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ch4_amplifier_09-16

ch4_amplifier_09-16 - Power Conversion Efficiency(C PL Ave...

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Last updated: 22-Nov-03 9 Power Conversion Efficiency ( η C ) Supply L C P P pplies su the from drawn Power load the to delivered Power Ave = = η P supply = 2 V CC I Q (constant) Assume the biasing current I R is small = = = CC sat CE Q CC Q sat CE CC Supply L c V V I V I V V P P ) ( ) ( max max 1 4 1 2 ] [ 2 1 η For V CE(sat) « V CC , η Cmax 25% (Not Good!) Also, standby Power = 2V CC I Q (very large!) Instantaneous Power of Q 1 When V ce1 increases, V e decreases, i.e. V O decreases I ce1 decreases (because I RL = I ce1 -I Q ) Power dissipated in the active device Q 1 P C1 = V ce1 I ce1 For maximum signal swing with R L = R L3 P C1 = V ce (1 + sin ω t) I Q (1 sin ω t) = V CC I Q (1 sin 2 ω t) = 2 Q CC I V (1 + cos2 ω t) P C1max = V CC I Q Note: To design an output stage, make sure V CC I Q is less than the rated power of a transistor. E.g. P rated = 1W, V CC = 10V I Q < 100mA Small Signal Properties 1 + + = β π S O R r R where R S is the source resistance 1 + + + = = β π S L L i o V R r R R v v A . Since r π depends on the quiescent current I Q , R o and A v are I Q dependent. Note: 1. EF Output Stage — no voltage gain but high power gain. 2. V omax = V CC – V CE(sat) for any v I . If v I is limited to V CC , V omax = V CC – V BE1 . Disadvantage in Class A — large power dissipation even in standby mode. Solve the problem by using 2 active devices (1 for each ½ cycle). Q 1 v i I ce1 R L C E B v o V CC = fixed voltage I Q t t t V ce1 0 V CC I ce1 0 I Q P c1 0 2 Q CC I V 2V CC 2I Q
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Last updated: 22-Nov-03 10 Class B (PUSH-PULL) Output Stage Note: 1. No-load current is zero (Both Q 1 & Q 2 are OFF). 2. Deadband of ~2V BE(on) lead to crossover distortion. 3. Bad for small signals. Class AB Output stage Note: 1. No dead band. 2. Small quiescent current (I Q is only used for making sure Q 3 & Q 4 are ON). 3. Output biasing current depends on the area ratio of Q 1 , Q 2 , Q 3 and Q 4 . Since = T BE S C V V I I exp , if Q 1 & Q 2 is 10 times larger, I output = 10 I Q . v i V BE(on) t v o t v o v i V CC + |V CE2(sat) | V BE(on) V CC + V CE1(sat) 0 Transfer Char.
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