LOTO Lesson 4: Practice of the common emitter amplifier circuit of transistor 2N3904

The basic amplifier circuits of triodes include common emitter amplifier circuit, common base amplifier circuit and common collector amplifier circuit, among which the common emitter amplifier circuit is the most widely used connection method. The figure below shows the principle of the triode common emitter amplifier circuit, with input on the left and output on the right. The emitter (e) is the common terminal of the input and output, so it is called a common emitter amplifier circuit.

Today's video is to build a transistor amplifier circuit like the one shown above, and observe the waveform of the input and output signals of the entire circuit with the help of the LOTO oscilloscope and signal generator module. Our physical circuit diagram is based on the 2N3904 transistor, Rb=300K, Rc=1K; after many experiments, we have adopted this pair of resistance values ​​to keep the transistor in the amplification area as much as possible and away from the cut-off area and saturation area. The physical diagram of the circuit is shown in the figure below.

The specific process of the video is as follows:

LOTO Lesson 4: Transistor Amplifier Circuit --- Using an Oscilloscope and a Signal Source to Measure Amplification, Cutoff and Saturation

After connecting our circuit, opening the upper computer software of the oscilloscope can clearly see that the output signal is amplified many times compared to the input signal. At the same time, due to the circuit characteristics of the common emitter amplifier circuit, the amplified output signal is the reverse amplification of the input signal. By comparing the waveform phase in the figure, it can be intuitively seen that the output signal and the input signal are amplified in reverse.

Think about it: Why is the output of the triode common-emitter amplifier circuit reversely amplified?

As the base input voltage increases, the base current increases, and the collector current also increases. As the collector current increases, the voltage on the collector resistor increases, and the collector output voltage = power supply voltage minus collector resistor voltage, so the collector output voltage decreases. In other words, as the base input voltage increases, the collector output voltage decreases. That is: reverse.