Crystal oscillator

Quartz Oscillator 3225 20M OSC

The multi-resonant circuit using a complementary transistor configuration is illustrated in Figure 3. This circuit consists of two stages of base-resistor-capacitor coupled inverters. Upon power-up, the transistors do not turn on immediately because the charging paths for capacitors CA and CB are: Ec→R2→CA→Rc1 and Ec→Rc2→CB→R1, respectively. Once CA and CB reach a certain voltage level, UCA and UCB provide forward bias to the base circuits of the transistors, increasing Ib1 and Ib2. Due to positive feedback, BG1 and BG2 quickly enter saturation, forming a temporary stable state.

Complementary multi-resonant circuit

At the start of saturation, CA discharges through the emitter junction of Rb2 and BG2, along with Rc1 (after CA discharges, it is charged back by Uc1 in reverse, making UcA positive on the left and negative on the right), while CB discharges through Rc2 and BG1’s emitter junction and Rb1. As CA and CB discharge, Ube1 increases, and Ube2 decreases continuously until both transistors exit saturation and return to the active region. This triggers a "avalanche" type positive feedback effect.

This causes BG1 and BG2 to turn off, allowing CA and CB to charge again, and the process repeats. The output pulse waveform shown in Figure (b) is obtained. Since the circuit is symmetric—CA=CB=C, Rb1=Rb2=Rb, R1=R2=R, Rc1=Rc2=R—the pulse width can be calculated as follows:

T1 = C(Rb + rbe) * ln{Ec / [Ubes + (Ec/Rb) * Rc]}

T2 ≈ 0.7 * Rc

The transistor's β should satisfy Rb < β * Rc. With t1 = 10 ms and t2 = 750 ms, the duty cycle (t1/t2) is approximately 1.3%, which can be determined based on the parameters of the circuit in Figure (a). This type of circuit is commonly used in applications requiring stable oscillation and precise timing control.