Because the current gain is typically unknown or varies greatly with temperature, time, collector–emitter
potential, and other factors, good designs should not depend on it. In this laboratory, we assume that is
sufficiently large (i.e.,amplification ≫ 1, where amplification ≈ 100 in our laboratory) so that
iB ≈ 0 and iC ≈ iE.
These simple rules are similar to the rules we use with operational amplifiers. The analysis approach usually
follows these steps:
1. Calculate the transistor base potential vB by assuming that no current enters the base (i.e., iB ≈ 0).
2. Calculate the potential vE at the emitter of the transistor using vB. For an npn transistor,
vE = vB − 0.65V,
and for a pnp transistor,
vE = vB + 0.65V.
3. Calculate the emitter current iE using the emitter voltage vE and the rest of the circuit.
4. Assume that iC ≈ iE and analyze the rest of the circuit.
• Because we know vE, we usually know iE as well. So our iE dictates what iC should be.
However, keep these notes in mind.
• For an npn transistor, active mode requires vC − vE > 0.2V. For a pnp transistor, active mode
requires vE − vC > 0.2V. If this condition is violated, the transistor is saturated, and the analysis
cannot continue using these simple rules. In design problems, change parameters (e.g., resistors, supply
rails, etc.) to prevent saturation.
• Sometimes it’s easier to find vE first and use it to calculate vB.
• How “small” iB must be to neglect its effect depends on the circuit. In particular, iB × RB must be
very small, where RB is the the Th´evenin equivalent resistance looking out of the transistor base.
