is a general bias scheme for a bipolar transistor. Figure 4. A general bias scheme for a bipolar transistor. 6
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Bias of Transistors (II) Bipolar Transistor Bias Values of the Inductors: The two inductors are used for preventing RF signal from leakage to the bias network, which are called as the RF chokes. The value of the inductors is chosen that its reactance at the operation frequency is much larger than the system impedance (generally 50 Ohm). For example, the reactance of a 100-nH inductor at 1 GHz is j628 Ohm, which is much larger than 50 Ohm. Values of the Capacitors: The three capacitors at each arm of the bias are also used for isolation between the RF signal and the DC network. The values of the capacitors ranging from 10 pF to 10 nF. The capacitors with small values are more effective to decouple the high frequency signal ( because series resonating frequency the capacitance and the inductance may appear around the operating frequency lead to zero Ohm to ground
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This note was uploaded on 05/01/2011 for the course ELECTRICAL EE5602 taught by Professor Xuequan during the Spring '11 term at City University of Hong Kong.

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Chapter 5 - Chapter 5 Small Signal Amplifier 1 2 Types of...

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