Lecture 6. OUTLINE BJT (cont d) PNP transistor (structure, operation, models) BJT Amplifiers General considerations. Reading: Chapter

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1 Lecture 6 ANNOUNCMNTS HW#3, Prob. 2: Re-draw -plots for W reduced by a factor of 2. n case of a major earthquake: Try to duck/crouch on the floor in front of the seats for cover. Once the earthquake stops, evacuate the building in an orderly manner. OUTLN JT (cont d) PNP transistor (structure, operation, models) JT Amplifiers General considerations Reading: Chapter Fall 2007 Lecture 6, Slide 1 Prof. Liu, UC erkeley

Current Flow in a Long-ase PN Junction The quasi-neutral N-type and P-type regions have low resistivity, whereas the depletion region has high resistivity.

2 Current Flow in a Long-ase PN Junction The quasi-neutral N-type and P-type regions have low resistivity, whereas the depletion region has high resistivity. When an external voltage D is applied across the diode, almost all of this voltage is dropped across the depletion region. J tot x A relatively small -field exists in the quasi-neutral regions drift current 0 a 105 Fall 2007 Lecture 6, Slide 2 Prof. Liu, UC erkeley J J n p x=0 = A -b Dn N L n N D D p L p = N N D A L L p n x D D n p

3 Review of JT Operation (Active Mode) The emitter junction is forward biased. Carriers diffuse across the emitter junction; thus, minority-carrier concentrations are enhanced (by D T 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. The collector junction is reverse biased (or not strongly forward biased). / Minority-carrier concentrations are ~0 (since e D T 0) at the edges of the emitter-junction depletion region. The minority-carrier concentration gradient in the quasi-neutral base region (of width W )results in minority-carrier diffusion toward the collector junction. f W 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. The collector current is primarily due to carriers collected from the base. 105 Fall 2007 Lecture 6, Slide 3 Prof. Liu, UC erkeley

4 Common-mitter Current Gain, β Assuming that no minority-carrier recombination occurs within the quasi-neutral base region: The collector current is equal to the current due to minority-carrier injection from the emitter into the base: C A qd 2 i ( 1) / e T = N W The base current is equal to the current due to minority-carrier injection from the base into the emitter: A qd N W n n ( ) / e 2 = i T 1 The current gain βcan thus be expressed as a function of the JT physical parameters: D N W β = D N W 105 Fall 2007 Lecture 6, Slide 4 Prof. Liu, UC erkeley C β

5 mpact of arly ffect on JT Currents For a fixed value of, W 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. Thus, C increases (slightly) with increasing C. + C i e n qd A T 1 / Fall 2007 Lecture 6, Slide 5 Prof. Liu, UC erkeley The base current is not impacted: Thus, the current gain β increases with increasing C. β C i e W N n qd A T = / 2 + A C i C e W N T 1 T S A C e / = β β A C A C W N D W N D + + = 0 β β

6 Small-Signal Models for ndependent Sources The voltage across an independent voltage source does not vary with time. ts small-signal voltage is always zero. Thus, it is regarded as a short circuit for the purpose of small-signal analysis. The current through an independent current source does not vary with time ts small-signal current is always zero. Thus, it is regarded as an open circuit for the purpose of small-signal analysis. 105 Fall 2007 Lecture 6, Slide 6 Prof. Liu, UC erkeley

7 PNP Transistor The operating principle of a PNP JT is the same as that of an NPN JT. Note that the bias-voltage polarities are reversed for the PNP device, compared to an NPN device. The emitter is biased at a higher potential than the base. The collector is biased at a lower potential than the base. 105 Fall 2007 Lecture 6, Slide 7 Prof. Liu, UC erkeley

8 NPN vs.pnp JTs The directions of current flow and operation modes for NPN andpnp JTs are shown below: 105 Fall 2007 Lecture 6, Slide 8 Prof. Liu, UC erkeley

9 PNP JT Terminal Currents S A C T S C = + = exp 1 exp β 105 Fall 2007 Lecture 6, Slide 9 Prof. Liu, UC erkeley T S A C T W N D W N D = + + = exp 1 β β β β

10 Large-Signal Model for PNP JT 105 Fall 2007 Lecture 6, Slide 10 Prof. Liu, UC erkeley

11 PNP JT iasing Note that the emitter is biased at a higher potential than the base and the collector. 105 Fall 2007 Lecture 6, Slide 11 Prof. Liu, UC erkeley

12 Small-Signal Analysis 105 Fall 2007 Lecture 6, Slide 12 Prof. Liu, UC erkeley

13 PNP JT Small-Signal Model The small-signal model for a PNP transistor is identical to that of an NPN transistor. Note that the polarity of the small-signal currents and voltages are defined to be in the opposite direction with respect to the large-signal model. This is OK, because the small-signal model is used only to determine changesin currents and voltages. 105 Fall 2007 Lecture 6, Slide 13 Prof. Liu, UC erkeley

14 Small-Signal Model xample Fall 2007 Lecture 6, Slide 14 Prof. Liu, UC erkeley

15 Small-Signal Model xample 2 Note that the small-signal model is identical to that in the previous example. 105 Fall 2007 Lecture 6, Slide 15 Prof. Liu, UC erkeley

16 Small-Signal Model xample 3 Note that the small-signal model is identical to that in the previous examples. 105 Fall 2007 Lecture 6, Slide 16 Prof. Liu, UC erkeley

17 Small-Signal Model xample Fall 2007 Lecture 6, Slide 17 Prof. Liu, UC erkeley

18 JT Amplifiers: Overview 105 Fall 2007 Lecture 6, Slide 18 Prof. Liu, UC erkeley

19 oltage Amplifier n an ideal voltage amplifier, the input impedance is infinite and the output impedance is zero. n reality, the input and output impedances depart from their ideal values. 105 Fall 2007 Lecture 6, Slide 19 Prof. Liu, UC erkeley

20 nput/output mpedances The figures below show how input and output impedances are determined. All independent sources are set to zero. impedance v i x x 105 Fall 2007 Lecture 6, Slide 20 Prof. Liu, UC erkeley

21 nput mpedance xample Note that input/output impedances are usually regarded as small-signal quantities. The input impedance is obtained by applying a small change in the input voltage and finding the resultant change in the input current: v x = i x 105 Fall 2007 Lecture 6, Slide 21 Prof. Liu, UC erkeley r π

22 mpedance at a Node When calculating /O impedances at a port, we usually ground one terminal. We often refer to the impedance seen at a node rather than the impedance between two nodes (i.e. at a port). 105 Fall 2007 Lecture 6, Slide 22 Prof. Liu, UC erkeley

23 mpedance seen at the Collector The impedance seen at the collector is equal to the intrinsic output impedance of the transistor, if the emitter is grounded. R = r out o 105 Fall 2007 Lecture 6, Slide 23 Prof. Liu, UC erkeley

24 mpedance seen at the mitter The impedance seen at the emitter is approximately equal to the inverse of its transconductance, if the base is grounded. v i x x R out ( = g A m 1 g = ) r m π 105 Fall 2007 Lecture 6, Slide 24 Prof. Liu, UC erkeley

25 Summary of JT mpedances 1. Looking into the base, the impedance is r π if the emitter is (ac) grounded. 2. Looking into the collector, the impedance is r o if emitter is (ac) grounded. 3. Looking into the emitter, the impedance is 1/g m if base is (ac) grounded and arly effect is neglected. 105 Fall 2007 Lecture 6, Slide 25 Prof. Liu, UC erkeley

Large Signal Model for Saturation Mode

Large Signal Model for Saturation Mode 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: sptiwari@iitj.ac.in ebpage: http://home.iitj.ac.in/~sptiwari/