In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as the voltage  ‎Emitter degeneration · ‎Characteristics · ‎Bandwidth · ‎Low-frequency voltage. Common-emitter amplifier: The input and output signals both share a connection to the emitter. Before, a small . BJT SPICE model parameters. type npn is. This course introduces students to the basic components of electronics: diodes, transistors, and op amps. It covers the basic operation and some common.


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Redraw the AC equivalent circuit without CE and derive the voltage gain. Capacitors are considered open circuit in DC and therefore are excluded.

  • Common Emitter BJT Applications
  • BJT Common Emitter Amplifier with emitter degeneration
  • The Common-emitter Amplifier
  • AC Analysis

This is to obtain maximum output voltage swing. At bjt common emitter amplifier power maximum light exposurethe solar cell will drive the transistor into saturation mode, making it behave like a closed switch between collector and emitter.

The result will be minimum voltage drop between collector and emitter, or almost zero output voltage.

Common emitter - Wikipedia

In actuality, a saturated transistor can never achieve zero voltage drop between collector and emitter because of the two PN junctions through which collector current must travel. For light exposure levels somewhere between zero and maximum solar cell output, the transistor bjt common emitter amplifier be in its active mode, and the output voltage will be somewhere between zero and full battery voltage.

An important quality to note here about the common-emitter configuration is that the output voltage is inverted with respect to the input signal. That is, the output voltage decreases as the input signal increases.

For this reason, the common-emitter amplifier bjt common emitter amplifier is referred to as an inverting amplifier.


A quick SPICE simulation Figure below of the circuit in Figure below will verify our qualitative conclusions about this amplifier circuit. At the beginning of the simulation in Figure above where the current source bjt common emitter amplifier cell is outputting zero current, the transistor is in cutoff mode and the full 15 volts from the battery is shown at the amplifier output between nodes 2 and 0.

Notice how the output voltage trace on the graph is perfectly linear 1 volt steps from 15 volts to 1 volt until the point bjt common emitter amplifier saturation, where it never quite reaches zero. This is the effect mentioned earlier, where a saturated transistor can never achieve exactly zero voltage drop between collector and emitter due to internal junction effects.

The Common-emitter Amplifier | Bipolar Junction Transistors | Electronics Textbook

What we do see is a sharp output voltage decrease from 1 volt to 0. The lowest the output voltage ever gets in this simulation is 0. In the solar cell light meter example, we were interested in amplifying the DC output of the solar cell to drive a DC meter movement, or to produce a DC output voltage.

However, this is not the only way in which a transistor may be employed as an bjt common emitter amplifier. Often an AC amplifier for amplifying alternating current and voltage signals is desired.

One common application of this is in audio electronics radios, televisions, and public-address systems. Earlier, we saw an example of the audio output of a tuning fork activating a transistor switch.

Transistor switch activated by audio. All we cared about here was turning the lamp on with a sound bjt common emitter amplifier from the microphone, and this arrangement sufficed for that purpose.

BJT Common Emitter Amplifier - Analog Electronics Course

But now we want to actually reproduce the AC signal and drive a speaker. Common emitter amplifier drives speaker with audio frequency signal.

The simulation plots Figure above both the input voltage an AC signal of 1.

What we see here is a full AC sine wave alternating in both positive and negative directions, and a half-wave output current waveform that only pulses in one bjt common emitter amplifier. If we were actually driving a speaker with this waveform, the sound produced would be horribly distorted.

The answer to this question is found by close inspection of the transistor diode current source model in Figure below.

Common emitter

The model shows that base current flow in on direction. Collector current is controlled, or regulated, through the constant-current mechanism according to the pace set by the current through the base-emitter diode.

Note that both current paths through the transistor are monodirectional: