Large Signal Model for Saturation Mode

Transcription

1 ndian nstitute of echnology Jodhpur, Year 2016 nalog lectronics (Course Code: 314) Lecture 8: PP J, Small Signal nalysis Course nstructor: Shree Prakash iwari mail: ebpage: / / Course related documents will be uploaded on ote: he information provided in the slides are taken form text books for microelectronics (including Sedra & Smith,. Razavi), and various other resources from internet, for teaching/academic use only 1 Large Signal Model for Saturation Mode 1

2 Large Signal Model for Saturation Mode J Output Characteristics 2

3 xample: cceptable CC Range n order to prevent the J from entering very deeply into saturation, the collector voltage must not fall below the base voltage by more than 400 m. CC C R C ( 400 m ) xample: cceptable CC Range n order to prevent the J from entering very deeply into saturation, the collector voltage must not fall below the base voltage by more than 400 m. R ( 400 m CC C C linear relationship can be derived for CC and R C and an acceptable region can be chosen. ) 3

4 Deep Saturation n deep saturation, the J does not behave as a voltage controlled current source. C is ~constant. Review of J Operation (ctive Mode) he emitter junction is forward biased. Carriers diffuse across the emitter junction; thus, minority carrier concentrations are enhanced (by D e / ) at the edges of the emitter junction depletion region. More minority carriers are injected into the base vs. emitter, because the emitter is more heavily doped than the base. he collector junction is reverse biased (or not strongly forward biased). / Minority carrier concentrations are ~0 (since e D 0) at the edges of the collector junction depletion region. he minority carrier concentration gradient in the quasi neutral base region (of width ) results in minority carrier diffusion toward the collector junction. f is much shorter than the minority carrier diffusion length, then most of the minority carriers injected from the emitter will reach the collectorjunction depletion region, and then drift into the quasi neutral collector. he collector current is primarily due to carriers collected from the base. 4

5 5 Common mitter Current Gain, ssuming that no minority carrier recombination occurs within the quasi neutral base region: he collector current is equal to the current due to minority carrier injection from the emitter into the base: he base current is equal to the current due to minority carrier injection from the base into the emitter: 1 / 2 i C e n qd C i e n qd 1 / 2 he current gain can thus be expressed as a function of the J physical parameters: e 1 D D mpact of arly ffect on J Currents For a fixed value of, decreases with increasing C (because the width of the collector junction depletion region increases with increasing reverse bias), so that the minoritycarrier concentration gradient in the quasi neutral base region increases. hus, C increases (slightly) with increasing C. he base current is not impacted: n qd 2 C i C e n qd 1 / 2 hus, the current gain increases with increasing C. C i e n qd / S C e / C C D D 0

6 Small Signal Models for ndependent Sources he voltage across an independent voltage source does not vary with time. ts small signal signal voltage is always zero. hus, it is regarded as a short circuit for the purpose of small signal analysis. he current through an independent current source does not vary with time ts small signal current is always zero. hus, it is regarded as an open circuit for the purpose of small signal analysis. PP ransistor he operating principle of a PP J is the same as that of an P J. ote that the bias voltage polarities are reversed for the PP device, compared to an P device. he emitter is biased at a higher potential than the base. he collector is biased at a lower potential than the base. 6

7 7 P vs. PP Js he directions of current flow and operation modes for P and PP Js are shown below: PP J erminal Currents C S C 1 exp S C S 0 exp 1 exp D D 0 0

8 Large Signal Model for PP J PP J iasing ote that the emitter is biased at a higher potential than the base and the collector. 8

9 Small Signal nalysis PP J Small Signal Model he small signal model for a PP transistor is identical to that of an P transistor. otethatthepolarityofthesmall signal that the of the signal currentsand voltages are defined to be in the opposite direction with respect to the large signal model. his is OK, because the small signal model is used only to determine changes in currents and voltages. 9

10 Small Signal Model xample 1 Small Signal Model xample 2 ote that the small signal model is identical to that in the previous example. 10

11 Small Signal Model xample 3 ote that the small signal model is identical to that in the previous examples. J mplifiers: Overview 11

12 J iasing and mplifiers hat next 12