diodes_bjt-6p.pdf - Diodes and BJT Transistors Contents Diode Model Diode Model Diodes and BJT Transistors Diodes are important elements in digital

diodes_bjt-6p.pdf - Diodes and BJT Transistors Contents...

This preview shows page 1 out of 5 pages.

Unformatted text preview: Diodes and BJT Transistors Contents Diode Model Diode Model Diodes and BJT Transistors Diodes are important elements in digital electronic circuits, as well as they are used to perform various logic operations, they are also used as variable capacitors, DC voltage level shifters and clamping diodes at logic circuit inputs. Diode Model Clamping Diodes Level-Shifting Diodes Symbols for PN junction diodes and MN junction diodes are shown in the gures left and right below, respectively. Ebers-Moll BJT Model BJT Modes of Operation Simplied NPN BJT Model IV Characteristics BJT Sub-Circuits PN junction diodes are formed from the combination of P-type and N-type regions. Usually, PN junctions in integrated circuits (ICs) are usually formed by utilizing the two out of the three regions of a bipolar junction transistor, instead of a separate device structure. Turn-on voltage for a PN junction diode is VD(ON ) = 0.7 V. MN junction (Schottky Barrier) diodes are formed from the combination of a metal and an N− -type semiconductor. Metal used in MN junction diodes is mostly platinum silicide (Pt5 Si2 ). As there are no holes present, MN junction diodes are much faster than PN junction diodes. Turn-on voltage for a Schottky Barrier (MN junction) diode is VSBD(ON ) = 0.3 V. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 1 / 27 Diode Model Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors Cross sections of some example PN and MN junction diodes as shown in the gures left and right below, respectively, in order to highlight some of the fabrication properties. 23-Nov-2017 2 / 27 Diode Model In the analysis of digital circuits, we are going to use the simplied diode model as shown in the gure and summarized in in the diode modes of operation table below Diode current-voltage (IV) characteristics are normally governed by the well-known Shockley's diode equation,   ID = IS eVD /γ − 1 where IS is the reverse saturation current (typically pA for PN junction diodes and µA for MN junction diodes) and γ = φT = kT /q is the thermal voltage (typically γ = 26 mV at 300 K) with k representing the Boltzman constant, T representing the temperature in kelvins and q representing the elementary charge. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 3 / 27 Diode Model The transition point from cuto mode to conduction mode (i.e., when the current is not yet owing) is called as edge of conduction (EOC). Diode Modes of Operation Junction Bias Mode of Operation Reverse Forward Cuto (OFF) Conducting (ON) Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 4 / 27 Clamping Diodes Clamping and Level-Shifting Diodes The large signal diode model used in SPICE in shown in the gure below. When the input to a gate is switched from high-to-low, the input voltage sometimes swings well beyond 0 V. This is called as ringing and may cause physical damage to the gate. Connecting clamping diodes to each input of a gate, as shown in the gure below, eliminates this problem by preventing inputs from falling below −0.7 V. The diodes will not aect the operation of the gate, as the diodes are open circuit for positive inputs. PN Junction capacitance can be utilized in ICs by applying a negative bias to a diode. Diodes used for this purpose are referred to as varactor diodes and have the modied circuit symbol presented in the gure below. Clamping diodes can be also connected to the output(s) of a gate. Most TTL/STTL families employ clamping diodes at their inputs and sometimes also at their outputs. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II 23-Nov-2017 5 / 27 Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II 23-Nov-2017 6 / 27 Diodes and BJT Transistors Level-Shifting Diodes Diodes and BJT Transistors It is often required to change the voltage level across particular portions of digital circuits, e.g., to level shift the output voltage. Another use of the diode forward voltage is to ensure that sub-circuits with complementary objectives are not conducting simultaneously. For example, TTL circuits employ two output drivers. Only one driver should be working for the output-low state, while only the other driver should be working for the output-high state. Placement of a voltage level-shifting device between the two drivers ensures the desired operation by allowing only one driver to be on at a time. Level-Shifting Diodes BJT Transistors Bipolar junction transistors (BJTs) are very important in digital circuits, e.g., TTL circuits are based on BJTs. Figure below shows a 3D cross-section (without metallization) of an NPN BJT fabricated with the junction isolated technology. Example 1: For the circuit below, determine the level-shifting voltage Vshift . In some BJT logic families (e.g., TTL), multiple inputs are achieved by using multi-emitter BJTs as shown in the gure on the left below. A multi-emitter Schottky-clamped BJT (SBJT) is shown in the gure on the right above. The base contact is extended over the N collector region, thus placing a Schottky Barrier (MN) diode in parallel with the base-collector PN junction. This device operates much faster than a normal BJT, and an SBJT does not go into saturation mode. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 7 / 27 Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) Level-Shifting Diodes ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 8 / 27 Ebers-Moll BJT Model Ebers-Moll BJT Model The most frequently used notation and symbols for BJT transistors are shown in the gure below for the NPN and PNP transistors.   ID,BE = IES eVBE /γ − 1   VBC /γ ID,BC = ICS e −1 IES : base-emitter reverse saturation current, ICS : base-collector reverse saturation current, γ : thermal voltage (kT /q = 26 mV at 300 K). IE = ID,BE − αR ID,BC IC = αF ID,BE − ID,BC IB = IE − IC αF and αR are the common base forward amplication factors. (typically αF ≈ 1 and 0.2 ≤ αR ≤ 0.6) and reverse Reciprocity theorem: IS = αF IES = αR ICS IS is known as the transport saturation current. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 9 / 27 Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) BJT Modes of Operation ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 10 / 27 BJT Modes of Operation Cuto (OFF) In the cuto (OFF) mode, both PN junctions (BE and BC) of the BJT are reverse-biased. If we assume simplied diode model for the PN junctions in the Ebers-Moll model, both ID,BE and ID,BC are zero. Consequently, IE(OF F ) = 0 and IC(OF F ) = 0 A BJT transistor has two PN junctions: the base-emitter PN junction (BE junction) and the base-collector PN junction (BC junction), as depicted in the gure above. As either junction can be forward or reverse biased, there are four modes of operation (or four transistor states) as shown in the table below. BE Junction Bias BJT Modes of Operation BC Junction Mode of Operation Bias Reverse Forward Reverse Forward Reverse Reverse Forward Forward Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) Cuto (OFF) Forward Active (FA) Reverse Active (RA) Saturation (SAT) (Forward Saturation (FSAT) or Reverse Saturation (RSAT) in reality) ELE315 Electronics II 23-Nov-2017 IB(OF F ) = 0 Forward Active (FA) In the forward active (FA) mode, the base-emitter PN junction (BE) is forward biased and the base-collector PN junction is reverse biased. In the Ebers-Moll model, ID,BC becomes zero. Consequently, VBE(F A) = 0.7 V IC(F A) = βF IB(F A) or IC(F A) = αF IE(F A) where βF is the common-emitter current amplication factor given by αF 1 − αF in terms of βF as βF αF = βF + 1 βF = Similarly, αF can also be expressed 11 / 27 Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II 23-Nov-2017 12 / 27 Diodes and BJT Transistors BJT Modes of Operation Reverse Active (RA) In the reverse active (RA) mode, the base-emitter PN junction (BE) is reverse biased and the base-collector PN junction is forward biased. In the Ebers-Moll model, ID,BE becomes zero. Consequently, VBC(RA) = 0.7 V Diodes and BJT Transistors BJT Modes of Operation Saturation (SAT) In the saturation (SAT) mode, both PN junctions (BE and BC) are forward biased. Normally, we only consider the case called forward saturation where base-emitter junction has a stronger bias (i.e., VBE ≥ VBC for NPNs). The opposite case (VBC > VBE for NPNs) is called reverse saturation and rarely occurs in digital circuits. Forward Saturation (FSAT): In this mode, base current is large and collector and emitter −IC(RA) = (βR + 1) IB(RA) (IC(RA) < 0) IE(RA) = αR IC(RA) = −βR IB(RA) (IE(RA) < 0) currents are saturated such that IC < βF IB . Note that, in this mode IC and IE are positive. or IC(F SAT ) < βF IB(F SAT ) VBE(F SAT ) = 0.8 V where βR is the reverse active current amplication factor (typically 0.1 ≤ βR ≤ 2.0) given by βR = VBC(F SAT ) = 0.6 V αR 1 − αR VCE(F SAT ) = 0.2 V A saturation parameter σ is dened to indicate the relationship between IC and IB as Similarly, αR can also be expressed in terms of βR as αR = βR βR + 1 σ= Note that, negative values for currents mean that currents ow in the reverse directions. In other words, negative IE and IC mean that the current is owing into the emitter and out of the collector for an NPN transistor, and into the collector and out of the emitter for a PNP transistor. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 13 / 27 where σ ≤ 1. Note that σ is not constant, it changes according to the operating point, and σ = 1 denotes forward active operation and/or edge of saturation operation. If it is not given, you may assume σmax = 1. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) BJT Modes of Operation ELE315 Electronics II Diodes and BJT Transistors Reverse Saturation (RSAT): In this mode, base-collector junction has a stronger bias, i.e., VBC > VBE for NPNs, and collector and emitter currents are saturated such that −IE < βR IB . Note that, in this mode IC and IE are negative. IC βF IB 23-Nov-2017 14 / 27 BJT Modes of Operation Summary −IE(RSAT ) < βR IB(RSAT ) −IC(RSAT ) < (βR + 1) IB(RSAT ) (for NPNs) VCE(RSAT ) < 0 In this course, we are going to refer forward saturation (FSAT) mode as the only saturation (SAT) mode, i.e., Cuto (OFF) Forward active (FA) SAT = F SAT. In all operation modes (FSAT, RSAT etc.) the following must hold: 1. 2. 3. 4. IC and IE always have the same sign, i.e., always in the same direction, Base current is always nonnegative, i.e., IB ≥ 0, KCL is satised, i.e., IE = IC + IB , KVL is satised, i.e., VCE = VBE − VBC . Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors State 23-Nov-2017 15 / 27 BJT Modes of Operation Forward Active (FA) Reverse Active (RA) Forward Saturation (FSAT) [ Saturation (SAT) ] Reverse Saturation (RSAT) IC = 0, VBE < VBE(F A) , IE = 0, IB = 0 VBC < VBC(RA) VBE = VBE(F A) , VBC < VBC(RA) , IC = βF IB VCE > VCE(F SAT ) > 0 VBC = VBC(RA) , VBE < VBE(F A) , IC = −(βR + 1)IB VCE < VCE(RSAT ) < 0 VCE = VCE(F SAT ) , IC < β F IB , VBE = VBE(F SAT ) IC > 0, IE > 0, VBC = VBC(F SAT ) VCE > 0, IB > 0. VCE = VCE(RSAT ) , −IC < (βR + 1)IB , VBE = VBE(RSAT ) IC < 0, IE < 0, VBC = VBC(RSAT ) VCE < 0, IB > 0. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 Saturation (SAT) 23-Nov-2017 16 / 27 BJT Modes of Operation IV Characteristics Simplied NPN BJT Model Circuit Behaviour Test Condition Cuto (OFF) Reverse Active(RA) Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) Figure above shows a set of IC versus VCE characteristics for changes in IB (of amount ∆). For equal increments in IB , the curves in the active regions are approximately evenly spaced, although the curves in the reverse active region are much closer than those in the forward active region. 17 / 27 Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II 23-Nov-2017 18 / 27 Diodes and BJT Transistors BJT Modes of Operation Diodes and BJT Transistors Example 2: For the circuit below, determine the state of the transistor and nd currents IB , IC and IE , given βF = 65. BJT Modes of Operation Example 4: For the circuit below, determine I and VB . Assume the BJT base current is negligible. Example 3: For the circuit below, determine the voltages at the base and emitter of each BJT. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 19 / 27 Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) BJT Sub-Circuits ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 20 / 27 BJT Sub-Circuits Output-High Pull-Up Driver BJT Sub-Circuits In order to provide a preview to succeeding chapters, this subsection introduces sub-circuits common to all TTL families summarized by the NAND block diagram in the gure below. Input Section A simple voltage driven resistor, also known as passive pull-up, would serve the purpose as shown in the gure (a) above. For this NAND diagram, input section consists of ANDing of all inputs either with a parallel diode conguration or with a multi-emitter BJT. Drive Splitter Depending on the result of ANDing, the drive splitter turns on one of the two output sections, namely output low and output high driver sections. A typical drive splitter is a BJT acting as a switch, when it is cuto mode it activate the output-high driver and when it is in saturation mode it activates the output-low driver. Driver splitter section also&provides an inversion operation. Dr. U. Sezen Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II 23-Nov-2017 21 / 27 Diodes and BJT Transistors As the output goes low-to-high, current is required to charge the equivalent input-capacitance of the load gates. Output-high pull-up driver provides the current for this charging. Some example pull-up driver sub-circuits are shown in the gure below. An emitter-follower shown in the gure (b) above is an active solution which provides a higher output current and hence provides faster switching time for the load gates. For even more sourcing current, a Darlington pair can be used as shown in the gure (c) above. Active pull-up circuitry also provides greater fan-out. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) BJT Sub-Circuits ELE315 Electronics II Diodes and BJT Transistors Output-Low Pull-Down Driver 23-Nov-2017 22 / 27 BJT Sub-Circuits Discharge Paths There are two purposes of output-low pull-down circuits: one is to discharge the capacitive load by providing a large sinking current, and another is to provide larger fan-out by sinking currents IIL from all the load gates as shown in the gure below. In order to turn o a saturated BJT, all of the stored charges in the base region must be removed. A path must therefore be available for base discharge. Some example discharge sub-circuits are shown in the gure below. Some example pull-down driver sub-circuits are shown in the gure below. Figure (a) above displays a circuit with an additional resistor RD that provides passive charge removal. A simple resistor connected to a negative power supply (or ground), also known as passive pull-down, would serve the purpose as shown in the gure (a) above. A BJT, as shown in the gure (b) above, will server as an active pull-down in saturation mode. Another advantage of active pull-down or pull-up circuits is that they can be activated and/or deactivated, apart from increasing fan-out. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II 23-Nov-2017 23 / 27 Figure (b) above shows an active conguration for stored charge removal, which provides a much faster discharge (i.e., higher discharge current) than RD itself. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II 23-Nov-2017 24 / 27 Diodes and BJT Transistors BJT Sub-Circuits Diodes and BJT Transistors Power Dissipation of BJT Logic Circuits Base Driving Circuitry BJT Sub-Circuits On the other hand, the turn-on time of a BJT is dependent on the time required to charge the base of the BJT. Active base driving current is often supplied to BJTs to ensure a shorter turn-on time. An emitter-follower BJT conguration, as shown in the gure below where QS drives base driving current to QO , usually supplies this driving current. When BJT logic circuits have a single power supply, as shown in the gure above, the power dissipation for a particular gate in a particular state is taken as the power supplied given by PCC = ICC VCC where ICC is the current drawn from VCC and is obtained by summing all the currents leaving the supply voltage source. For example, for the gure above, the current supplied by VCC is ICC = IRB + IRC + IRCP . Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II Diodes and BJT Transistors 23-Nov-2017 25 / 27 BJT Sub-Circuits Consequently, the average power dissipated in a logic circuit with two output states (output-low and output-high) is dened as PCC(avg) = ICC(OL) + ICC(OH) 2 VCC Example 5: For the circuit below, calculate the average power dissipation for this gate, if IRB(OH) = 1.55 mA, IRC(OH) = 24.7 µA, IRCP (OH) = 1.21 mA, IRB(OL) = 1.14 mA, IRC(OL) = 4.48 mA and IRCP (OL) = 104 µA. Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II 23-Nov-2017 27 / 27 Dr. U. Sezen & Dr. D. Gökçen (Hacettepe Uni.) ELE315 Electronics II 23-Nov-2017 26 / 27 ...
View Full Document

  • Fall '16
  • Umut Sezen

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Ask Expert Tutors You can ask You can ask ( soon) You can ask (will expire )
Answers in as fast as 15 minutes