Download ENGR 311 - Bipolar Junction Transistors

ENGR 311 - Bipolar Junction Transistors
PSpice Simulation and Lab Instrumentation - Laboratory # 5 (Part 1)
The objective of this lab is to investigate the characteristics and operation of the bipolar junction
transistor (BJT). See page 4 for some simple rules of operation.
1.
2.
3.
4.
Generating the static characteristics for a simulated transistor using PSpice.
The determination of .
Gathering data for the static characteristics
The dc analysis of some BJT circuits.
Experiment 1
Generate a set of three common emitter characteristic curves for the 2N3904 transistor using PSpice. The curves
should be for IB = 20A, 60A, and 100A and VCE = 0 - 10V.
Use nested simulations
Experiment 2
For the circuit below investigate the collector current as a function of VBE and ambient
temperature.
Experiment 3
For the circuit below investigate the collector current as a function of the VCE and VBE.
you can estimate VA (Early Voltage) from PSpice Simulation.
See if
Experiment 3
a - Verify the solution of example Exercise 4.8.
b - Investigate the temperature dependence of the circuit.
c – Explain differences between calculated and simulated results.
d – Investigate transistor model (Webpage Specs and PSpice Model)
Experiment 4
Solve the circuit below (beta = 100) analytically (use MathCAD), and then verify solution via
PSpice. Follow instructions from example 4.7 (page 248). Use Thevenin’s theorem to simply
circuit and then the loop equation for IE.
Experiment 5
Implement the MathCAD iterative solution for solving the circuit below. Assume beta 1 and
beat 2 = 100. Use values from previous case for the first part of the circuit (since they are
identical).
Follow Example 4.8 (very carefully) for understanding the nature of the solution of multitransistor circuits.
Transistor Model: Current Amplifier
A Summary For Clarification (assume npn for the following general rules/properties – for pnp
reverse polarities)
Rules / Properties
1 – The collector must be positive than the emitter.
2 – The base-emitter and base-collector circuits behave like diodes. Normally the base-emitter
diode is conducting and the base-collector diode is reverse-biased
3 – When 1 and 2 are obeyed Ic is proportional to Ib (Ic = beta . Ib)
Both Ib and Ic follow to the emitter.
Note: the collector current is not due to forward conduction of the base-collector diode; that
diode is reverse-biased. Just think of it as “transistor action.”
Property 3 gives the transistor its usefulness: a small current flowing into the base controls a
much larger current flowing into the collector.
Note the effect of property 2. This means you can’t go sticking a voltage across the base-emitter
terminals, because an enormous current will flow if the base is more positive than the emitter by
more than about 0.6 to 0.8 volt. This rule also implies that an operating transistor has Vb = ~ Ve
+ 0.6 (Vb = Ve + Vbe) (for an npn).
Let me emphasize again that you should not try to think of the collector current as diode
conduction. It isn’t, because the collector-base diode normally has voltages applied across it in a
reverse direction. Furthermore, collector current varies very little with collector voltage (it
behaves like a not-too-great current source), unlike forward diode conduction, where the current
rises very rapidly with applied voltage.
Current flow
The forward bias on the base-emitter junction will cause current flow across this junction.
Current will consist of two components: electrons injected from the emitter into the base, and
holes from the base into the emitter.
The electrons injected from the emitter into the base are minority carriers in the p-type base
region. Because the base is usually very thin the excess minority carriers (electron) concentration
in the base will have an almost straight-line profile. The electrons will reach the boundary of the
collector-base depletion region. Because the collector is more positive than the base these
electrons will be swept across the CB junction region into the collector. They are then “collected”
to constitute the collector current. By convention the direction of ic will be opposite to that of the
electron flow; thus ic will flow into the collector terminal.