About Passive Filtering (II)

2. Inductive filter circuit

The principle of the inductive filter circuit is similar to that of the capacitor filter, also because of the direct blocking characteristics and energy storage characteristics of the inductor. The explanation from the aspect of energy storage is the same principle as the capacitor. When the filter circuit of the inductor is explained from the aspect of the straight-through resistance characteristic, the inductor blocks the AC voltage portion which is decomposed by the unidirectional pulsating DC voltage, and The capacitor is shorted to ground. The larger the inductance, the better the filtering effect. The inductor is used as a separate filter circuit. It is usually used in combination with a capacitor.

3. L-shaped RC filter circuit

The L-shaped RC filter circuit is to add a resistor in front of the capacitor in the common capacitor filter circuit. The resistor is connected in series in the circuit, and the capacitor is connected in parallel in the circuit. At this time, the resistor and the capacitor form an L shape, so They are L-shaped RC filter circuits. Its filtering principle and filtering effect are similar to those of ordinary capacitor filtering circuits. At this time, capacitors and resistors also constitute a voltage dividing circuit. Because the capacitive reactance of the capacitor is small, the voltage division of the AC component is greatly attenuated. The amount is grounded short-circuited by a capacitor to achieve the purpose of filtering. For the DC voltage part, since the capacitor is isolated from the DC current, the capacitor does not have a voltage dividing effect on the resistor, and DC does not flow through the capacitor. In this filter circuit, if the resistance of the resistor is constant, increasing the capacity of the filter capacitor can improve the filtering effect, and the larger the capacity of the filter capacitor, the better. If the capacity of the filter capacitor is constant, increasing the resistance of the resistor can also improve the filtering effect, but the resistance of the filter resistor can not be too large, because the resistance of the filter resistor is too large, the DC output voltage will become smaller.

The LC filter mainly has a small inductance and a small DC loss. The inductance of the alternating current is large and the filtering effect is good. The disadvantage is that it is bulky and cumbersome. high cost. Used in power circuits that require high levels.

The resistor in the RC filter consumes a part of the DC voltage, and R cannot be made large, and is used in a circuit where the current is not required to be high. The RC is small in size and low in cost. The filtering effect is not as good as the LC circuit.

LC filtering is generally used in high frequency circuits or power circuits, while RC is used in low frequency circuits. LC filters are used in the frequency range of 1 kHz to 1.5 GHz. Due to the Q value of the inductor, the frequency response is not steep enough. .

1.) The RC filter is easier to miniaturize or integrate with respect to the LC filter, and the relative volume of the LC is mostly;

2.) The RC filter is depleted, and the LC filter can theoretically be free of loss, so the power supply part circuit is generally an LC circuit;

3.) RC is smaller than LC, and the cost is lower;

4.) RC is used in low frequency circuits, and LC filtering is generally used in high frequency circuits;

5.) The resistor in the RC filter consumes a part of the DC voltage. R can't be made very large. It is used in circuits with low current requirements. The RC is small in size and low in cost. The filtering effect is not as good as the LC circuit. The LC filter is mainly inductive. The resistance is small, the DC loss is small. The inductance of the alternating current is large, and the filtering effect is good. The disadvantage is that it is bulky, bulky, and costly. It is used in a power circuit with high requirements;

6.) The more the number of filtering stages, the better, but the higher the loss and cost, so it is not recommended to exceed 3 levels;

7.) RC filters are often used in combination with operational amplifiers to form an active filter, mostly as a filter for low frequency signals. For example, used as a loop filter in a phase-locked loop;

4. π-shaped RC filter circuit

First of all, from the structural point of view, this filter circuit is composed of two capacitors and a resistor. It is actually a π-shaped RC filter circuit in front of the resistor in the L-shaped filter circuit and a capacitor is grounded. The two capacitors simultaneously filter, and the latter filter capacitor can further filter the DC voltage of the front capacitor without filtering, so that the two capacitors are simultaneously filtered, and the filtering effect is of course more ideal. The capacity of the first filter capacitor can be increased to improve the filtering effect, but the capacity of the first filter capacitor cannot be too large, because the charging time will be too long when the capacity of the first filter capacitor is too large when the power is turned on. This charging current flows through the rectifier diode. When the charging current is too large and the duration is too long, the rectifier diode is damaged. Therefore, when the π-shaped RC filter circuit is used, the capacity of the first capacitor can be slightly reduced. The filtering effect is improved by adjusting the L-shaped RC filter circuit behind.

5. Multi-section π-shaped RC filter circuit

The multi-section π-shaped RC filter circuit is followed by an L-shaped RC filter circuit formed by a common π-shaped RC filter circuit to form a multi-section π-shaped RC filter circuit. The filtering principle is the same as the ordinary π-shaped RC filter circuit above, except that the filter circuit has multiple DC voltage output terminals, and the DC output filter effect of the latter output is better. The first filter output has the highest voltage and the last filter output has the lowest voltage, mainly because each resistor has a voltage drop. The multi-section π-shaped RC filter circuit is the most used one filter circuit in the whole circuit.

6. π-shaped LC filter circuit

This filter circuit is basically the same in structure as a conventional π-shaped RC filter circuit, except that the resistor is replaced with an inductor. Because the resistor has the same resistance to DC and AC, and the inductor has large resistance to AC inductor and low resistance to DC inductor, it can improve the AC filtering effect and not reduce the DC output voltage because the inductor does not exist for DC. Inductive reactance does not have a voltage drop on DC power like a resistor. The direct blocking characteristic of the inductor is the biggest advantage of this filter circuit, but the cost of the inductor is high, so this filter circuit is much more versatile than the π-shaped RC filter circuit.