Filter Circuit: Types, Characteristics and Principles

Filter circuit types

The filter circuit mainly has the following: Capacitive filter circuit, which is the most basic filter circuit; π-type RC filter circuit; π-type LC filter circuit; electronic filter circuit.

Filter principle

1. Characteristics of one-way pulsatile DC voltage

Figure 1 Decomposition of unidirectional pulsatility voltage

Fig. 1 (a) shows a one-way pulsatile DC voltage waveform. It can be seen from the figure that the directionality of the voltage is consistent, but is fluctuating in the voltage amplitude, that is, on the time axis, the voltage exhibits a periodic change, so it is pulsation.

However, according to the principle of waveform decomposition, this voltage can be broken down into an alternating current voltage different from a set of frequencies, as shown in FIG. 1 (b). In Fig. 1 (b), the dashed portion is the DC component in the one-way pulse DC voltage UO, and the solid portion is an alternating component in UO.

2. Capacitive filter principle

According to the above analysis, the one-way pulsatile DC voltage can be decomposed into the AC and DC two parts. In the filter circuit of the power supply circuit, the characteristics and energy storage characteristics of the "compact communication" of the capacitor can be filtered off the alternating current component in the voltage using the characteristics of the inductance "separating traffic". Figure 2 shows the schematic of the capacitive filter.

Figure 2 Capacitive filter principle

Figure 2 (a) is an output circuit of a rectifier circuit. The output of the AC voltage after the rectifier circuit is a unidirectional pulsating DC or Uo in the circuit.

Figure 2 (b) is a capacitive filter circuit. Since capacitor C1 is equivalent to an open circuit for DC, the DC voltage output from the rectifier circuit cannot pass through C1 to the ground but is only added to the load RL. For the AC component output from the rectifier circuit, the AC component flows through C1 to the ground because of the larger capacity and smaller capacitive resistance of C1, and cannot be added to the load RL. In this way, the required DC voltage +U is removed from the one-way pulsating DC through the filtering of capacitor C1.

The larger the capacity of the filter capacitor C1, the smaller the capacitive resistance of the AC component, the smaller the alternate component remaining on the load RL, the better the filter effect.

3. Inductive filter principle

Figure 3 Schematic diagram of inductive filtering

Figure 3 shows the schematic of the inductor filter. Since the inductance L1 pairs the direct current corresponds to the path, the DC voltage output by the rectifier circuit is directly applied to the load RL.

For the alternating current ingredient output from the rectifier circuit, the amount of inductance of L1 is large, the sensing resistance is large, and the alternating constant occurs, the alternating power is prevented from flowing to the load RL. Thus, the desired DC voltage + u is taken from the one-way pulsatic DC power by the filtering of inductance L1.

The larger the inductor amount of the filter inductor L1, the larger the sensing resistance of the AC component, the smaller the alternating gene remaining on the load RL, the better the filter effect, but the DC resistance will increase.

π-type RC filter circuit reading method

Figure 4  π-type RC filter circuit

Figure 4 shows the π-type RC filter circuit. C1, C2, and C3 in the circuit are 3 filter capacitors, R1 and R2 are filter resistors. C1, R1, and C2 constitute the first section of the π-type RC filter circuit. C2, R2, and C3 constitute a second π-type RC filter circuit. Since this filter circuit is like Greek alphabet π and a resistor and capacitor are used, it is called a π-type RC filtering circuit.

The principle of π-type RC filter circuit is as follows:

The filtering principle of this circuit is: the voltage output from the rectifier circuit is first passed through the filtering of C1, and most of the AC components are filtered out, and then added to the filter circuit composed of R1 and C2. The capacitive resistance of C2 and R1 constitute a voltage divider circuit. Because C2 is small, the voltage division decrease in the AC component is large, and the purpose of filtering is achieved. For DC, since C2 has a fault effect, the R1 and C2 voltage pressing circuits have the action of different pressure attenuation of DC, so that the DC voltage can be output by R1.

When the R1 size is constant, the increase of the capacity in C2 can improve the filtering effect, and the value of the R1 can increase the filtering effect when the C2 capacity is not changed. However, the resistance value of the filter resistor R1 cannot be too large, since the DC current flowing through the load flows through R1, the DC pressure drop is generated on R1 so that the DC output voltage UO2 is reduced. The larger the resistance of R1, the larger the current flowing through the load, the larger the pressure drop on R1, the lower the DC output voltage.

C1 is the first quarter filter capacitor, which increases the capacity to improve the filtering effect. However, the C1 is too large, and the charging time of C1 is very long when it is turned on, and this charging current is flowing through the rectifier diode. When the charging current is too large, the time is too long, it will damage the rectifier diode. Therefore, this π-type RC filter circuit can make the C1 capacity, and the filtering effect is further improved by reasonable design R1 and C2.

This filter circuit has three DC voltage outputs, which outputs three sets of DC voltages UO1, UO2, and UO3. Among them, UO1 is only filtered by capacitor C1; UO2 has passed the filtering of C1, R1, and C2 circuits, so the filtering effect is better, and the alternating current in UO2 is smaller; UO3 passes the filtering circuit of the 2 filter circuit, the filter effect is the best.

The three DC output voltages are different. UO1 voltage is the highest, up to the power amplifier circuit, or added to the circuit required to DC operating voltage and maximum operating current; UO2 voltage is slightly lower, because the resistance R1 pairs a voltage drop in the DC voltage; UO3 voltage is the lowest, this voltage is generally supplied as a DC operating voltage because the DC operating voltage is relatively low, and the AC ingredient in the DC operating voltage is required.

π-type LC filter circuit reading method

Figure 5 π-type LC filter circuit

Figure 5 is a π-type LC filter circuit. The π-type LC filter circuit is basically the same as the π-type RC filter circuit. This circuit only converts the filter electrical resistance to filter inductance because the filter resistor has the same resistance on DC and AC power, while filtering inductance is large, the sensing resistance of direct current is small, so it can improve the filtering effect, and the DC output voltage will not be reduced.

In the circuit of FIG. 5, the unidirectional pulsating DC voltage output from the rectifier circuit is filtered by the capacitance C1, and most of the AC components are removed, and then the L1 and C2 filter circuits are added.

For the AC component, L1 is very large, so that the AC voltage on the L1 is large, and the alternate component is added to the load.

For direct current, since L1 does not present sensance, it is equivalent to the path. And the wire diameter employed by the filter inductor is thick, the DC resistance is small so that the DC voltage is substantially no voltage drop. So the DC output voltage is relatively high, this is the main advantage of the inductor filter.

Electronic filter reading method

1. Electronic filter

Figure 6 Electronic filter circuit

Figure 6 is an electronic filter. The VT1 in the circuit is a triode, which functions as a filter tube, and C1 is the base filter capacitor of VT1. R1 is the base bias resistance of VT1, and RL is a load of this filter circuit, and C2 is a filter capacitor of the output voltage.

The working principle of the electronic filter circuit is as follows:

The VT1, R1, and C1 in the circuit constitute an electronic filter circuit, which corresponds to a capacitor having a capacity of C1 × β1 size. The current amplifier of β1 is VT1, and the current amplification of the transistor is relatively large, so it is equivalent. The capacitance is very large, and the filtering performance of the electronic filter is very good. The equivalent circuit is shown in Figure 6 (b). The C is an equivalent capacitor in the figure.

In the electronic filter, the filtering is mainly realized by R1 and C1, which is also an RC filter circuit, but it is different from the RC filter circuit introduced earlier. The DC current flowing through the load in this circuit is the emitter current of VT1, and the current flowing through the filter resistor R1 is the base current of VT1. The base current is very small, so the resistance value of the filter resistor R1 can be set very large (good filtering effect), but it will not make the DC output voltage drop much.

The resistance value of R1 in the circuit determines the base current of VT1, which in turn determines the tube voltage drop between the collector and emitter of VT1. It also determines the output DC voltage of the VT1 emitter. Therefore, changing the size of R1 can adjust the size of the DC output voltage +V.

2. Electronic voltage regulator filter

Figure 7 shows another electronic voltage regulator filter. Compared with the previous circuit, a voltage regulator diode VD1 is connected between the base of VT1 and the ground terminal. The principle of electronic voltage regulation is as follows.

Figure 7: Electronic voltage regulator filter circuit

After the voltage regulator diode VD1 is connected between the base of VT1 and the ground terminal, the input voltage makes the voltage regulator diode VD1 in reverse bias state by R1, and the voltage regulator characteristic of VD1 makes the base voltage of the VT1 tube stable at this time so that the DC voltage output from the emitter of VT1 is also stable. Note: This voltage stabilization characteristic is determined by the voltage stabilization characteristic of VD1, and has no relationship with the electronic filter circuit itself.

R1 is also a current limiting protection resistor of VD1. After the regulator diode Vd1 is added, the size of the change R1 cannot be changed to the VT1 emitter output voltage size. Since the transmittance of the VT1 is present, the emitter output voltage is slightly smaller than the voltage stability value of the VD1.

C1, R1, and VT1 can also form an electronic filter circuit to filter.

In some cases, in order to further improve the filtering effect, a two-tube electron filter circuit can be employed, and 2 electron filtering tubes constitute a composite tube circuit. This total current amplification is the product of each tube current amplifier, clearly improving the filtering effect.