Capacitor three-point oscillator circuit diagram

Capacitor three-point oscillator circuit

This circuit is suitable for generating signals above tens of megahertz and is often used as a radio frequency oscillator. The second figure is the equivalent circuit diagram of the LC oscillation circuit. It can be seen from the figure that the oscillation frequency of the circuit is determined by L, C, C1, and C2. The base has a large capacitor (1000~2000pF) that serves as an AC ground. Since the values ​​of inductance and capacitance are relatively small, in some cases the inter-electrode capacitance of the transistor and the inter-turn capacitance of the inductor coil cannot be ignored. Their contribution to the total capacitance is about several pico-farads.

Suppose the total equivalent capacitance of the transistor inter-electrode capacitance, the inductor coil inter-turn capacitance and other distributed capacitance is C ₀ . For the convenience of calculation, we choose C ₀ = 2pF. Then

    The capacitor three-point oscillation circuit is also called the Colpitts oscillation circuit, as shown in Figure Z0808. Its structure is similar to the inductor three-point oscillation circuit, except that the inductor and capacitor are interchanged. In order to form the DC path of the collector loop, a resistor RC is added . The AC path of the circuit is shown in Figure Z0809. It can be seen that it conforms to the construction principle of the three-point oscillation circuit "emitter-base-reflector" and meets the phase balance condition of self-excited oscillation.
     Under the condition that the Q value of the LC resonant circuit is high enough, the oscillation frequency of the circuit is . The characteristic of this     oscillation circuit is that the oscillation frequency can be made higher, generally reaching more than . Since C2 has a small impedance to high-order harmonics, the high-order harmonic components in the feedback voltage are small, so the oscillation waveform is better. The disadvantage of the circuit is that the frequency adjustment is inconvenient. This is because when adjusting the capacitor to change the frequency (even if C1 and C2 use double-connected variable capacitors) , C1 and C2 are difficult to change proportionally, which causes the instability of the circuit working performance. Therefore, this circuit is only suitable for generating fixed-frequency oscillations.     The capacitor three-point oscillator circuit composed of an integrated operational amplifier is shown in Z0810. It can be proved that its oscillation frequency is: