Simple audio amplifier (LM307, μA741)

As shown in the figure, it is a simple audio amplifier circuit. The audio amplifier composed of integrated operational amplifiers has stable operation and is easy to add negative feedback, so it is easy to obtain balanced characteristics. In addition, the crosstalk interference between channels is small, and it is also easy to mix multi-channel inputs. The circuit shown in Figure (a) uses dual power supplies (Vs=±15V), and the static resistances of the in-phase and inverting input terminals to the ground are basically balanced. Its input signal Vi is sent to the in-phase input terminal (pin 3) of LM307N by a 0.1μF coupling capacitor. For the signal source, its input resistance is about 1MΩ, so its equivalent load is very light. The voltage amplification factor of this circuit is about 101 times, and it has good dynamic characteristics. The circuit shown in Figure (b) uses a single power supply mode, because there is no DC level problem of the signal in the AC amplifier, so this power supply mode is desirable. In this circuit, the audio signal K is sent to the inverting input terminal (pin 2) of μA741C through the coupling capacitor C and the resistor R1. As can be seen from the figure, the circuit introduces 100% negative feedback to the DC component (c is quite open circuit to the DC component), so there is no amplification effect on the offset voltage. The voltage amplification factor of this circuit is: Av=-R2/R1=-1000 (i.e. 60dB). However, it should be emphasized that: R1, R2 and capacitor C have low-frequency attenuation and phase shift effects. When the signal frequency is low, the capacity of capacitor C can be appropriately increased.