Harmonic signal generator with soft limiter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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Most oscillator circuits have a non-linear signal level control that maintains oscillation of a certain amplitude with minimal output distortion. Some circuits use the amplitude of the output signal to control the resistance of a circuit element (eg, a JFET) operating in the triode region, while others control it through an amplitude limiter circuit. For example, Encarna Garcia (Spain) proposed a limiter circuit with a "soft" characteristic to minimize the non-linear distortion of the output signal.
The circuit (Fig. 1) contains a simple step-phase-shifting RC oscillator and an amplitude limiter circuit. The frequency F0 of the output voltage Vout is described by the formula:
F0=SQRT(6)/(2*PI*R*C) = SQRT(6)/(2*PI*10кОм*1нФ).
Fig. 1
The inverting amplifier block contains a differential transistor pair Q1 and Q2 which defines the non-linear transfer characteristic. On the operational amplifier IC1, a current-to-voltage converter is made.
For oscillation to occur, the gain of the inverting amplifier must be greater than 29. Choosing the appropriate bias current IEE and emitter pair resistors RE1, RE2, and RE3 provides gain with a non-linear transfer characteristic Vout from Vin (Figure 2).
Fig. 2
Rice. 3. Generator output voltage reaches full amplitude in approximately 400 µs (15 cycles after startup)
Rice. 4. The oscillator output spectrum shows only a small amount of third harmonic output
At low values of the input voltage, the transfer characteristic of the amplifier is almost linear, at large values, the operation of transistors Q1 and Q2 occurs in a non-linear region, reducing the gain of the amplifier (creates a gradual bend in the transfer characteristic).
The current regulator on transistors Q3 and Q4 controls the operation of the operational amplifier IC1, which generates the output signal Vout.
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