Meaning and mechanism
Proper AC/DC operating points help maintain the active region even when the input signal changes in both directions. It is necessary to differentiate between the changes in the signal and the DC value of the operating point.
In the experimental circuit, an increase in the input causes an increase in the collector current and a decrease in the collector output voltage. Consequently, an inversion amplification, where the direction of change in the input and output is opposite, is observed.
Relationships and conditions
The voltage gain within the active region of the given simple model is approximately −Rc/Re. However, when the output reaches the cutoff or saturation boundaries, this linear relationship of the same gain is no longer maintained.
The actual amplifier is affected by the bias circuit, frequency response, and deviations in component current. The ratio gain of the model is an approximation based on defined circuit conditions.
What to observe in the simulation
Active-region voltage gain is −Rc/Re; cutoff and saturation clip the waveform.
The actual bias divider, base current, junction dynamics and bypass capacitor are omitted. Experiment on screen; do not reproduce this with real power supplies, outlets or high voltage.
Key relationships and symbols
Ic ≈ max(0,(Vb−0.7)/Re) · Vout = clamp(Vcc−IcRc,0.2,Vcc)
| Symbol | Meaning | Unit |
|---|---|---|
| Rc, Re | Collector and emitter resistances | Ω |
| Av | output change/input change | Dimensionless |
| Clipping | Waveform clipping at output limits | Phenomenon |
Worked example
With a 6 V supply, what is the output when base voltage falls to 0.55 V and cuts off the transistor?
6 V
Common misconceptions
- Amplification is not the process of creating energy from nothing, but rather the process of controlling a signal using an existing power source.