Timers & Filters Study Notes for GATE Electronics and Communication Engineering Exams

By BYJU'S Exam Prep

Updated on: September 25th, 2023

In this article, you will find the Study Notes on Timers and Filters which will cover the topics such as  The 555 Timer IC Circuit and its various operating modes, Passive and Active filters, and types of filters.

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 The 555 Timer Circuit


            Pin diagram of  555 Timer 


Block diagram representation of the internal circuit of the 555-IC timer

A block diagram representation of the 555-timer circuit is shown in figure . The circuit consist of two comparators, an SR flip-flop and a transistor Q1 that operates as a switch. One power supply VCC is required for operation.  A resistive voltage divider, consisting of the three equal valued resistor R1 which is equal to 5kΩ is connected across VCC and establishes the reference or threshold voltages for the two comparators.


SR flip-flop works as a bistable circuit having complementary outputs, denoted as Q and\ 

In the set state, the output at Q is ‘high’ (approximately equal to VCC) and that at \ is ‘low’ (approximately equal to 0 V).In the reset state, the output Q is low and\is high.

The flipflop is set by applying a high level (VCC) to its set input terminal S and reset by applying to the reset input terminal R. The outputs of comparator 1 and comparator 2 respectively are connected to the set and reset input terminals of the flip flop.

The positive input terminal of comparator 1 is connected to an external terminal of the 555 IC is labelled as Threshold.  Similarly, the negative input terminal of comparator 2 is connected to an external terminal labelled as Trigger and the collector of transistor Q1 is connected to a terminal labelled discharge. finally, the output of flip flop Q is connected to the output terminal.

Implementation of Monostable Multivibrator using 555 Timer


 2.a.The 555 Circuit connected as Monostable Multivibrator


        2.b Waveform


 The external circuitry and waveform of 555 IC as a monostable multivibrator is shown in Figure 2(a) & (b). Before applications of trigger pulse VT , The voltage at trigger input is high which is equal to +VCC.  output\ and output voltage V0 is equal to 1. With \ and output voltage V0 is equal to 1. When  \ the discharging transistor Q1 undergoes to saturation and across the timing capacitor the voltage will be zero i.e., VC(t) = 0.

At t = 0, the application of trigger VT (negative going pulse) CC/3 causes the output of comparator C2 to be high i.e. S = 1. This will set the flip flop with \ This makes output voltage V0 = 0. Due to \the discharge transistor Q1 will get turned off. After the termination of the trigger pulse, the flipflop will remain in\ the state, since S = 0 and R = 0, So no change in state. The timing capacitor charges up exponentially toward the final value of V+ through resistor R.

The capacitor voltage is given by


When v(t) = 2/3 V+, the threshold comparator output goes high, resetting the flip flop. Output \ then goes high and the output of the 555 goes low. The high output at \ 

turns on the discharge transistor, allowing the timing capacitor to discharge to near zero volts. The circuit thus returns to its Quiescent state.

The width of the output pulse is determined from equ. (i) by putting

v(t) = 2/3 v+ and t = T, then


The width of the output pulse is a function of only the external time constant RC, it is independent of supply voltage V+ and any internal circuit parameters.

Implementation of Astable Multivibrator using 555 Timer


Astable Multivibrator 555  Circuit



In this, the threshold and trigger input is connected together. In astable mode, the timing capacitor C charges through RA = RB until v(t) reaches 2/3 V+. The threshold comparator output then goes high, forcing the flip flop output \ to go high. The discharge transistor turns on, and the timing capacitor C discharges through RB and the discharge transistor.

The capacitor voltage decreases until it reaches (1/3)V+, at which point the trigger comparator switches stages and sends\ low.

The discharge transistor turns off, and the timing capacitor begins to recharge. When v(t) reaches the threshold level of (2/3) V+, the cycle repeats itself.



Therefore, charging time is given as


When the timing capacitor is discharging, during the time 0 < t’ < TD, the capacitor voltage is


Duty cycle: It is defined as the percentage of time the output is high during one period of oscillation. during the changing time TC, the output is high and during discharging time TD, the output is low.


The duty cycle of the circuit is always greater than 50%.


Active Filters

  • An active filter means using amplifiers to improve the filter. An active filter generally uses an operational amplifier (op-amp) within its design.
  • An electronic circuit that modifies the frequency spectrum of an arbitrary signal is called a filter.
  • A filter that modifies the spectrum-producing amplification is said to be an active filter.
  • Filtering components are resistance and capacitance only. Doesn’t include inductors
  • Active components like op-amp, FET, transistors etc. are used
  • It requires a biasing voltage
  • Its bandwidth is limited due to active component
  • Gain is limited to the gain of active components used
  • Small in size due to resistors and capacitors are used
  • Due to active components like op-amp and FET input impedance is high
  • Output impedance is low
  • Basically used to suppress unwanted frequency components from information signals
  • The main function of filters is to filter out required frequency components from mixed frequency signals. They allow
  • They allow a range of frequencies to pass known as the pass band and reject (or suppress) other frequencies known as the stop band.
  • The cut-off frequency is the parameter that separates these two bands. So depending upon these pass bands and stop bands there are four types of filters: Low pass filters, High pass filters, Bandpass filters, and Band reject filters.


  • An advantage of active filters:
    • They can provide gain
    • They can provide isolation because of the typical characteristic impedances of amplifiers
    • They can be cascaded because of the typical characteristic impedances of amplifiers
    • They can avoid the use of inductors greatly simplifying the design of the filters.
  • Disadvantages of active filters:
    • They are limited by the amplifier’s bandwidth and noise
    • They need power supplies
    • They dissipate more heat than a passive circuit.

Low pass filter (LPF)

  • It allows passing all the frequencies lower than its cut-off frequency and stops all other frequencies.
  • A low-pass filter has a constant gain (=Vout/Vin) from 0 Hz to a high cut-off frequency fH.
  • This cut-off frequency is defined as the frequency where the voltage gain is reduced to 0.707, that is at fH the gain is down by 3 dB; after that (f > fH) it decreases as f increases.
  • The frequencies between 0 Hz and fH are called pass band frequencies, whereas the frequencies beyond fH are the so-called stop band frequencies.
  • A common use of a low-pass filter is to remove noise or other unwanted high-frequency components in a signal for which you are only interested in the dc or low-frequency components.
  • Low-pass filters are also used to avoid aliasing in analogue-digital conversion (which we will encounter in a few weeks). Correspondingly, a high-pass filter has a stop band for 0 < f < fL and where fL is the low cut-off frequency.
  • A simple, single-pole, low-pass filter (the integrator) is often used to stabilize amplifiers by rolling off the gain at higher frequencies where excessive phase shifts may cause oscillations.

First Order Low Pass Filter Circuit:



  • The transfer function of the circuit: 



  • Gain:


  • Cutoff Frequency:


High pass filter (HPF)

  • It allows passing all the frequencies higher than its cut-off frequency and stops all other frequencies.
  • A common use for a high-pass filter is to remove the dc component of a signal for which you are only interested in the ac components (such as an audio signal).
  • A simple, single-pole, high-pass filter can be used to block dc offset in high-gain amplifiers or single supply circuits.
  • Filters can be used to separate signals, passing those of interest, and attenuating the unwanted frequencies.

First Order High Pass Filter Circuit:



  • The transfer function of the circuit:



  • Gain:


  • Cutoff Frequency:


Bandpass filter (BPF)

  • It allows a passing band of frequencies between its higher cut-off and lower cut-off frequencies.
  • If a high-pass filter and a low-pass filter are cascaded, a band-pass filter is created.
  • A bandpass filter has a pass band between two cut-off frequencies fH and fL, (fH > fL), and two stop bands 0 < f < fL and f > fH.
  • The bandwidth of a bandpass filter is equal to fH -fL
  • The simplest band-pass filter can be made by combining the first-order low-pass and high-pass filters.


  • This circuit will attenuate low frequencies (w<<1/R2C2) and high frequencies (w>>1/R1C1) but will pass intermediate frequencies with a gain of -R1/R2.  However, this circuit cannot be used to make a filter with a very narrow band.


  • First-order band pass filter Circuit: Bandpassfilter-2
  • Transfer function:Transferfunction-bandpassfilter-1

Band reject filter (BRF)

  • It stops the band of frequencies between its higher cut-off and lower cut-off frequencies.
  • A complement to the bandpass filter is the band-reject or notch filter.
  • The passbands include frequencies below fL and above fH. The band from fL to fH is in the stop band.


  • First order Band Stop filter Circuit:  
  •   Transfer function:Bandstopfilter-transferfunction


Passive Filter

  • A passive filter is made up entirely of passive components such as resistors, capacitors and inductors.
The difference between Active and Passive Filters:
  • Passive filters consume the energy of the signal, but no power gain is available; while active filters have a power gain.
  • Active filters require an external power supply, while passive filters operate only on the signal input.
  • Passive filters are constructed using only passive components (resistors, capacitors and inductors). Active filters may contain passive as well as active components.
  • Active filters may contain passive as well as active components.
  • Only passive filters use inductors.  Active filters do not contain inductors.
  • Only active filters use elements like op-amps and transistors, which are active elements.
  • Theoretically, passive filters have no frequency limitations while active filters have limitations due to active elements.
  • Passive filters can be used at high frequencies by using inductors.
  • For Active filters, the frequency range is dependent on the bandwidth of the amplifier. Typically, to filter high-frequency signals, passive filters are used.
  • Passive filters have better stability and can withstand large currents.
  • Passive filters are relatively cheaper than active filters.

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