Common-emitter BJT · small signal → large signal
A bipolar junction transistor (BJT) uses a small base current to control a much larger collector current. In a common-emitter amplifier, a correct DC bias sets the operating point and a small AC input voltage rides on top of it. The result is a large, inverted copy of the input at the output — until you push it too hard and the peaks clip.
Ic = beta · Ib gm = Ic / Vt
Av = −gm · Rc Vout = Vcc − Ic · Rc
The first working transistor amplifier was demonstrated at Bell Labs in December 1947. The same common-emitter principle shown here scales from a single tiny signal stage to the power amplifiers driving loudspeakers and radio transmitters around the world.
Ten common questions about how a common-emitter BJT amplifier turns small signals into large ones.
A bipolar junction transistor (BJT) lets a small base current control a much larger collector current. In a common-emitter amplifier this current-controlling action turns a tiny input voltage into a large output voltage swing across the collector resistor.
The Q-point is the steady DC voltage and current the transistor sits at with no signal applied. It is set by the DC bias. The AC input then wiggles the transistor around this point. A good Q-point sits roughly midway on the load line so the signal can swing up and down equally.
Voltage gain A_v is how many times larger the output voltage is than the input.
For a common-emitter stage Av = −gm · Rc, where gm is the
transconductance and Rc is the collector resistor. The minus sign means the
output is inverted relative to the input.
When the input voltage rises, base current rises, collector current rises, and the voltage dropped across Rc rises. Because the output is taken between Rc and the transistor, the collector voltage falls. So a rising input produces a falling output: a 180-degree phase inversion.
Clipping happens when the signal tries to swing beyond the supply rails. If the transistor saturates, the output cannot drop below about 0.2 V; if it cuts off, the output cannot rise above the supply. The peaks of the waveform get flattened, adding harmonic distortion.
Transconductance gm = Ic / Vt relates a small change in
base-emitter voltage to the resulting change in collector current, where Vt is
the thermal voltage (~26 mV at room temperature). Larger collector current gives
larger gm and therefore larger gain.
Beta is the current gain of the transistor: Ic = beta · Ib. A
typical small-signal BJT has a beta of around 100, meaning the collector current
is 100 times the base current. Beta lets a tiny base current control a large
collector current.
Centre the Q-point so the quiescent collector voltage is about half the supply, then increase the input amplitude until the peaks just begin to flatten. Backing off slightly from that point gives the maximum clean swing.
Gain is gm · Rc, so a larger Rc produces a bigger output voltage
swing for the same change in collector current. The trade-off is that a large Rc
lowers the quiescent collector voltage and reduces the available output swing
before clipping.
It uses the standard small-signal and large-signal BJT equations:
Ic = beta · Ib, gm = Ic / Vt, and
Av = −gm · Rc, with saturation and cutoff limits. Real designs add
bias networks, emitter degeneration and coupling capacitors, but the core
amplifying behaviour shown here is genuine.