RL Filter Circuit

Simple RC circuits are not used in switching circuits, especially, in integrated circuit. However, inductors are still used, as in the case of current sweep generation, tunnel diode monostable and astable circuits. The inductor is used if a large time constant is required because a large value of inductance can be obtained with an iron core inductor. Because of non­linear properties of the iron core cause distortion, which may be undesirable far the circuit.

If it is desired to pass a very low frequency through a circuit in which inductor is, a shunt element, and then the size of the inductor may become very large. Usually air-core inductor is used in low time constant applications.

Just like RC low-pass and high-pass circuits. RL circuits consist of resistor and inductor. RL filter circuits can be used as a low-pass and high-pass filter circuits.

Step-Voltage Input:

For a step input of Fig. 1.16 (a), the output voltage VR across the resistor R and voltage across the inductor VL are:

VR = E(1 – e(tR/L))…………….. Eq.1


VL = E – VR = Ee(tR/L) ………………Eq.2

Step Voltage Input
Figure 1: Step Voltage Input

Compare equation (1) and (2) with

Vo = Ve-1/RC               and              Vo = V(1 – e-1/RC) ....(For RC Circuits)

observed that the exponential rise or fall controlling quantity ( T ) is equal to R x C in RC circuit and is equal to L / R in RL circuit. Also equation for VR in RC circuit is similar to equation for VL in RL circuit. Thus the response of the circuit shown in Fig. 2(a) is similar to the response of circuit shown in Fig. 2(b). Also, the response of the circuit shown in Fig. 2(c) is similar to the response of the circuit shown in Fig. 2(d).


RL Filter Circuit Diagram
Figure 2: RL Filter Circuit Diagram

Comparison of RC and RL Circuits:

The comparison between RC and RI circuits are given as under:

  1. RC circuits occupy small space as compared to RL circuit. It is not possible to fabricate inductors on the surface of semiconductor chip.
  2. The response of charging and discharging of RL circuit is not better than RC circuit.
  3. Capacitors are less expensive and can work at high frequencies. However, inductors are more expensive and cannot handle higher frequencies due to distributed capacitance.      
  4. RC circuit used in high time constant applications. While RL circuit used in low time constant applications. Also non-linear properties of iron cause distortion.

RL Peaking coil:

Fig 3(a) shows a RL Peaking coil circuit. It consists of a  transistor, a coil and a battery circuit 3(b) shows a linear, equivalent circuit for RL peaking coil. The circuit shows how a square wave may be converted in to pulses by means of the peaking coil L. It is assumed that the bias voltage and the magnitude of the input are such that the transistor operates linearly.

RL Peaking Coil
Figure 3: RL Peaking Coil

Since the circuit used is common-emitter and if the base input is a square wave of current whose peak-to-peak value is I, then the output voltage has the some waveform as shown in Fig. 3(b). The peak voltage is now hfe I/hoe and the time constant is hoeL. Fig. 3 shows the input and output waveform of the RL peaking coil.