Adjustment and test of RC bridge oscillator circuit

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1. General method for designing RC bridge oscillator circuit (1) Selection of integrated op amp The choice of op amp, except for high input resistance and low output resistance, the most important thing is that the gain-bandwidth product of the op amp should meet the following conditions. , which is

Since the amplitude of the oscillation output is relatively large, the integrated operational amplifier operates in a large signal state, so the conversion rate S _{R is required to be} satisfied.

S _R ≥ω _o U _om

(2) Determination of the value of the frequency selective network component should be based on the oscillation frequency Æ’ ₀ = 1/2 RC to choose the size of the RC. In order to reduce the influence of the input impedance of the integrated op amp on the oscillation frequency, a smaller R should be selected; however, in order to reduce the influence of the output impedance of the integrated op amp on the oscillation frequency, it is desirable to have a larger R. Usually, the input resistance of the integrated operational amplifier is relatively large, so R can be taken larger, and generally can take several thousand ohms to several tens of kilo ohms. Capacitor C should generally be larger than a few hundred picofarads to reduce the influence of circuit parasitic capacitance on the oscillation frequency, but the capacitance value is too large to use electrolytic capacitors. Therefore, C can be selected between several hundred picofarads to one microfarad. In order to improve the stability of the oscillation frequency, it is generally preferred to use a resistor with good stability and high precision (such as E24 series RJ type resistor and E96 series high precision resistor) and a capacitor with low dielectric loss.
(3) Determination of the value of the negative feedback circuit component The size of the negative feedback circuit component parameter will determine the gain after the closed loop. The closed-loop gain is large, the starting is easy, the output amplitude is large, but the oscillating waveform is easy to produce distortion; the closed-loop gain is small, the output waveform is good, but the amplitude is small and the vibration is easy to stop. In order to obtain a stable oscillation waveform with a certain amplitude and little distortion, a nonlinear resistor is usually used to form a negative feedback resistor. The selection of the stabilizing diode should be noted that silicon diodes should be used in consideration of the temperature stability of the amplitude.
2. Debugging steps of RC bridge type oscillating circuit (1) Check the circuit wiring. Special check should be made on whether there is a short circuit at the output of the integrated operational amplifier. If the positive and negative power supplies are connected incorrectly, confirm that there is no error and then close the DC power supply.
(2) Observe the voltage waveform at the output with an oscilloscope.
(3) If the oscillation waveform is severely distorted, check the diode characteristics.
(4) Adjustment of the oscillation frequency. Fixed capacitor C, change resistor or fixed resistor R, change C (serial-parallel R and C should be adjusted synchronously) until the oscillation frequency reaches the required level.