Drive Capability:We have seen that in order of assurance that the transistor in the simple inverter remains in saturation whenever needed, the base is overdriven by injecting more base current than is required to bring the transistor to the edge of saturation. A usual base overdrive factor of σu = 5 has been utilized. There is another significant reason for such a high degree of overdrive and that is to provide the inverter output drive capability. Let consider the inverter with a load joined to the output as shown in figure below.
Figure: Transistor Inverter Driving a Multiple Resistive Load
In this case, the collector current flowing via the transistor whenever it is turned on is determined not just by the collector resistor, RC, however as well by the effective load in parallel with it. This effective load, in practice, might represent N similar loads, each representing an equal resistance, RL. Of interest is the maximum number of such loads which can be driven by the inverter. In this case, the overall transistor collector current is as:iC = iRC + iLTOTIn saturation, iC << βFiB, that is guaranteed by overdrive. The limiting condition occurs whenever so many phases are added that the transistor comes out of saturation into forward active region since the collector current is no longer limited by the RC. Critical point is if the transistor is at the edge of saturation. At this point:iC = βFiBiC = (VCC – VCESAT)/RC + (VCC – VCESAT)/RLOTAnd, iB = (VCC – VCESAT)/RB
And hence,(VCC – VCESAT) [(1/RC) + (1/RLOT)] = βF (VCC – VBESAT) (1/RB)For N phases of load RL, in parallel: RLOT = RL/NIn the limit, with maximum number of load phases, NMAX and taking:VCC – VCESAT ≈ VCC – VBESAT then,(1/RC) + (NMAX/RL) = βF/RBThis gives:(RL/RC) + NMAX = (βF RL)/RBNMAX = [(βF RL)/RB] - (RL/RC)By slightly modifying it,NMAX = [(βF RL)/RB] . (RC/RC) - (RL/RC)NMAX = [{(βF RC)/RB} - 1] (RL/RC) But as βF (RC/RB) = σu then,NMAX = (σu - 1) (RL/RC)Notice that the number of load phases which can be driven by inverter depends on the value of load resistor RL, in relation to RC however as well on the degree of base overdrive applied to unloaded inverter phase. When no overdrive was present, then σu – 1 and NMAX = 0 that would mean no extra loading could be tolerated. The conclusion is that overdrive of transistor is as well essential to enable it to drive loads.Fan-Out:Fan-Out is a direct, practical measurement of drive capability of a logic gate.Fan-Out is generally defined as the number of gate inputs similar to its own, joined in parallel, that a gate output is capable to drive devoid of malfunction. Let consider the single transistor inverter loaded by numerous identical phases as shown in figure below. The question is how many of such phases can be joined in parallel as load before an inaccurate value of VO outcomes. Whenever the output VO is LO, the load transistors are OFF and therefore there is no demand for current therefore this condition doesn’t limit the fan-out. Whenever the output VO is HI, the load transistors are ON and each one demands a base current adequate to saturate it. The question is how many load transistors can be tolerated prior to VO falls to the minimum level which will be seen by the load transistor as HI input, ViH MIN. In another words, how many load transistors can be joined in parallel before:Vo = VIH MIN = VBE SAT + [RB/(βFRC)] (VCC - VCESAT)
Figure: A Bipolar Transistor Inverter Loaded by Multiple similar Inverters
Figure: Equivalent Circuit of Loaded Transistor Inverter
An equivalent circuit can be drawn symbolizing the N load transistors in parallel as shown in figure above. Each and every load transistor consists of a base-emitter potential drop VBE SAT and draws a base current:IB = (VO – VBESAT)/RBWhenever the driving inverter output is high, then from equivalent circuit of figure shown above:Vo = VBESAT + (RB/N)/{(RB/N) + RC} (VCC - VBESAT)
In limit Vo = ViH MIN and N = NMAX therefore equating the two expressions provides:VBESAT + [(RB/NMAX)]/[(RC + RB)/NMAX] (VCC - VBESAT) = VBESAT + (RB/βFRC) (VCC - VCESAT)When we consider, VCC – VBESAT ≈ VCC - VCESAT then,RB/(RB + NMAX RC) = RB/βF RCTherefore, RB + NMAX RC = βF RCAnd hence,NMAX = (βF RC - RB)/RCNMAX = βF (1 - RB/ βF RC) However as βF RC/RB = σu, we can state the Fan-Out, F = NMAX is:F = βF (1 – 1/σu)Again if σU = 1 then F = 0.This can be seen that a high transistor βF gives a high gate Fan-out. When βF = 50, σU = 5 then F = 50(1 – 0.2) = 50 x 0.8 = 40 that is a little higher than manufacturers assurance in practice. A usual value is of the order of 10.
Latest technology based Electrical Engineering Online Tutoring Assistance
Tutors, at the www.tutorsglobe.com, take pledge to provide full satisfaction and assurance in Electrical Engineering help via online tutoring. Students are getting 100% satisfaction by online tutors across the globe. Here you can get homework help for Electrical Engineering, project ideas and tutorials. We provide email based Electrical Engineering help. You can join us to ask queries 24x7 with live, experienced and qualified online tutors specialized in Electrical Engineering. Through Online Tutoring, you would be able to complete your homework or assignments at your home. Tutors at the TutorsGlobe are committed to provide the best quality online tutoring assistance for Electrical Engineering Homework help and assignment help services. They use their experience, as they have solved thousands of the Electrical Engineering assignments, which may help you to solve your complex issues of Electrical Engineering. TutorsGlobe assure for the best quality compliance to your homework. Compromise with quality is not in our dictionary. If we feel that we are not able to provide the homework help as per the deadline or given instruction by the student, we refund the money of the student without any delay.
it is significant to take great care throughout the bearing removal to make sure that the bearing, shaft and housing are not broken. bearing removal is best accomplished through using a bearing puller for standard outer and inner rings.
Thermosetting Plastics are Aminos (Urea formaldehyde resins, Melamine, Phenolics) Heat and solvent resistant. Non-conductors of electricity. Employed like a Electrical appliance handles, TV and Radio cabinets.
tutorsglobe.com plant physiology assignment help-homework help by online botany tutors
TutorsGlobe.com Liquids Assignment Help-Homework Help by Online Access Chemistry Tutors
tutorsglobe.com dialysis assignment help-homework help by online renal failure tutors
www.tutorsglobe.com offers Types of Budgets homework help, assignment help, case study, writing homework help, online tutoring assistance by accounting tutors.
tutorsglobe.com shifts in supply assignment help-homework help by online shift in demand and supply tutors
Online ACT Exam Preparation course and online ACT tutoring package offered by www.tutorsglobe.com are the most comprehensive and customized collection of study resources on the web, offering best collection of ACT practice papers, quizzes, ACT test papers, and guidance.
Theory and lecture notes of Markov algorithm simulates Post machine all along with the key concepts of markov algorithm simulates post machine, homework help, assignment help, equivalence of tms, pms and markov algorithms tutors. Tutorsglobe offers homework help, assignment help and tutor’s assistance on Markov algorithm simulates Post machine.
tutorsglobe.com types of crystals assignment help-homework help by online solid state chemistry tutors
Management Accounting uses the standard and practices of financial accounting and cost accounting additionally to other modern management methods for effective operation of a company.
tutorsglobe.com taxes of state governments assignment help-homework help by online direct and indirect taxes tutors
tutorsglobe.com federal finance assignment help-homework help by online subject matter of public finance tutors
tutorsglobe.com hypermetropia assignment help-homework help by online errors of refraction tutors
tutorsglobe.com eye infections assignment help-homework help by online receptor organs tutors
1964540
Questions Asked
3689
Tutors
1441186
Questions Answered
Start Excelling in your courses, Ask an Expert and get answers for your homework and assignments!!