Amplifier Configurations and Their Limits

The Three Fundamental Configurations

Configuration	Common Terminal	Voltage Gain	Current Gain	Ze	Zs	Typical Use
Common emitter	emitter	high (about -100)	high	medium	medium/high	general amplification
Common collector (follower)	collector	close to 1	high	high	low	impedance matching
Common base	base	high	close to 1	low	high	high frequencies

Bandwidth and Distortion

A real amplifier is linear with constant gain only within a range of frequencies (bandwidth, defined at -3 dB) and for a limited input amplitude. Beyond a certain amplitude, the instantaneous operating point leaves the active zone: clipping occurs, a source of unwanted harmonics and distortion.

Operating Classes

• Class A: the transistor conducts over the entire cycle (360 deg), high linearity but low efficiency (less than 25 percent in theory).
• Class B: conduction over only half a period, better efficiency but crossover distortion.
• Class AB: a compromise between A and B, widely used in audio.

Negative Feedback

Injecting a fraction of the output signal, in phase opposition, back into the input (negative feedback) reduces the gain but improves stability, linearity, and bandwidth: this is the principle used in most transistor amplifiers and operational amplifiers.

Common Pitfall

Believing that increasing Rc increases the gain Av = -Rc/re indefinitely: beyond a certain value, Vce0 becomes too small, the transistor enters saturation, and the output signal is clipped before even reaching the expected amplitude.