Load resistor is an output testing device or component which is used as ideal output while designing or testing the electrical circuit.

To figure out what is a load resistor  in detail you should know about the term load, where it came from? what is its practical use? Various other forms of it in detail below.

What is a load?

The load is any device that can dissipate a considerable amount of power from the source to give the required output.
So this load can be resistive, capacitive, inductive load or combination of any two.

And the power consumed by them can be in the range of a fraction of Watts to several kiloWatts (uW to kW). The term load is broad in nature so we will try to cover that too in further discussion.

The name load is because it is seen as an added load to the circuit, moreover its characteristic is Load impedance.

Characteristics of the load:
a) It can be passive or active in nature.
b) It can be linear or nonlinear in nature.

There are three basic types of load impedance (Z):

i) Resistive load [Xr]
iii) Capacitive load [Xc]
iii) Inductive load [Xl]

What is a load resistor? -Resistive load:

The load in the circuit which is resistive in nature is called a resistive load or load resistor. The property of a material or a device to resist the flow of electrons through it is called resistivity.

The resistive loads exhibit the property of pure resistivity (there is no reactance or admittance which are the property of capacitor and inductor, XR ≠ 0, XC = 0 and XL = 0 which implies Z = XR )

Load resistor symbol:

Load resistor formula:

There is no such particular formula, but if you want to find it then refer to the output resistance of amplifier (in the case of the amplifier only)

• Furthermore, the load resistor is temporary and a mocked output device for designing and testing purposes only.
  You will find this ‘TERM‘ in educational curriculum and tutorials but never in real life practical circuits.
• In other words, it is simply a resistor that’s been used as a load in various circuits especially which are under test or for experimental purposes.

Additionally, in practical circuits, there is nothing like a load resistor. This is because output devices with a specific name is used everywhere instead of it.

 

Example of load resistor:

• This can be seen as a speaker is used as an output device in an amplifier circuit in the place of load resistor (RL).
• Besides this LED/ lamp/ light bulbs is used as an indicator in the power converter circuits as an output device instead of (RL).
• Note that the load resistor value can be anything that can handle the output power from the source circuit.

 

What is a load resistor in circuit?

Below are those things that can be load resistance:

1) Simple resistor
2) Audio speakers
3) LED lights
4) All types of the light indicator
5) Resistive sensors
6) Buzzers
7) The output stage of the radio frequency amplifier or audio frequency amplifier.
8) Antennas
9) High wattage resistor

 

It also recognized as a discharge load resistor, this type of device has its application in discharging the stored energy in the output capacitor to avoid electric shock or damage of output devices.

USB mini discharge load resistor is commercially available for sale online. Its objective is to discharge USB source circuits so as to avoid accidental electric shock.

 

Variable Load resistor:

As the name suggests, variable + load + resistor. It is the load that is having variable resistance that can be controlled either manually or by some other means. This image showing a variable load resistor available for the experimental purpose in labs. It comes with various wattage ratings from 5 Watts to 100 Watts and so on. It is also called as a potentiometer.

Generally, a variable load resistor is used to test the effect of Load resistance on the power consumed by the circuit and several other parameters.

 

ii) Capacitive load:

The load in the circuit which is capacitive in nature can be termed as capacitive load.
The capacitive load can be the combination of capacitor and inductor in which capacitance should be greater than that of inductance (XC > XL). Or the circuit must contain the capacitor only.

 

iii) Inductive load:

The load connected in the circuit which is inductive can be termed as inductive load.
The inductive load can be the combination of an inductor and capacitor in which inductance should be greater than that of the capacitor (XL > XC). Or the circuit must be purely made of inductors.

 

FAQ:

 

What if we don’t use load resistance?

In such case we can not understand the nature of circuit when used load connected and thus circuit analysis is affected.

 

What if we use a very small load in the power/electrical circuit?

The circuit will deliver a large current and may cause a short circuit which in results can destroy the driver circuit.

Unlike other filters, a high pass filter has its unique importance. The basic mode of communication is voice calling. Of all forms of communication, voice calling is still the most preferred form of communication.

Also, it has challenges while communicating a long distance over the telephone, the only information/data required is voice signal and therefore, it is necessary to maintain the quality of transmitted and received voice signal.

However, a lot of noise interference of low frequency is added to the communication channel. To avoid this highpass filter is used (HPF because the human voice is having a higher frequency than that Of noise).

What is the high pass filter?

A high pass filter is a circuit that allows the higher frequency above cutoff frequency and attenuates all the frequency below the cutoff frequency (ƒc).

As we have already studied low pass filter which blocks low frequency passing through it and only allows higher frequencies to the output, but on the other hand, high pass filter is opposite to that of low pass filter.

The high pass filter circuit is essential in the day-to-day application in the field of audio applications such as voice recording voice filtering music tone control and so on.
It is widely used for eliminating ‘hiss’ noise, and noise of wind while recording through the microphone in an open environment. We have discussed this in the application section below in detail.

Ideal Vs Practical High Pass Filter

The figure is showing the frequency versus attenuation response curve of the high pass filter. Ideal having very flat response i.e., infinite loss/ attenuation at stop-band and Zero attenuation at passband frequency, but practically due to ohmic losses and lossy components, practical HPF has finite attenuation at stopband and few dB of loss at passband.

The frequency response of High Pass filter:

A) -3d dB cutoff frequency is nothing but half of the power at output than at input.
Alternatively, (1/√2=) 70.7{42041a7992ac3be9e9e29c856254fb498d8c7935d7cf8512da6802e8688e734a} of Vin

B) If you have values given in voltage, can be to convert them in dB using this formula:
dB = -20log (Vout/Vin)

Types of high pass filter:

This filter is classified on the basis of the component used to implement the circuit as LC or RC.

A) HPF using R & C:

There are two schematics generally used in the filter. The filter that uses a resistor in the circuit is used in combination with an external power supply called active filter, otherwise passive filter.

The Other method is to design filter using RC and then amplify the signal, that filter without any external power source is called a passive filter.

The figure showing the schematic of HPF using RC along with the formula:

 

The capacitor is in series with the input, and the resistor is parallel to the input. Also, the output is taken across the resistor, as shown in the figure. The role of a capacitor here is to block the DC signals (lower frequency signals) which is determined using the formula.

The product of capacitance C and resistor R is time constant (Tau)

ƒc = 1/(2πRC)

where, fc is in hertz,
R in ohm,
C in farad,
therefore (τ)is in sec.

B) High pass filter using L and C:

You might have observed that the formula for deriving the component values in the filter is the same for all the types of filters i.e. HPF, LPF, and BPF.
Then, How are you getting different responses for different types of filter?
The answer is the position of capacitor C in the schematic.

• When the capacitor is kept in series with the source, then it is a high pass filter.
• On the contrary, if you placed the capacitor in parallel with the source, then it is a low pass filter.

Figure showing LC high pass filter schematic and formula:

Why is LC filter better than RC or RL filter?

The answer is simple: it offers theoretically zero power loss due to two non-resistive elements and makes no ohmic losses.
Therefore, it is preferable in the circuit at the receiver end, where we have to pick very small RF signals. It is quite complex to design inductor but it worths it.

Higher-order filters:

A higher-order filter can be achieved by cascading two or more stages of the filter. These stages of the filter are either schematic of RC or LC. The order of the filter is increased so that we get a very sharp response curve that is near to the Ideal filter. The first-order filter gives a slope of 20 dB/decade.
If we add 2nd stage in series with the first stage then 40 dB/ decade is achieved.

Practical example to design High pass filter:

Design a high pass filter fc=1.2 kHz using RC?

Given- fc=1.2kHz

R=100Ω (assumed)

find C using formula:  ƒc = 1/(2πRC). And it came out to be 1.32uf

 

Design a high pass filter having cutoff frequency fc=3 MHz using RC components?

Given- fc=3MHz

R=100Ω (assumed)

find C using formula:  ƒc = 1/(2πRC). And it came out to be 0.53nf

 

Design a high pass filter having cutoff frequency fc=90 MHz using LC components?

Given- fc=90MHz

C=25pf (assumed)

find L using formula:

ƒc = 1/(2π√LC).

And it came out to be ~62nH

 

What is high pass filter used for? Its applications.

1) While recording audio outdoor using a mic, causes the sound of wind also gets recorded as noise. The frequency of the human voice is slightly higher than that of the noise of the wind.
The sound of the wind is of lower frequencies which are unnecessary, therefore, high pass filter is used to lower the noise.

2) In pre-amplifier to avoid amplification of noise

3) To reduce ‘Hiss’ sound in the audio circuit (caused due to power lines nearby).

4) For Waveshaping in different applications in analog and digital electronics.

5) It is used as differentiator circuit(converts square wave/ step response to spike/impulse)

6) As an audio equalizer, then this filter is called treble boost filters.

Bonus Tips/Overview:

In short, High filter makes it difficult for a low frequency to pass through it and eases high frequency to pass through it.

The trick to identifying whether the filter is a highpass filter or low pass filter?

a) if the position of the capacitor is lower that is a low pass filter (parallel to the input)
b) if the position of the capacitor is upper then it is high pass filter (in series with input)

 

What is a Band pass filter?

Band pass filter circuit is an electronic circuit that is used to pass the frequencies in a particular frequency range so that only a certain frequency is obtained for further processing in the application circuit.”

Alternatively, it can be said that “Band pass filter attenuates all the signal outside the particular frequency range allowing only certain signals to receive.”

The frequency which is allowed, that range of that acceptable frequency is known as pass band. The difference of higher allowed frequency to the lower allowed frequency is called bandwidth of the filter.

This circuit is useful for removing unwanted noise signals by blocking them. The Band pass filter has a lot of applications ranging from audio circuits to the radio frequencies circuits.

Generally, RF bandpass filters are used in transmitters and receivers so that we transmit the data signals to the required destination without interfering with other signals. Because interfering with another signal may cause confusion and cross-talks among the transmitter and receiver.

 

Importance:

• Filtering is one of the most used methods to get rid of noise in the circuit besides the modulation technique and different other methods.
• These circuits are the weapon for electronic circuit designer who uses this to deal with noise in the circuits special RF receivers in communication devices.
• You can find the simple use of this type of filter in the audio amplifier for driving loudspeakers.
• There you can control the tone of the output music/sound, controlling tone is nothing but the controlling of a pass frequency to the output loudspeaker.
  Later on, we will discuss in brief the area of application of the Band pass filter below in the application section

 

Working of Band Pass Filter :

We have already discussed the working of a low pass filter. The similar is the working of a band-pass filter except for a few extra circuits.

It is the combination of a low pass filter and high pass filter so the formula for cutoff frequency we already discussed {here}

The basic idea of implementing bandpass filter is as shown in the figure:

• The ideal bandpass filter and the practical bandpass filter frequency response is as shown in figure. As you see that the Band pass filter has the infinite attenuation for the blocked frequencies and zero attenuation for passband frequencies.
• But in practical this is not possible, the practical bandpass filter offers of zero to one dB attenuation for passive filters which also depends on the type of filter, whether it is RC filter or LC filter? and /or it is passive filter or active filter, which we will discuss below.

Types of Band Pass filter:

Ultrabasic classification is Analog filter and Digital filter. We are discussing here is analog(check below at last for digital filter). Also, basic types of filters are active filters and passive filters.

Active filters are those which require some sort of external power supply(and active components like transistors) to give the required outcome.

On the contrary,
Passive filter does not require any type of active components to do its work and thus preferable in Low-cost circuits.

Now the real bandpass filter is classified into two types based on its bandwidth as follows:

1) Wideband filter.
2) Narrow-band filter /tuned filter.

 

WideBand pass filter BPF :

The wide-band filter is implemented using One circuit of low pass filter and high pass filter.

• The low pass filter blocks the lower frequencies which are not required and passes all the other frequencies, at the same time the high pass filter blocks the higher frequency than required and passes the frequencies lower than that. Take a look at the following diagram to understand it in a better way.

(BPF using resistor and capacitor,)

• The wide band filter has quite a lower Q factor. The BPF can be implemented using RC or LC, ie. using a resistor-capacitor for inductor-capacitor.
• The best is to use the Inductor capacitor filter as it has a low power loss. We have discussed the RC filter in low pass filter so we will discuss the LC here in the example below.

Narrow band filter:

• It is also called a tuned filter. As its name suggests that it will pass only a narrow range of frequency through it. The construction of a narrow band filter is quite different.
• The resonance circuit is used to implement a narrow band filter. This is the most favorite filter in all classes of the filter this is because it uses only two components the one is inductor and another is a capacitor in a very simple configuration as shown in the figure. This configuration is called a tank circuit.

• In this circuit the inductor and capacitor value is chosen such a way that it resonates at a particular frequency, rather say the transfer of charges (current/ energy) from the inductor to capacitor and capacitor to inductor depending on their capacities to hold charges. (This is practically used in various application but lesser taught in the curriculum)

Important definitions:

1) SNR: Signal to noise ratio, it is very important down while designing filters and RF circuits especially transmitters and receivers, also equally important in audio circuits. SNR is the ratio of signal power to noise power.

• Further for ease, the factor used to determine the quality of signal received at the application end is signal to noise ratio.
• The power of the signal must be greater then that of the noise, real noise is the frequencies of unrequired signals which may be natural or artificial for propose full or unintentional.
• Generally, the value of 20 dB for higher than that is considered a good network signal. Similarly, 3dB is a minimum acceptable Value below that value it is very difficult Orra impossible to detect the required signal with accuracy.

2) Q factor: It is lease also called a damping factor it is a dimension list that gives information about the bandwidth with respect to its central frequency. It is used in the circuit having LC in parallel( tank circuit).

 

Band pass filter example (practical):

 

Design a bandpass filter having frequency range 80Mhz to 120Mhz?

Given- fc1 and fc2    (where, fc1 =fl =lower cutoff frequency & fc2 = fh = higher cutoff frequency)

The above circuit is designed using the inductor and capacitor as we have already discussed the upper hand of the LC filter over the RC filter, so we will design the bandpass filter using the inductor.

step1: for LPF
L2 = 10 nH  (assumed) & C2 = 0.395 nF (using formula).

step2: for HPf
L1 = 10 nF & C1 = 0.395 nF

(for both LPF and HPF the values came out to be same because for LC filters formula for finding L&C is same only the position of inductor and capacitor interchanges “look fig.”)

 

Design a narrow bandpass filter having Centre frequency Fc=100Mhz?

Use narrow band pass filter circuit,

Fc=1/2π.sqrt(LC)

Here, the only frequency value is given then how can we calculate the value of L & C ?

-You have to assume the value of any one component either it is an inductor ‘L’ or a capacitor ‘C’.

-Let’s assume, C=1.58 nF (assume the standard value which is available to you)

-then using formula, find L, it came out to be 1.56 nH.

Hence, the circuit is completed. (see image for schematic)

4th order bandpass filter and 5th order Bandpass filter:

The effect of the orders of filters on the response is displayed in the image below. Just observe the attenuation of the pass frequency and stop frequencies.

• The attenuation roll-off of each is different in different orders of filters.
• The picture is showing 1st order bandpass filter, 2nd order bandpass filter, 4th order bandpass filter, 5th order bandpass filter.
• 1st order filter has -20dB/decade gain roll-off than that of -40dB/ decade for 2nd order filter and so on.
• The order of the filter is achieved by cascading the number of stages of the same filter and sometimes using modern topologies.

Application of Band Pass Filter:

1) Radio transmitter and receiver widely use BPF.

2) Optimization of signal to noise ratio so as to reduce the probability of error in the receiver.

3) In almost every Audio Tone control circuit to increase or decrease the tone in the music.

4) It is an important circuit in demodulators in the receiver circuit.

5) Band selection in FM radio receivers, also in mobile communication systems.

6) In pre-emphasis and de-emphasis circuits to increase S/N.

7) In RADAR to transmit signals in a different spectrum.

 

What is a low pass filter?

Before understanding the low pass filter let’s look at what is a filter. Have you ever heard of strainer? if yes then you must know the use of strainer, it is used for straining solids from liquids, or for separating coarser particles from finer particles, in short, the strainer is used to filter out the unwanted impurities in the solution or liquid and allow only what is required.

Similarly, filters are the device or circuits which are used where only the required range or a frequency is required.

The frequency range can be all frequency less than particular frequency, the difference between two predetermined frequency, or frequencies above particular frequency.
The main categories of the filters are high pass filter low pass filter Bandpass filter, notch filter/band-reject filter.

Definition: Low pass filter

Low pass filter (LPF) is a filter that allows signals with a frequency lower than a particular frequency (that particular frequency is called cutoff frequency).
And does not allow the signals of frequencies higher than the cutoff frequency.

In other words:

• LPF is a circuit that is designed to reject unwanted higher frequency of electromagnetic signal, audio signals, electrical signals and accepts only those signal which is required in the applicational circuits.
• A low pass filter is a circuit that attenuates all the signal components above cutoff frequency to a considerable level.
• Technically, any filter can be classified as the ideal filter and practical filter the figure below showing the ideal and practical response of low pass filter:

Ideal LPF can be defined as the filter having the ideal response of input versus output frequencies, i.e, it must have zero attenuation for all pass vacancy and infinite attenuation for or blocked frequencies as shown in the figure.

Idea LPF shows a flat response. But practically it is not possible and we get a slightly curved response, this is due to non-ideal components we use in making LPF.

Types of low pass filters:

a) Active low pass filter
b) Passive low pass filter

Active low pass filter:

It is a low pass filter that uses the external power supply to give required output frequencies with the specific Gain.

This is because the filter circuit itself consumes some power which is not desirable for the circuits which use very low input power and therefore unable to process the input signals.

Generally, active low pass filter is used in “Amplifier with equalizer” and Critical radio frequency circuit designs.

Passive low pass filter:

It is LPF which does not use any external power supply and just filters out the higher frequency to give the lower frequencies.
It is used in audio circuits, power supply circuits to eliminate the noise, and radio circuits to select lower frequencies and avoid high-frequency noise in the receiver end.

 

let’s discuss passive low pass filter in detail:

A passive low pass filter can be implemented in multiple ways using the given schematic. A few of them are RC filter, LC filter, RL filter, and some topology includes, Butterworth filter, Chebyshev filter, π filter, T filter, and so on.
Butterworth filter, Chebyshev filter, π filter, T filter, k filter, etc come under modern filter design. Detail analysis of these filters will be published in the next post.

In this post, we will be discussing the traditional and basic ways of implementing the low pass filter.

 

Terminology Used :

1) Pass band frequency: Frequencies that are allowed through the filter without/low attenuation are called passband frequencies.

2) Stop band frequency: Frequencies that are completely blocked, face high attenuation are called stopband frequencies.

3) Bandwidth: It is the range of particular frequencies.

4)Cutoff frequency (higher cutoff frequency/ lower cutoff frequency): The frequency at which the filter offers half of the power loss which is equal to 3db loss which is equal to (0.707 Vi).

 

RC low pass filter:

In the RC low pass filter, we use two components namely Resistor and Capacitor.
This is the most used low pass filter schematic for Audio proposes and Rectifier filter purposes.

The reason behind this is very simple as they are available at low cost and therefore the first choice for mass production.

The figure showing the schematic of the RC low pass filter:

The basic circuit of the RC low pass filter consisting of the resistor in series and capacitor in parallel with the load.

(not the position of the capacitor because this is the component that decides whether the filter is low pass or high pass)

Working:

So how this circuit blocks higher frequencies?

This circuit used the capacitor’s property of Reactance Xc which acts as a short circuit for higher frequencies and therefore higher frequency cannot travel to the Load.

On the contrary it blocks the low-frequency signals through it, which in results pass through the load.

Another way to understand this is through learning the charging and discharging time of the capacitor which is responsible for behavior two different frequencies.
The combination of R and C creates a charging and discharging effect on the capacitor called as its Time Constant ( τ ) of the circuit.

τ = RC seconds

For lower frequencies, there is sufficient time for the capacitor to charge at the same voltage that of input and results in an open circuit.

For higher frequencies, there is less amount of time for the capacitor to charge Before the negative cycle come and results in short circuit.

What determines the passing frequency and blocking frequency?

fc=1/(2 π R C)

The tau ( τ ), τ= RC ,is the time constant is dependent on the cut-off frequency ƒc as above.

Here frequencies lower than ‘fc’ are passed and higher than that is blocked see example below.

(check figure for all formulae)

RL low pass filter

RL LPF uses of resistor and inductor in the same configuration as that of RC LPF.

Generally, RL low pass filter is avoided for implementation as a filter because of large size and wt. of the inductor (particularly for higher values of components), non-linear frequency response over the change in temperature, also quite complex to implement.

LC low pass filter

LC-LPF Is made up of inductor and capacitor, this is another most used LPF. The reason behind this it has simplicity and accuracy of frequency response. a very e sharp cutoff frequency is achievable using an LC filter.

The radio circuit the receivers, the transmitters, the modulators use the LC filters. Or you can say that any RF circuit which is required to inhibit low power loss and better stability uses LC filters.

The formula and schematic for the LC low pass filter:

         

Let’s analyze a low pass filter with a practical example :

Q. Design a low pass filter having cutoff frequency ‘fc‘ = 75MHz and Vin = 5 volts using RC filter?
Solution: given -> f = 75Mhz.
R = 100 Ω(assumed)——-<assume either the value of R or C>
C = (to find)

Formula, fc = 1/(2 π R C)
Putting the values we get , C = 21.2 pF. Hence the design

Further analysis,

Find Xc= ? —–[using formula Xc= 1/(2π f C) ]
Therefore, Xc1 = 100.14 (75Mhz) ;.
Xc2 = 8.23 (900Mhz)

Vout = Vin[Xc/sqrt(R²+Xc²)]

putting the value

We get,       Vout = 3.54 Volts for freq. 75 Mhz—-(i)

Vout = 0.41 Volts for freq. 900Mhz—-(ii)

From result i), ii) we observed that for ‘designed the frequency’ we get is almost zero loss than that of ‘out of designed freq’ range. Hence LPF designed according to requirement.

There are other ways too for implementing it, which we will discuss in the next post.

 

Improving the frequency response:

Improving frequency response means steps towards achieving to near ideal frequency response curve. We can implement this using increasing the number of orders.

Here order means the number of stages of the same circuit. if two to RC filter is used in series there it is second-order filter. if we use three RC filters side by side there it is a third-order filter.

And this goes on, but there is a certain limitation to use the higher-order filter. Those imitations are attenuation(insertion loss) of passband frequencies, noise insertion, etc

By increasing the order of the filter, the dB gain roll-off decreases and thus gives a sharp response.
The figure below shows a second-order filter (1st order+ 1st order):

 

Applications of LPF:

1) In receiving-end of radio receiver television receiver etc specifically in the superheterodyne receivers.
2) In power electronics to filter out high-frequency noises.
3) In demodulation circuits to recover the original signals eg. FM/AM receiver.
4) R-C type LPF is used as an integrator.
5) In phase detection in the phase-locked loop.

 

Modern filter design:

Why there is a need for modern filter design?

Modern filter design is gaining popularity within the circuit design engineer due to better frequency response and stability. But the disadvantage of the modern filter is its complexity.

Even though these designs are complex it is preferred among RF IC designers.

 

Below are the most popular topologies:
a) Butterworth filter
b) Chebychev filter
c) Bessel filter
d) K-filter, etc