chapter2 - Chapter 2 Stability Considerations 1 The...

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Chapter 2 Stability Considerations 1
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The Importance of The Stability of an Amplifier When an amplifier is unstable, it is out of order at all. It’s no longer an amplifier; it’s an oscillator out of control. An unstable amplifier can make the whole RF or microwave system collapse. Most of microwave transistors are potentially unstable. It’s important to choose suitable source impedance and load impedance to attain stable amplifiers. 2
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Definition of Stability (1) An amplifier maintains effectiveness in its normal operating characteristics in spite of large changes in the environment. `` Unstable balance Stable balance 3
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Definition of Stability (2) When there is no input signal, a stable amplifier has no output signals except white noise. If signals can be detected at the output port of an amplifier without input signal, the amplifier is oscillating. An oscillating amplifier suffers from degradation in gain, noise figure, efficiency, and linearity. 4
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Definition of Stability (3) The reflection coefficients ( IN Γ and OUT Γ ) of the input and output ports of a stable amplifier are less than unity. z IMN-Input Matching Network z OMN-Output Matching Network Figure 1 A single stage amplifier circuit 5
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Definition of Stability (4) The real part of the input and output impedances are greater than zero. A port with a negative impedance means it creates power or signal. It’s unstable. A port with a negative impedance leads to reflect coefficient greater than 1. It’s unstable. 6
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Conditional Stability .vs. Unconditional Stability Our study of amplifier stability will focus on the stability of amplifier in presence of different passive source or load impedance . A network is said to be unconditionally stable in a frequency range if, and only if it’s stable at any passive source or load impedance. A network is said to be conditionally stable in a frequency range if it’s stable only for a limited range of values of passive source or load impedance 7
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Criteria of Stability (1) Please have a look at the general two-port network. Figure 2 A general two port network Z S Z L 8
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Criteria of Stability (2) The criteria of the stability of an amplifier can be: 0 ) Re( > IN Z or (1) 0 ) Re( > OUT Z They can be expressed equivalently as: 1 < Γ IN or 1 < Γ OUT (2) 9
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Reflection Coefficients of a 2-port Network According to the S-parameter calculation, the reflection coefficients of a general 2-port network in figure 2 are given by: L L IN S S S S Γ Γ + = Γ 22 21 12 11 1 (3) S S OUT S S S S Γ Γ + = Γ 11 21 12 22 1 (4) 10
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Definition of S-Parameters V 2 + , I 2 + V 2 - , I 2 - V 1 + , I 1 + V 1 - , I 1 - 22 21 12 11 S S S S 0 1 1 11 2 = + + = V V V S , 0 2 1 12 1 = + + = V V V S 0 1 2 21 2 = + + = V V V S , 0 2 2 22 1 = + + = V V V S 11
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Special Case: Unilateral 2-Port Network When , we call the 2-port network unilateral.
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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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chapter2 - Chapter 2 Stability Considerations 1 The...

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