LC oscillator and common circuits

This article introduces the frequency selection network of the LC oscillator and three common circuits of the LC resonant circuit used in the LC oscillator, namely the transformer feedback LC oscillation circuit, the inductor three-point oscillation circuit, and the capacitor three-point oscillation circuit.

LC oscillator and common circuits

The frequency selection network of the LC oscillator is the LC resonant circuit. Their oscillation frequencies are relatively high, and there are three common circuits.

1. Transformer feedback LC oscillation circuit

Figure 1 (a) is a transformer feedback LC oscillation circuit. Transistor VT is a common emitter amplifier. The primary of transformer T is an LC resonant circuit that plays a frequency selection role, and the secondary of transformer T provides a positive feedback signal to the amplifier input. When the power is turned on, a weak transient current appears in the LC loop, but only the current with the same frequency as the loop resonant frequency f0 can generate a higher voltage at both ends of the loop. This voltage is sent back to the base of transistor V through the coupling of the transformer primary and secondary L1 and L2. As can be seen from Figure 1 (b), as long as the connection method is correct, the feedback signal voltage is the same phase as the input signal voltage, that is, it is positive feedback. Therefore, the oscillation of the circuit is rapidly strengthened and finally stabilized.

The characteristics of the transformer feedback LC oscillation circuit are: wide frequency range, easy to start oscillation, but low frequency stability. Its oscillation frequency is: f0 =1/2π LC. It is often used to generate sine wave signals from tens of kilohertz to tens of megahertz.

2. Inductor three-point oscillation circuit

Figure 2 (a) is another commonly used inductor three-point oscillation circuit. In the figure, inductors L1, L2 and capacitor C form a resonant circuit that plays a frequency selection role. The feedback voltage is taken from L2 and added to the base of the transistor VT. From Figure 2 (b), it can be seen that the input voltage and feedback voltage of the transistor are in phase, satisfying the phase balance condition, so the circuit can oscillate. Since the three poles of the transistor are connected to the three points of the inductor respectively, it is called an inductor three-point oscillation circuit.

The characteristics of the inductor three-point oscillation circuit are:

The frequency range is wide and it is easy to oscillate, but the output contains more high-order harmonics and the waveform is poor.

Its oscillation frequency is: f0=1/2π LC, where L=L1+L2+2M. It is often used to generate sinusoidal wave signals below tens of megahertz.

3. Capacitor three-point oscillation circuit

Another commonly used oscillation circuit is the capacitor three-point oscillation circuit, as shown in Figure 3 (a). In the figure, the inductor L and the capacitors C1 and C2 form a resonant circuit that plays a frequency selection role. The feedback voltage is taken from the capacitor C2 and added to the base of the transistor VT. As can be seen from Figure 3 (b), the input voltage and feedback voltage of the transistor are in phase, satisfying the phase balance condition, so the circuit can oscillate. Since the three poles of the transistor in the circuit are connected to the three points of the capacitors C1 and C2 respectively, it is called a capacitor three-point oscillation circuit.

The characteristics of the capacitor three-point oscillation circuit are: high frequency stability, good output waveform, and the frequency can be as high as 100 MHz or more, but the frequency adjustment range is small, so it is suitable for a fixed frequency oscillator. Its oscillation frequency is: f0=1/2π LC, where C=C1C2C1+C2.

The amplifiers in the above three oscillation circuits all use common emitter circuits. The oscillator with common emitter connection has higher gain and is easy to start. The amplifier in the oscillation circuit can also be connected in the form of a common base circuit.

The oscillation frequency of the common base connected oscillator is relatively high and the frequency stability is good.