PWM variable analog signal (integration circuit)
An integrating circuit is a simple electronic circuit that processes pulse signals. It maintains a constant frequency, and the output voltage level corresponds to the average of the high-level voltage over one cycle. When the duty cycle is 50%, the output voltage is 50% of the maximum voltage. For a 30% duty cycle, the output would be 30% of the peak voltage. If the PWM signal is at 5V, the resulting DC voltage would be 2.5V for 50% duty cycle and 1.5V for 30%.
Integrating circuits are commonly used in scanning circuits for both black-and-white and color televisions. Their structure resembles a resistor divider but differs from differential circuits in that the positions of resistance and capacitance are swapped. This forms a voltage divider using a capacitor and a resistor, but instead of a sinusoidal input, it processes a pulse signal—making it unique compared to other voltage dividers.
A key requirement for an integrating circuit is that the RC time constant (the product of resistance and capacitance) must be significantly larger than the width of the input pulse. This ensures that the capacitor charges and discharges slowly, allowing the circuit to effectively integrate the input signal over time.
Functionally, integrating circuits perform the opposite of differential circuits. While differential circuits extract high-frequency components, integrating circuits extract the average or low-frequency components of the input signal. This makes them similar to capacitor filter circuits, where high-frequency noise is filtered out, and the output becomes more stable.
The principle of an integrating circuit is based on the exchange of resistance and capacitance in the circuit design. Imagine a basic differential circuit, and then swap the positions of the resistor and capacitor. In this setup, a rectangular pulse signal is applied to the resistor, and a triangular waveform is output across the capacitor. This occurs because the capacitor charges and discharges through the resistor, converting the square wave into a triangle wave. The capacitor stores energy during charging and releases it gradually during discharging, leading to a smoother output.
By observing the circuit with an oscilloscope, you can see a rectangular pulse before the resistor and a triangular wave before the capacitor. This behavior mimics the function of a filter capacitor, which helps smooth out voltage fluctuations. However, unlike a simple filter, the integrating circuit specifically processes pulse signals to generate a more stable, averaged output.
The analysis of the integrating circuit can be broken down into several stages:
1. Before the input pulse arrives, the output voltage is zero.
2. When the pulse starts, the capacitor initially acts as a short circuit, so the output voltage remains near zero.
3. As the capacitor charges, the output voltage increases proportionally to the integral of the input signal, giving the circuit its name.
4. When the pulse ends, the capacitor begins to discharge through the resistor, causing the output voltage to decrease.
5. Due to the large RC time constant, the capacitor does not fully discharge before the next pulse arrives. This results in a continuous charging and discharging process, producing a steady triangular waveform over multiple cycles.
This behavior makes integrating circuits ideal for applications requiring signal averaging, such as in power supplies, control systems, and signal processing.
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