Modulator circuit principle

1. Overview The modulator is an important link in the modulated DC amplifier circuit. It can be seen from the box in the figure below: after the DC signal ui to be amplified passes through the modulator, it becomes an AC signal UA; after being amplified by the AC amplifier, it is finally converted into a DC output signal UO by the demodulator; the oscillator generates a switching signal UC ; Used to control the sampling action of the modulator. Since the signal amplification task is mainly completed by the AC amplifier, and the zero drift of the AC amplifier is so small that it can be ignored, the zero drift of the modulator and demodulator can also be made very small. Therefore, the modulated DC amplifier can be used to amplify weak signals. For DC signals, modulators usually come in three forms: mechanical modulator (mechanical chopper), transistor modulator, and field effect tube modulator. According to the circuit form, it can be divided into two types: parallel modulator and series and parallel modulator. The latter has superior performance than the former, but has a complicated structure. 2. Modulator Principle The following figure is the schematic diagram of the modulator. Because the switch K load is connected in parallel, it is called a parallel modulator. The working process is as follows: If K is disconnected within 0-T/2 time, point A obtains the level UmA ; If K is turned on within (T/2)-T time, point A will be grounded; after that, the random switch K will be on and off periodically, and a pulsating DC voltage UA will be obtained at point A (as shown in the figure below). UA can It is decomposed into a DC component UAO and an AC component UA-O. After passing through the DC blocking capacitor C, UAO lands on the capacitor, and the AC component UA- is sent to the load RL, that is, UO=UA-O 3. Parallel modulator 1. Transistor modulator The transistor modulator uses a transistor as a switch. Its circuit and waveform are as shown in the figure below. The base of transistor BG is connected to the control voltage Ua (square wave). When Ua is a negative half-wave, BG stops , then Ui charges C, and the charging current flows through RL from top to bottom, so UO is positive; conversely, when Ua is a positive half-wave, BG is saturated, then C is discharged through BG and RL, and the discharge current flows through RL from bottom to top. , so UO is negative. As UO changes alternately, the output terminal UO gets an alternating square wave voltage, whose value is proportional to the input voltage and whose frequency is the same as Ua. 2. Field effect tube modulator The field effect tube modulator uses a field effect tube as a switch. Its circuit and waveform are as shown in the figure below. It can be seen from the figure that when the negative pulse voltage Ua is applied to the gate of BG, it can regularly The ground controls the opening and closing of the field effect transistor, thereby changing the input DC voltage Ui into the AC output voltage UO. The working process is the same as that of the transistor modulator. 4. Series-parallel modulator The figure (a) below shows the series-parallel modulator and its waveform diagram. BG1 is connected in series with load RL, and BG2 is connected in parallel with RL. The two field effect transistors BG1 and BG2 are controlled by voltages Ua1 and Ua2 respectively; and Ua1 and Ua2 have opposite phases relative to the location. If at 0-(T/2) time When Ua1 is positive, Ua2 is negative, causing BG1 to be turned on and BG2 to be turned off. At this time, Uw=Ui. If Ua1 is negative within (T/2)/T time, Ua2 is positive, making BG1 to be turned off and BG2 to be turned on. , at this time, UW=0. After passing through the coupling capacitor C2, the DC component in UW is filtered, and the AC modulated square wave is obtained at both ends of the negative-cut RL. In the figure (b) below, the series and parallel modulators of the dual DC input signals (Ui and Uf) and Its waveform diagram shows that it can complete the two tasks of modulation and comparison at the same time. The output voltage UO is proportional to the amplitude difference of the two input signals. The working principle is the same as above.