[Nanjing Post and Telecommunications School Competition] Three-level amplifier circuit

Question requirements: To achieve the amplification of a small signal with a peak-to-peak value of 20mv, it is required that the amplified peak-to-peak value is greater than 2V and there is no obvious distortion.
Award information: First prize at the school level.
Team information: Bai Ruixin, Cong Zhigang, Chen Tianhao
Instructors: Hao Xueyuan, Xue Mei
 

2019 National College Student Electronic Design Competition Design Report

1. Design Content Summary
Design and produce an amplifier nonlinear distortion research device. Transistor amplifiers are only allowed to have one input port and one Output port, and can respectively output sinusoidal voltage without obvious distortion, "top distortion" voltage, "bottom distortion" voltage, "bidirectional distortion" voltage, and "crossover distortion" voltage, and can measure and display five outputs Total harmonic distortion of the voltage.
2. Working principle and circuit design
1. Common-emitter amplification circuit
In order to achieve 100 times amplification of small signals, we use a two-stage transistor common-emitter amplifier circuit, and the inter-stage coupling adopts resistance-capacitance coupling, as shown in Figure 2-1 Show:

Figure 2-1 The highlight of the design of the two-stage common-emitter amplifier circuit
is the change of the emitter resistance under high-frequency and low-frequency conditions. We know that the impact of the emitter resistance on the common-emitter amplifier circuit lies in the common emitter resistance. The amplification factor of the radio amplifier will be greatly reduced. The amplification factor of the common emitter amplifier circuit with emitter resistance is:
                      (2.1)
       Under low frequency conditions, the electrolytic capacitor and open circuit, the emitter resistance becomes larger, and the low frequency amplification factor decreases, so that Under low-frequency conditions, the distortion is greatly reduced; under high-frequency conditions, , and are turned on, the emitter resistance is reduced, and the amplification factor is greatly increased. However, under high-frequency conditions, the distortion of the output signal is very small, thus achieving high-low frequency conversion. .
The simulation waveform of the circuit in Multisim is shown in Figure 2-2 below:
 
 

Figure 2-2 Common emitter amplifier circuit output waveform
Figure 2-3 shows the total harmonic distortion measurement of the output waveform:

Figure 2-3
It can be seen from the distortion analysis The distortion is very small, beyond the discernible range of the human eye, and the magnification meets the requirements.
The theoretical calculation of the static operating point of the first-stage common emitter circuit is as follows:
It can be seen that the static operating point is ideal.
The static operating point of the second-stage common emitter circuit is the same and will not be described again here.
2. Distortion circuit
The design of the distortion circuit is shown in Figure 2-4 below:

Figure 2-4 Distortion circuit
It can be seen from the figure that the top distortion, bottom distortion and crossover distortion are all generated by diodes, and the forward conduction of the diode is used. The voltage drop and reverse resistance tend to be infinite, so diode filtering can be used to produce distortion.
3. The relay switching circuit
is shown in Figure 2-5. A triode is used as the driver of the microcontroller. The pin of the microcontroller outputs a voltage signal of 3.3V. The resistor divides the voltage so that the triode reaches a turn-on voltage of 0.7V, causing the diode to conduct. The multimeter The two ends of the relay are connected to both ends of the relay, so that the switch circuit can be turned on when the microcontroller outputs a high level.

Figure 2-5 Relay switching circuit
3. System software design and analysis
The flow chart of the entire system is shown in Figure 3-1:

Figure 3-1 System flow chart