Crystal oscillator

Quartz Oscillator 3225 20M OSC

The multi-resonant circuit using a pair of complementary transistors is illustrated in Figure 3. The circuit consists of two stages of base-resistor-capacitor coupled inverters. When the power is turned on, the two transistors cannot immediately turn on due to the charging paths of capacitors CA and CB: Ec→R2→CA→Rc1 for CA, and Ec→Rc2→CB→R1 for CB. Once these capacitors are charged to a certain level, the voltages UCA and UCB act as forward bias voltages for the base circuits of the transistors, increasing the base currents Ib1 and Ib2. Due to the positive feedback effect, BG1 and BG2 quickly reach saturation, creating a temporary stable state.

Complementary multi-resonant circuit

At the start of saturation, capacitor CA discharges through the emitter junction of BG2 and resistor Rb2, along with Rc1. After CA discharges, it gets recharged by the reverse voltage from Uc1 (at this point, the left side of CA is positive and the right side is negative). Similarly, CB discharges through Rc2 and the emitter junction of BG1, along with Rb1. As CA and CB discharge, Ube1 increases while Ube2 decreases continuously until both transistors exit saturation and return to the active region. This triggers an "avalanche" type positive feedback effect:

As a result, BG1 and BG2 are turned off, and capacitors CA and CB begin charging again, repeating the cycle. This produces the output pulse wave shown in Figure (b). The circuit is symmetric, meaning CA = CB = C, Rb1 = Rb2 = Rb, R1 = R2 = R, and Rc1 = Rc2 = Rc. 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. Using the parameters from Figure (a), we can calculate t1 = 10 ms, t2 = 750 ms, and the duty ratio (t1/t2) = 1.33%. This circuit is widely used in applications requiring stable oscillation and precise timing control.