Amplitude modulation circuit is to add modulation signal and carrier signal to a nonlinear element (such as crystal diode or triode) at the same time and transform it into a new frequency component through nonlinear transformation, and then select the required frequency component by resonance circuit.
The amplitude modulation circuit is divided into diode amplitude modulation circuit, transistor base amplitude modulation circuit, emitter amplitude modulation circuit and collector amplitude modulation circuit.
1. Base amplitude modulation circuit
Figure 1 shows the transistor base amplitude modulation circuit. The carrier signal is added to the base of BG through high-frequency transformer T1. The low-frequency modulation signal is connected in series with high-frequency carrier through an inductance coil L. C2 is a high-frequency bypass capacitor, C1 is a low-frequency bypass capacitor, and R1 and R2 are bias voltage dividers. Due to the nonlinear effect of ic = f (Ube) relationship curve of transistor, collector current IC contains various harmonics The amplitude component is selected through the collector tuning loop. The advantage of the AM amplitude modulation circuit is that the low frequency modulated signal power is low, so the low frequency amplifier is relatively simple. Its disadvantage is that it works under voltage condition, and its collector efficiency is low, so it can not make full use of the energy of DC power supply.
2. Emitter amplitude modulation circuit
Fig. 2 is the emitter amplitude modulation circuit. Its principle is similar to that of the base, because the voltage applied between the base and the emitter is about 1V, while the collector power supply voltage is more than ten to dozens of volts. The influence of modulation voltage on the collector circuit can be ignored. Therefore, the working principle and characteristics of emitter amplitude modulation and base amplitude modulation are similar.
3. Collector amplitude modulation circuit
Figure 3 is the collector amplitude modulation circuit. The low-frequency modulation signal is introduced from the collector. Because it works under the over-voltage state, it has high efficiency, but the nonlinear distortion of the modulation characteristics is serious. In order to improve the modulation characteristics, nonlinear compensation measures can be introduced in the circuit to make the input excitation voltage change with the collector power supply voltage. For example, when the collector power supply voltage decreases, the excitation voltage will change On the contrary, when the collector power supply voltage increases, it will not enter the under voltage area. Therefore, the amplitude modulator always works in the weak over-voltage or critical state, which can not only improve the modulation characteristics, but also have high efficiency. The circuit to achieve this measure is called dual collector amplitude modulation circuit.
The modulation characteristics can also be improved by using the Collector Emitter dual amplitude modulation circuit in Fig. 4. Pay attention to the transformer with the same name. When modulating the positive half wave of the signal, although the collector power supply voltage increases, the base bias also becomes positive, which prevents the transformer from entering the undervoltage working state; when modulating the negative half wave of the signal, although the collector voltage decreases, the base extreme bias voltage also becomes negative, which will not enter the strong overvoltage area, so as to maintain the operation under critical and weak overvoltage conditions.
Figure 1. Base amplitude modulation circuit
Figure 2. Emitter amplitude modulation circuit
Figure 3. Collector amplitude modulation circuit
Figure 4. Dual amplitude modulation circuit