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# How to get positive and negative power from a single power supply loop

【Introduction】Because the input terminal and the output terminal are shorted together, the output voltage of the NOT gate is equal to the input voltage (Vi=VO); in this way, the NOT gate is forced to work at the center point of the transfer characteristic curve, so the output voltage It is limited to the threshold level of the gate circuit, and its size is equal to half of the power supply voltage. If we use the output terminal of the NOT gate as the DC ground terminal, the power supply voltage VCC can be converted into a dual power supply voltage of ?VCC/2; at this time The non-gate of the circuit acts as a voltage regulator for storing current, and the output impedance of the circuit is low, so the output voltage is relatively stable.

A special method for obtaining positive and negative power in a single power supply circuit

The center of gravity device of the polarity conversion circuit shown in Figure 1 is an ordinary NOT gate. Since the input terminal and the output terminal are shorted together, the output voltage of the NOT gate is equal to the input voltage (Vi=VO); in this way, the NOT gate is forced to work at the center point of the transfer characteristic curve, so the output voltage is limited to The threshold level of the gate circuit is equal to half of the power supply voltage. If we use the output terminal of the NOT gate as the DC ground terminal, the power supply voltage VCC can be converted into a dual power supply voltage of ?VCC/2; the NOT gate at this time. It acts as a voltage regulator for storing current, and the output impedance of the circuit is low, so the output voltage is relatively stable.

The non-gate in the figure can choose common gate circuit such as 74HC00 or CD4069. Considering the limited ability of CMOS non-gate to drive the load, it is best to use several non-gates in parallel to increase its effective output current. Capacitors C1 and C2 in the figure For decoupling, the capacity can be appropriately larger. In the circuit shown in Figure 2, the non-inverting input terminal of the operational amplifier is connected with a symmetrical series resistance voltage divider, and the operational amplifier itself is connected in the form of a voltage follower; it is not difficult to see that the output terminal of the operational amplifier is connected to the The potentials between the voltage dividing points are strictly equal. Since the output terminal of the operational amplifier is grounded, the power supply VCC of the operational amplifier is correspondingly divided into two sets of symmetrical positive and negative power supplies VCC/2. When the output current of the op amp cannot meet the actual demand, it cannot be simply used in parallel like a gate circuit; at this time, the general-purpose low-power op amp can be replaced with a power amplifier-like op amp device with a larger output current, such as the common TDA2030A. Similar to Figure 1, C1 and C2 are both decoupling capacitors, and the capacitor C3 that loads the non-inverting output of the op amp plays the role of suppressing interference and filtering. For most OTL power amplifier devices, a symmetrical bias is generally set inside. According to the above principle, we can use the integrated amplifier to convert the single power supply into bipolar positive and negative power supplies of equal size, the specific circuit is shown in Figure 3 shown. In fact, due to the discreteness of the content parameters and the influence of the bootstrap circuit structure, the voltage at the output end of the integrated amplifier is not the VCC/2 of the jue pair, resulting in an unbalanced positive and negative output voltage. For this, we need to connect a 10-100kΩ potentiometer in series between the positive and negative power supplies, and connect the input end of pin 3 of the LM386 to the middle tap of the potentiometer, while pin 2 remains floating. After the above improvements are made to the circuit, by adjusting the DC input level of the power amplifier, the positive and negative voltage values ​​that are very close in magnitude can be obtained at the output end of the chip.