Adjustment and test of RC bridge oscillator circuit

GW MB10F 0.8A 1000V rectifier bridge

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

DFN1006 TP-RCLAMP3321P ESD Diode Low Capacitance

1. General Method for Designing an RC Bridge Oscillator Circuit

The design of an RC bridge oscillator circuit involves several key steps, starting with the selection of an operational amplifier (op-amp). The op-amp should have high input impedance and low output resistance. More importantly, its gain-bandwidth product must be sufficient to meet the requirements of the circuit. This is typically calculated using the formula:

Since the oscillation output amplitude can be quite large, the op-amp will operate in a large-signal condition. Therefore, it's essential that the slew rate (S_R) of the op-amp meets the following requirement:

S_R ≥ ω_o U_om

2. Determining the Value of Frequency Selective Network Components

The oscillation frequency (f₀) is determined by the RC network, given by the formula: f₀ = 1/(2Ï€RC). When choosing the values of R and C, you need to consider the impact on the oscillation frequency. A smaller R helps reduce the effect of the op-amp's input impedance, while a larger R minimizes the influence of the output impedance. Typically, since the input resistance of the op-amp is high, R is often selected to be in the range of several thousand to tens of kiloohms. The capacitor C should be larger than a few hundred picofarads to minimize the effects of parasitic capacitance, but not too large to use electrolytic capacitors. Therefore, C is generally chosen between a few hundred picofarads and one microfarad.

3. Determining the Value of Negative Feedback Circuit Components

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

4. Debugging Steps for an RC Bridge Oscillator Circuit

(1) Check the circuit wiring carefully, especially for any short circuits at the op-amp output. If the power supply connections are incorrect, make sure to correct them before powering up the circuit.

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

(3) If the waveform is severely distorted, check the characteristics of the diode in the feedback loop.

(4) Adjust the oscillation frequency by changing either the resistor or the capacitor. If using series or parallel combinations of R and C, adjust them simultaneously until the desired frequency is achieved.