1EE 334Midterm ReviewDiode: Why we need to understand diode?• The base emitter junction of the BJT behaves asa forward bias diode in amplifying applications.• The behavior of the diode when reverse bias is the key to the fabrication of the integrated circuits.• The diode is used in many important nonamplifer applications.Departure from ideal behaviorThe four major reason why the actual diode do not correspond exactly to the ideal.1. Ohmic resistance and contact resistance in series with the diode cause the VIcurve to become linear at high forward current.2. Avalanche or Zener breakdown take place at high reverse voltage, causing anabrupt increase in reverse current.3. Surface contaminants cause an ohmic layer to form across the junction, which isIncreasing the reverse current as reverse voltage is increased.4. Recombination of current carrier in the depletion region take place due to traps.The purpose of modeling• Nonlinear problems are much more difficult than linear ones. These problems could be impossible to solve manually and could require huge amount of time if solved on a computer.• One possible solution of the above mentioned problem is to approximate the nonlinear relationship with a model that has a linear relationship.• The trust of nonlinear modeling is direct towards this end.• The modeling not only simplifies the solution, it also allows the designer to understand how the circuit behaves. Modeling often increases the conceptual understanding of the circuit operation.2Schottky Barrier DiodeOne semiconductor region of the pn junction diode can be replaced by a non-ohmicrectifying metal contact.A Schottky contact is easily formed on n-type silicon. The metal region becomes the anode. An n+region is added to ensure that the cathode contact is ohmic.Schottky diodes turn on at a lower voltage than pn junction diodes and have significantly reduced internal charge storage under forward bias.Reverse BreakdownIncreased reverse bias eventually results in the diode entering the breakdown region, resulting in a sharp increase in the diode current. The voltage at which this occurs is the breakdown voltage, VZ. 2 V <VZ< 2000 V3Half Wave RectificationFigure 2.7A full-wave bridge rectifier: (a) circuit showing the current direction for a positive input cycle, (b) current direction for a negative input cycle, and (c) input and output voltage waveforms4Half-wave rectifier with filter5% regulation is used to measure how well the regulator isPerforming its function.Voltage regulation is the measure of circuit’s ability to maintaineda constant output even when input voltage or load current varies.Demonstration of Zener diode as a voltage regulator67The large value of VCEdecreases theeffective base width W.Since ISis inverselypropositional to W,which cause increase in IC.Bipolar NOR logic gateExample 3.11 Determine current and voltage in the circuit 3.43(b)Rc=1KΩRB=20KΩVBE(on)=0.7VVCE(sat)=0.2Vβ=50Lecture #3 The process by which the quiescent output voltage is caused to fall somewhere the cutoff and saturated values is referred to as biasing.8Example 3.13Q-point has shifted Substantially.Q-point is not stabilizedAgainst the variationβ.910Chapter 4Small-Signal Modeling and Linear Amplification11DC and AC Analysis• DC analysis:– Find dc equivalent circuit by replacing all capacitors by open circuits and inductors by short circuits.– Find Q-point from dc equivalent circuit by using appropriate large-signal transistor model.• AC analysis:– Find ac equivalent circuit by replacing all capacitors by short circuits, inductors by open circuits, dc voltage sources by ground connections and dc current sources by open circuits.– Replace transistor by small-signal model– Use small-signal ac equivalent to analyze ac characteristics of amplifier.– Combine end results of dc and ac analysis to yield total voltages and currents in the network.DC Equivalent for BJT Amplifier• All capacitors in original amplifier circuits are replaced by open circuits, disconnecting vI, RI, and R3from circuit.AC Equivalent for BJT Amplifierk Ω100k Ω3.43k Ω30k Ω1021====RCRRRRBR•Find ac equivalent circuit by replacing all capacitors by short circuits,12Hybrid parameter I: diffusion resistance/input impedance• The diffusion resistance rπis define as the reciprocal of the iB-vBEcurve, which can be find as,Output terminal characteristics of the bipolar transistor: transconductance• If we assume constant collector-emitter voltage the,•As we know• By using the two hybrid parameters (rπ, gm), we can develop a simplified small signal hybrid-π- equivalent circuit for the npn transistor.•Voltage -controlled current source gmvbecan be transformed into current-controlled current source,From equivalent circuit we can write asVoltage gain13• If we include the early effect then collector current in terms of early voltage as,•thenSummary of hybrid-π-model parametersDiffusion resistancetransconductanceCurrent gainOutput resistanceCharacteristics of a CE amplifier• It has moderately low input impedance (1K to 2K)• Its output impedance is moderately large(50K or so)• Its current gain is high• It has very high voltage gain of the order of 1500 or so• It produce very high power gain of the order of 10,000 times or 40dB• It produce phase reversal of input signalUses: many applications because of Large gain in voltage, current and power(by voltage dividerRule)14Small-Signal Analysis of Complete C-E Amplifier: AC Equivalent• Ac equivalent circuit is constructed by assuming that all capacitances have zero impedance at signal frequency and dc voltage source is ac ground.•Assume that Q-point is already known.21RRBR =Small-Signal Analysis of Complete C-E Amplifier: Small-Signal Equivalent3RCRorLR =If we include an emitter resistance in the circuit, the Q-point of the circuit will be less dependant on the transistor current gain β.In order to determine the input impedance Rib, which is the resistance looking into the base of the transistor. We can write the following loop equationThe overall input impedance to the amplifier is nowVoltage gain is less dependant on β15The voltage gain is Substantially reducedWhen an emitter resistoris included!!How can we Improve the voltage gain ?A common collector amplifier has following chracteristics:1 High input impedance (20-500K)2 Low output impedance (50-2000 Ohms)3 High current gain (50-300)4 Voltage gain of less