In one line
In common emitter (CE) configuration, the input characteristic is \( I_B \) versus \( V_{BE} \) at constant \( V_{CE} \), and the output characteristic is \( I_C \) versus \( V_{CE} \) at constant \( I_B \), showing cut-off, active and saturation regions.
Explained simply
"Common emitter" means the emitter is shared by input and output: the signal goes in between base and emitter and comes out between collector and emitter. It is the most used arrangement because it amplifies both current and voltage, a bit like a gear system that multiplies both speed and force.
Technically, CE gives current gain \( \beta \), high voltage gain, high power gain, moderate input resistance (around 1 kΩ) and moderate-high output resistance, and its output is 180° out of phase with the input.
How it works
- Circuit: \( V_{BB} \) with a series resistor feeds the base; \( V_{CC} \) with a collector resistor or meter feeds the collector; emitter is common to both. Meters measure \( V_{BE}, I_B, V_{CE}, I_C \).
- Input characteristic: keep \( V_{CE} \) fixed, vary \( V_{BE} \), record \( I_B \). It looks like a forward diode curve: almost no current below about 0.5–0.7 V, then a steep rise. A higher \( V_{CE} \) shifts the curve slightly right (base-width modulation / Early effect).
- Output characteristic: keep \( I_B \) fixed, vary \( V_{CE} \), record \( I_C \). \( I_C \) rises quickly for very small \( V_{CE} \), then becomes nearly flat at about \( \beta I_B \). Repeat for several \( I_B \) values to get a family of curves.
- Active region: flat portion; \( J_E \) forward, \( J_C \) reverse; used for amplification.
- Saturation region: very small \( V_{CE} \) (about 0.2 V); both junctions forward biased; transistor acts as a closed switch.
- Cut-off region: below the \( I_B = 0 \) curve; only a tiny leakage \( I_{CEO} \) flows; transistor acts as an open switch.
Key formula
\( \beta_{dc} = \dfrac{I_C}{I_B}, \quad r_i = \dfrac{\Delta V_{BE}}{\Delta I_B}\Big|_{V_{CE}}, \quad r_o = \dfrac{\Delta V_{CE}}{\Delta I_C}\Big|_{I_B} \)
\( r_i \) = input dynamic resistance (\( \Omega \)), \( r_o \) = output dynamic resistance (\( \Omega \)), \( \beta_{dc} \) = DC current gain (no unit).
Quick example
Reading two output curves: at \( V_{CE} = 5 \) V, the \( I_B = 20\,\mu \)A curve gives \( I_C = 2 \) mA and the \( I_B = 40\,\mu \)A curve gives 4 mA, so \( \beta = \Delta I_C/\Delta I_B = 2\text{ mA}/20\,\mu\text{A} = 100 \).
Common mistakes
- Swapping the axes or the constant parameter of input and output characteristics.
- Labelling the steep initial part of the output curve as the active region (it is saturation).
- Saying CE has no voltage gain (that is common collector).
How MU asks it
Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Draw the circuit and explain the input and output characteristics of a CE configuration" (5–8 marks). For full marks draw the test circuit with meters, draw both characteristic families with axes and parameters labelled, mark the cut-off, active and saturation regions, and state the properties of CE (gain, phase reversal, resistances).