ENGR 311 – Electronic Devices and CircuitsOctober 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 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.

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

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