Adjustment and test of RC bridge oscillator circuit

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1. General Method for Designing an RC Bridge Oscillator Circuit

When designing an RC bridge oscillator circuit, the selection of an operational amplifier is crucial. In addition to having a high input impedance and low output resistance, the most important factor is that the gain-bandwidth product (GBP) of the op-amp must meet certain conditions, which are:

Since the oscillation output amplitude is relatively large, the integrated operational amplifier operates in a large-signal state. Therefore, the slew rate (SR) of the op-amp must satisfy the following condition:

SR ≥ ω₀U₀ₘ

2. Determination of Frequency Selective Network Components

The value of the frequency-selective network components should be chosen based on the desired oscillation frequency, given by the formula: f₀ = 1/(2πRC). To minimize the effect of the op-amp's input impedance on the oscillation frequency, a smaller R is preferable. However, to reduce the impact of the op-amp’s output impedance, a larger R is better. Typically, since the input resistance of an op-amp is quite high, it is common to use resistors in the range of several thousand ohms to tens of kiloohms. The capacitor C should generally be larger than a few hundred picofarads to reduce the influence of parasitic capacitance, but it shouldn’t be too large, as electrolytic capacitors may not be suitable. Therefore, C is usually selected between hundreds of picofarads and one microfarad. For better frequency stability, it's recommended to use high-precision resistors (such as E24 or E96 series) and capacitors with low dielectric loss.

3. Determination of Negative Feedback Component Values

The values of the negative feedback components determine the closed-loop gain. A higher gain makes it easier to start oscillation and produces a larger output amplitude, but can lead to waveform distortion. On the other hand, a lower gain results in a cleaner waveform but smaller amplitude and potential instability. To achieve a stable waveform with minimal distortion, a nonlinear resistor is often used in the feedback loop. When selecting a stabilizing diode, it's important to choose silicon diodes for better temperature stability in amplitude control.

4. Debugging Steps for an RC Bridge Oscillator Circuit

(1) Check the circuit wiring carefully, especially whether there is a short at the op-amp output. Ensure the power supply is connected correctly before turning on the DC power supply.

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

(3) If the waveform is severely distorted, check the characteristics of the diodes in the circuit.

(4) Adjust the oscillation frequency by changing either the resistor or the capacitor (if using series-parallel combinations, adjust both simultaneously) until the desired frequency is achieved.