2W×2 power amplifier circuit using LM1877

The circuit uses the dual integrated operational amplifier LM1877 as the amplifier device. As can be seen from the figure, the circuit is a symmetrical structure with two signals added to the in-phase input terminals of the two operational amplifiers of LM1877 respectively. The output terminals are connected in the same way: a high-frequency filter circuit is composed of a 2.7Ω resistor and a 0.1μF capacitor to prevent high-frequency self-excitation; the coupling capacitor Co (470μF) and the load resistor ZL (speaker: 8Ω) convert the output power into sound output, and the size of Co and ZL also determines the lower limit cutoff frequency fL of the circuit. The relationship between the three is: fL=1/(2πZLCo). For the parameters shown in the figure, fL=42Hz. In the circuit, the 100kΩ potentiometer at the input end is the volume control element, and the 0.1μF capacitor is the coupling element (blocking DC and passing AC). Since the two operational amplifiers in LM1877 use the same-phase input mode, their input impedance is very high, and the voltage amplification factor of the circuit is determined by the feedback branch resistors R2 and R1, that is, Av≈R2/R1=100kΩ/2kΩ=50. The power supply voltage of the circuit is 20V, the output power of each channel is 2W, and the power supply current is about 500mA, so when there are two outputs, the power supply current can reach 1A.

The operating range of the power supply voltage of LM1877 is very wide, about 6~24V. When low power output is required, the power supply voltage can be appropriately reduced. Its open-loop input differential resistance is about 4MΩ.