CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.67 CHAPTER 5: SMALL-SIGNAL MIDFREQUENCY FET Table of Contents 5.1. INTRODUCTION 68 5.2. SMALL-SIGNAL EQUIVALENT CIRCUITS FOR THE FET 68 5.3. CS AMPLIFIER ANALYSIS 70 5.4. CD AMPLIFIER ANALYSIS 72 5.5. CG AMPLIFIER ANALYSIS 74 Table of Figures Fig. 5.1 Drain charactersistics 69 Fig. 5.2 Small-signal models for the CS FET 70 Fig. 5.3 CS Amplifier 71 Fig. 5.4 CD Amplifier 73 Fig. 5.5 CG Amplifier 75 CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.68 CHAPTER 5: SMALL-SIGNAL MIDFREQUENCY FET 5.1. INTRODUCTION In this chapter, all voltage and current signals are considered to be in the midfrequency range, where all capacitors appear as short circuits. There are three basic FET amplifier configurations: the common- source (CS), common-drain (CD) or source-follower (SF), and common- gate (CG) configurations. The CS amplifier, which provides good voltage amplification, is most frequently used. The CD and CG amplifiers are applied as buffer amplifiers (with high input impedance and near-unity voltage gain) and high-frequency amplifiers, respectively. 5.2. SMALL-SIGNAL EQUIVALENT CIRCUITS FOR THE FET CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.69 Fig. 5-1 Drain charactersistics From the FET drain characteristics, it is seen that if D i is taken as the dependent variable, then , D GS DS i f v v For small excursions (ac signals) about the Q point, D d i i ; thus, application of the chain rule: 1 d D D m gs ds ds i i di g v v r Where m g and ds r are defined as follows: Transconductance: D D m GS GS Q Q i i g v v Source-drain resistance: 1 D D ds DS DS Q Q i i r v v or DS DS ds D D Q Q v v r i i As long as the JFET is operated in the pinchoff region, 0 G g i i , so that the gate acts as an open circuit. This leads to the current-source equivalent circuit. Either of these models may be used in analyzing an amplifier, but one may be more efficient than the other in a particular circuit. CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.70 Fig. 5-2 Small-signal models for the CS FET 5.3. CS AMPLIFIER ANALYSIS A simple common-source amplifier is shown in Fig. 5-3(a) and its associated small-signal equivalent circuit is displayed in Fig. 5-3(b). Source resistor s R is used to set the Q point but is bypassed by s C for midfrequency operation. CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.71 Fig. 5-3 CS Amplifier Example 5.1 In the CS amplifier, let 3 D R k , 60 , 30 ds r k . (a) Find an expression for the voltage-gain ratio o v i v A v . (b) Evaluate v A using the given typical values. CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.72 Solution (a) By voltage division, D o gs D ds R v v R r Substitution of gs i v v and rearrangement give : o D v i D ds v R A v R r (b) The given values lead to 5.45 v A Where the minus sign indicates a 0 180 phase shift between i v and o v . 5.4. CD AMPLIFIER ANALYSIS CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.73 Fig. 5-4 CD Amplifier A simple common-drain (or source-follower) amplifier is shown in Fig. 5-4(a); its associated small-signal equivalent circuit is given in Fig. 5-4(b), where the voltage-source equivalent of Fig. 5-2(b) is used to model the FET. Example 5.2 In the CD amplifier, let 5 S R k , 60 , 30 ds r k . (a) Find an expression for the voltage-gain ratio o v i v A v . (b) Evaluate v A using the given typical values. Solution (a) By voltage division, / 1 1 1 S gd S o gd S ds S ds R v R v v R r R r CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.74 Replacement of gd v by i v and rearrangement give 1 o S v i S ds v R A v R r (b) Substitution of the given values leads to 0.895 v A Note that the gain is less than unity; its positive value indicates that o v and i v are in phase. 5.5. CG AMPLIFIER ANALYSIS Its small-signal equivalent circuit, incorporating the current-source model of Fig. 5-2(a), is given: (a) CG amplifier CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.75 (b) Small-signal equivalent circuit Fig. 5-5 CG Amplifier Example 5.3 In the CG amplifier, let 1 D R k , 3 2 10 m g S , 30 ds r k . (a) Find an expression for the voltage-gain ratio o v i v A v . (b) Evaluate v A using the given typical values. Solution (a) By KCL, r d m gs i i g v . Applying KVL around the outer loop gives: o d m gs ds gs v i g v r v But gs i v v and o d D v i R ; thus o o m i ds i D v v g v r v R CHAPTER 5: Small-Signal Midfrequency FET Val de Loire Program p.76 And : 1 m ds D o v i D ds g r R v A v R r (b) Substitution of the given values yields 1.97 v A . Fig. 5. 3 CS Amplifier 71 Fig. 5. 4 CD Amplifier 73 Fig. 5. 5 CG Amplifier 75 CHAPTER 5: Small- Signal Midfrequency FET Val de Loire Program p.68 CHAPTER 5: SMALL- SIGNAL MIDFREQUENCY. CHAPTER 5: Small- Signal Midfrequency FET Val de Loire Program p.67 CHAPTER 5: SMALL- SIGNAL MIDFREQUENCY FET Table of Contents 5. 1. INTRODUCTION 68 5. 2. SMALL- SIGNAL EQUIVALENT. Fig. 5- 2(a), is given: (a) CG amplifier CHAPTER 5: Small- Signal Midfrequency FET Val de Loire Program p. 75 (b) Small- signal equivalent circuit Fig. 5- 5 CG Amplifier Example 5. 3 In