Radio Frequency Electronics

Transcription

1 Radio Frequency Electronics Active omonents III Samuel Morse Born in 1791 in Massachusetts Fairly accomlished ainter After witnessing various electrical exeriments, got intrigued by electricity Designed the first single-wire telegrah Invented the concet relay what we now call reeaters reated Morse ode (digital communications?) Held several atents related to the telegrah Dies in 1872 Image from Wikiedia 1

2 Junction & Diffusion aacitance j V 1 V j0 R bi Junction caacitance mjc Junction caacitance j V 1 V j0 R bi mje Diffusion caacitance associated with current flowing through the baseemitter junction d I T V T g m T 2

3 BJT Hydrid π Model The basic hybrid-π model for a BJT becomes more and more inadequate as the oerating frequency increases. 3

4 Exanded Hybrid- Equivalent ircuit ~100 ~ M We can view these elements as arasites that surround the basic BJT. There are called arasitic elements. ~ 1 2 Parasitic elements 4

5 SPIE SPIE was not designed for RF work er se and has serious limitation in some areas. However, with roer modeling, one can do quite good RF simulations with SPIE SPIE use more comlex models than hybrid- Some SPIE arameters match u with hybrid- arameters, while other don t 5

6 SPIE ~ 1 2 6

7 SPIE RB J R IS JE RE BF VAF ~ 1 2 7

8 Exanded Hybrid- Equivalent ircuit Deending on the situation, we can ignore some of the arasitic elements. In RF work we can seldom ignore highlighted elements. π is normally >> µ However, because of the feedback from to B the effect of µ can be much bigger than that of π Both π, and µ are functions of Q-oint 8

9 Short ircuit Gain Ignore effect of R 1 and R 2 ouling caacitor R L 0 urrent amlifier 9

10 A. Kruger 10 Radio Frequency Electronics The University of Iowa Short-ircuit urrent Gain: BJT Frequency Resonse s V s V r V I b KL at inut KL at outut s g V I m c 0 1 s V I V g m s r s g h I I A m fe b c i 1 r s r g s r g h m m fe 1 1 With tyical values for µ and g m fe b c i h I I A

11 h fe g 1 s 1 s m r g m r r r Recall that at dc we used r π = β g m, β = g m r π f 2 r 1 ( ) f T f o Beta cutoff frequency Transition frequency 11

12 Short-ircuit urrent Gain: BJT Frequency Resonse Short-ircuit urrent Gain: BJT Frequency Resonse 12

13 Miller Effect and Miller aacitance B B E E Small (~ 1 F), but can have significant effect on frequency resonse B E 13

14 Physical Origin of Miller Effect Inverting amlifier 1 g ( R R )] M [ m L Voltage gain from B to (i.e., across μ?) M 1 voltage gain across Answer g R m L 14

15 Inherent Resistances and aacitances in n-hannel MOSFET Small Small Small gs gd 1 WL 2 ox 15

16 Equivalent ircuit for n-hannel ommon Source MOSFET 16

17 Unity-Gain Bandwidth Unity gain-band width is defined as the frequency where the magnitude of the short circuit current gain goes to 1. KL at inut node KL at outut node I i 1 Vgs j 1 gs V gs j gd I d g m V gs 1 V gs j gd A i I I d i g m j m j gs j gd gd g gs gd Set to 1 f T gm 2 ( gs gd) Similar to BJT f T gm 2 ( ) 17

18 MOSFET Miller aacitance Inverting amlifier M gd [ 1 gmrl ] Voltage gain from G to D (i.e., across gd?) Answer g R m L 18

19 Base Sreading Resistance EB Junction is forward-biased Emitter ++ n Base ollector B Junction is reverse-biased 19

20 Base Sreading Resistance EB Junction is forward-biased This resistance can often be ignored Emitter ++ n Base ollector B Junction is reverse-biased 20

21 Base Sreading Resistance EB Junction is forward-biased This resistance can often be ignored ++ Emitter n Base This resistance can often be ignored ollector This resistance is the base-sreading resistance r b, large, and can often not be ignored B Junction is reverse-biased 21

22 Base Sreading Resistance EB Junction is forward-biased Emitter ++ n Base Because of the base-sreading resistance, most of the hole injection occurs at the corners ollector Problematic with high-ower and high frequency devices. B Junction is reverse-biased 22

23 Emitter rowding in n BJT EB Junction is forward-biased Emitter ++ n Base ollector Emitter crowding at edges Problematic with high-ower and high frequency devices. B Junction is reverse-biased 23

24 Emitter rowding in nn BTT 24

25 Solution to Emitter rowding Since crowding occurs at edge, create base- and emitter connections with many edges 25

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