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ENGR 311 – Electronic Devices and Circuits
October 16, 2000
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 baseemitter 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 baseemitter 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.
Ic – Vce Characteristic for an npn Transistor
Ic- Vbe Characteristics
Exercises
Solutions
Example 1 Beta = 100
Example 2 - Beta = 100
Example 3 - Beta = 100
Example 5
Determine the voltages at all nodes and current through all branches.
Assume beta 1 and beta2 = 200. Assume Q1 is in the active mode.
Tolerances in Bias Circuits
Worst Case Analysis
V = +12 V
IC
CC
22k 
R
2
R
R
C
EQ
R
C
22k 
IB
36 k 
V
12 k 
VEQ
Q1
R
4V
18 k 
CC
IE
16 k 
R
1
16 k 
R
+12 V
E
E
Study the operation of the transistor considering tolerances (worst case anaysis) in the
circuit. Assume that the 12V power supply has a 5% tolerance and the resistors have 10%
tolerance. Assume also that the voltage drop in REQ can be neglected, and beta is large.
VEQ (max, min)
IC (max, min)
VCE (max, min)
Monte Carlo Analysis
Perform Monte Carlo Analysis on previous circuit assuming the random values to
Vcc, R1, R2, Rc, Re, and beta. (Use Excel and/or Pspice).
Calculate
VEQ
REQ
IB
IC
IE
VC = VCC – IC.RC – IE.RE
Electronic Devices and Circuits – 11/5/00
Monte Carlo Analysis – Using Pspice
Probe Output
Ic(Q), Ib(Q), Vce