Capacitor three-point oscillator circuit diagram

Crystal oscillator

Quartz Oscillator 3225 20M OSC

Capacitor three-point oscillator circuit

This circuit is suitable for generating signals above tens of megahertz and is commonly 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, C2, and the base has a large capacitance (1000~2000pF), which is AC grounded. effect. Since the values ​​of the inductor and the capacitor are relatively small, the inter-electrode capacitance of the triode and the inter-turn capacitor of the inductor cannot be ignored in some cases. Their contribution to the total capacitance is approximately a few picofarads.
Let the interelectrode capacitance of the triode and the inter-turn capacitor of the inductor and the total equivalent capacitance of the other distributed capacitors be C0. For the convenience of calculation, choose C0=2pF.


The three-point oscillating circuit of the capacitor is also called the Cobbitz oscillating circuit. As shown in Figure Z0808, the structure is similar to the three-point oscillating circuit of the inductor, except that the inductor and capacitor are interchanged. In order to form a DC path of the collector circuit, a resistor RC is added. The AC path of this circuit is shown in Figure Z0809. It can be seen that it conforms to the principle of "shooting the same base" of the three-point oscillating circuit and satisfies the phase equilibrium condition of the self-excited oscillation.
Under the condition that the Q value of the LC resonant circuit is sufficiently high, the oscillation frequency of the circuit is

among them

The characteristic of this oscillating circuit is that the oscillating frequency can be made higher, generally up to 100MHz.

In the above, since the impedance of the higher harmonics of C2 is small, the higher harmonic components in the feedback voltage are smaller, and thus the oscillation waveform is better. The disadvantage of the circuit is that the frequency adjustment is inconvenient. This is because when the capacitance is adjusted to change the frequency (even if C1 and C2 are double-connected variable capacitors), C1 and C2 are also difficult to scale, which causes instability of the circuit performance. Therefore, the circuit is only suitable for generating a fixed frequency oscillation.
A three-point capacitor circuit with an integrated op amp, as shown in Z0810. It can be proved that the oscillation frequency is: