Chapter 3 - Chapter 3 Gain Considerations 1 The Importance...

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Chapter 3 Gain Considerations 1
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The Importance of the Amplifier Gain The purpose of an amplifier is to amplify the signal. The gain is the measure of the amplification of an amplifier; High gain with low noise helps improve the receiving sensitivity of RF and microwave receivers. High gain with large saturate output power helps improve the communications range of a transmitter; The gain suppression with the increasing of input signal causes nonlinearity. 2
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Definition of Gain (I) The gain in RF and microwave amplifiers is generally defined in term of power ration. So we will define the various power levels in an amplifier first. Figure 1 depicts the power in an amplifier. P AVN P AVS Figure 1. A transistor amplifier 3
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Definition of Gain (II) 1. P IN , power input to the transistor or the input-matching network. 2. P L , power delivered to the load or the output matching network. 3. P AVS , power available from the source under matched condition, or when Г IN = Г * S, P IN = P AVS . 4. P AVN , power available from the transistor under matched condition, when Г L = Г * OUT, P L = P AVN . 4
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Definitions of Gain (III) Transducer power gain G T : AVS L T P P G = (1) Operating power gain or power gain G P : IN L P P P G = (2) Available power gain G A , AVS AVN A P P G = (3) Different definitions of gain are used for convenient of analyzing different characteristics of amplifiers. 5
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The Transducer Power Gain G T (I) According to the definitions of gain from equations (1), with the help of S- parameter calculations, we have the transducer gain G T as: 2 22 2 2 21 2 2 1 1 1 1 L L S IN S T S S G Γ Γ Γ Γ Γ = (4) If we denote G S , G 0 , and G L as: 2 2 1 1 S IN S S G Γ Γ Γ = (5) 2 21 0 S G = (6) 2 22 2 1 1 L L L S G Γ Γ = (7) Equations (4) can be written as: L S T G G G G 0 = (8) 6
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The Transducer Power Gain G T (II) From equation (8), we can attribute G 0 to the gain of the transistor, while G S and G L are attributable to effective gains of the input and output matching networks. G T can also be written in terms of Г OUT (rather than Г IN ) as: 2 2 2 21 2 11 2 1 1 1 1 L OUT L S S T S S G Γ Γ Γ Γ Γ = (9) 7
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The Transducer Power Gain G T (III) In equations (4) and (9), Г IN and OUT are: L L IN S S S S Γ Γ + = Γ 22 21 12 11 1 (10) S S OUT S S S S Γ Γ + = Γ 11 21 12 22 1 (11) If the transistor is unilateral, means S 12 =0 , we have IN = S 11 OUT = S 22 . So that IN and OUT in (4) and (9) can be replaced by S 11 and S 22, respectively. 8
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Operating Power Gain G P and Available Power Gain G A Again, according to the definitions of gain from equations (2) and (3), with the help of S-parameter calculations, we have the operating power gain G P and the available power gain G A as : 2 22 2 2 21 2 1 1 1 1 L L IN P S S G Γ Γ Γ = (12) 2 2 21 2 11 2 1 1 1 1 OUT S S A S S G Γ Γ Γ = (13) 9
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The Source Mismatch Factor M S The source-mismatching factor M
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Chapter 3 - Chapter 3 Gain Considerations 1 The Importance...

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