Single-supply principle of operational amplifier

Most operational amplifiers require dual power supplies (positive and negative power supplies), and only a few operational amplifiers can work under single power supply conditions, such as LM358 (dual op amp), LM324 (quad op amp), CA3140 (single op amp), etc. It should be noted that single-power-supply operational amplifiers can work not only under single power supply conditions, but also under dual power supply conditions. For example, LM324 can work under +5~+12V single power supply conditions, and can also work under +5~±12V dual power supply conditions.

In some AC signal amplification circuits , a power bias circuit can also be used to reduce the static DC output voltage to half of the power supply voltage. A single power supply is used for operation, but both the input and output signals need to add AC coupling capacitors . An inverting amplifier powered by a single power supply is shown in Figure 1(a), and its op amp output waveform is shown in Figure 1(b).

The gain of this circuit is Avf = -RF/R1. When R2 = R3, the static DC voltage Vo (DC) = 1/2Vcc. The values ​​of coupling capacitors Cl and C2 are determined by the required low-frequency response and the input impedance (for C1) or load (for C2) of the circuit. Cl and C2 can be determined by the following formula: C1 = 1000/2πfoRl (μF); C2 = 1000/2πfoRL (μF), where fo is the required minimum input frequency. If R1 and RL are expressed in kΩ and fO is expressed in Hz, the units of C1 and C2 are μF. Generally speaking, R2 = R3 ≈ 2RF.

[page] Figure 2 is a single-power supply summing operational amplifier. The output voltage of this circuit is Vo = -RF (V1/R1 + V2/R2 + V3/R3). If R1 = R2 = R3 = RF, then Vo = -(V1 + V2 + V3). It should be noted that using a single power supply has a certain price. It is a Class A amplifier, and the loss is large when there is no signal input.

Questions to think about (1) Figure 3 is a single-supply inverting amplifier circuit with a gain of 10, an input impedance of 10kΩ, a low-frequency response of approximately 30Hz, and a driving load of 1kΩ. What is the peak-to-peak value of the undistorted input voltage of this circuit? (Hint: The typical input and output characteristics of a general operational amplifier are shown in Figure 4); (2) Figure 5 is a single-supply differential amplifier. If the input voltage is a 50Hz AC voltage, V1 = 1V, V2 = 0.4V, what is its output voltage?