Adjustment and testing of RC bridge oscillation circuit

 1. General methods for designing RC bridge oscillation circuit

(1) Selection of integrated operational amplifier

When selecting an op amp, in addition to requiring 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:

Because the oscillation output amplitude is relatively large and the integrated operational amplifier works in a large signal state, the conversion rate SR is required to meet

SR≥ωoUom

(2) Determination of frequency selection network component values

At this time, the oscillation frequency should be ƒ0=1/2

RC to select the size of RC. In order to reduce the impact of the input impedance of the integrated operational amplifier on the oscillation frequency, a smaller R should be selected; however, in order to reduce the impact of the output impedance of the integrated operational amplifier on the oscillation frequency, R should be larger. Usually the input resistance of integrated operational amplifiers is relatively large, so R can be larger, and generally the resistance can be from several thousand ohms to tens of kiloohms. Capacitance C should generally be larger than several hundred picofarads to reduce the impact of circuit parasitic capacitance on the oscillation frequency. However, it is inappropriate to use electrolytic capacitors if the capacitance value is too large. Therefore, C can be chosen from a few hundred picofarads to 1 microfarads. In order to improve the stability of the oscillation frequency, resistors with better stability and higher precision (such as E24 series RJ-type resistors and E96 series high-precision resistors) and capacitors with smaller dielectric losses are generally selected.

(3) Determination of negative feedback circuit component values

The size of the parameters of the negative feedback circuit components will determine the gain after closed loop. The closed-loop gain is large, the oscillation is easy to start, the output amplitude is large, but the oscillation waveform is prone to distortion; the closed-loop gain is small, the output waveform is good, but the amplitude is small and the oscillation is easy to stop. In order to obtain a stable oscillation waveform with a certain amplitude and small distortion, a nonlinear resistor is usually used to form a negative feedback resistor. When choosing amplitude stabilizing diodes, attention should be paid to: considering the temperature stability of the amplitude, silicon diodes should be selected.

2. Debugging steps of RC bridge oscillation circuit

(1) Check the circuit wiring, especially check whether the output terminal of the integrated operational amplifier is short-circuited, whether the positive and negative power supplies are connected incorrectly, and turn on the DC power supply after confirming that there are no errors.

(2) Use an oscilloscope to observe the voltage waveform at the output terminal.

(3) If the oscillation waveform is seriously distorted, the diode characteristics should be checked.

(4) Adjustment of oscillation frequency. Fix the capacitor C and change the resistor or fix the resistor R and change C (series and parallel R and C should be adjusted synchronously) until the oscillation frequency reaches the required level.