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## EE 334 Midterm Review

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**Diode: Why we need to understand diode?**• The base emitter junction of the BJT behaves as a 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 behavior**• The four major reason why the actual diode do not correspond exactly to the ideal. • Ohmic resistance and contact resistance in series with the diode cause the VI • curve to become linear at high forward current. • Avalanche or Zener breakdown take place at high reverse voltage, causing an • abrupt increase in reverse current. • 3. Surface contaminants cause an ohmic layer to form across the junction, which is • Increasing 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.**Schottky Barrier Diode**One semiconductor region of the pn junction diode can be replaced by a non-ohmic rectifying 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 Breakdown**Increased 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 V**Figure 2.7**A 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 waveforms**Voltage regulation is the measure of circuit’s ability to**maintained a constant output even when input voltage or load current varies. % regulation is used to measure how well the regulator is Performing its function.**The large value of**VCE decreases the effective base width W. Since IS is inversely propositional to W, which cause increase in IC.**Bipolar NOR logic gate**Example 3.11 Determine current and voltage in the circuit 3.43(b) Rc=1K RB=20K VBE(on)=0.7V VCE(sat)=0.2V β=50 Lecture #3**The process by which the quiescent output voltage is caused**to fall somewhere the cutoff and saturated values is referred to as biasing.**Q-point has shifted**Substantially. Q-point is not stabilized Against the variation .**DC 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 R3 from circuit.**AC Equivalent for BJT Amplifier**• Find ac equivalent circuit by replacing all capacitors by short circuits,**Hybrid parameter I: diffusion resistance/input impedance**• The diffusion resistance r is define as the reciprocal of the iB-vBE curve, 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 (r, gm), we can**develop a simplified small signal hybrid-- equivalent circuit for the npn transistor. • Voltage -controlled current source gmvbe can be transformed into current-controlled current source,**From equivalent circuit we can write as**Voltage gain**If we include the early effect then collector current in**terms of early voltage as, then**Summary of hybrid--model parameters**Diffusion resistance Current gain transconductance Output resistance