Design ✅ Certified Course

Active Filter Design Techniques

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🎯 What you'll get

MODULE 1 — Introduction to Active Filters

1.1 What is a Filter?

Purpose of filtering
Desired vs unwanted frequencies
Frequency-selective circuits
Analogue vs digital filters

1.2 What is an Active Filter?

Passive filter
Active filter
Advantages of active filters
Limitations of active filters

1.3 Components of an Active Filter

Resistors
Capacitors
Op-amps
Feedback network

1.4 Applications

Audio
Sensor signal conditioning
ADC anti-aliasing
Instrumentation
Communications
Automotive electronics
Power electronics
Control systems

Online activity

Students identify filter applications in real electronic systems.

MODULE 2 — Filter Terminology & Specifications

Students learn the terminology used when specifying a real filter.

Topics

Passband
Stopband
Transition band
Cut-off frequency
Centre frequency
Bandwidth
Attenuation
Gain
Roll-off
Selectivity

Important parameters

-3 dB frequency
Q factor
Damping factor
Insertion loss
Stopband rejection

Practical exercise

Given a filter specification, students identify:

Passband → Stopband → Transition region → Required attenuation

MODULE 3 — Frequency Response Fundamentals

3.1 Frequency Response

Magnitude response
Phase response

3.2 Bode Plots

Frequency axis
dB scale
Phase plot
Logarithmic frequency

3.3 Decibels

Voltage gain in dB
Power gain in dB
Gain/attenuation

3.4 Poles & Zeros

Introduction to poles
Introduction to zeros
Effect on magnitude
Effect on phase

Online simulation

Students create a Bode plot for a simple RC network and identify its cut-off frequency.

MODULE 4 — First-Order Active Filters

4.1 First-Order Low-Pass

Circuit topology
Transfer function
Cut-off frequency
Gain

4.2 First-Order High-Pass

Circuit topology
Frequency response
Gain

4.3 Buffering

Voltage follower
Loading effects

4.4 Non-Inverting Active Filter

Design exercise

Specification:

Design an active low-pass filter with a 1 kHz cut-off frequency and a gain of 2.

Students:

Select topology.
Calculate component values.
Build circuit in SPICE.
Run AC sweep.
Generate Bode plot.
Verify the design.

MODULE 5 — Second-Order Filter Fundamentals

Second-order filters are the foundation of practical active filter design.

Topics

Second-order response
Natural frequency
Damping
Q factor
Resonance
Peaking

Standard form

Students learn how the second-order transfer function relates to:

Natural frequency
Q
Gain

Response shapes

Overdamped
Critically damped
Underdamped

Simulation

Students change Q and observe the response.

MODULE 6 — Second-Order Low-Pass Filters

Topics

Second-order low-pass topology
Gain
Cut-off frequency
Q factor
Component selection

Design parameters

R1
R2
C1
C2
Op-amp gain

Online design exercise

Design a:

2nd-order 2 kHz low-pass filter

with specified:

Gain
Q
Component constraints

MODULE 7 — Second-Order High-Pass Filters

Topics

Second-order high-pass topology
Cut-off frequency
Q factor
Gain
Phase response

Design exercise

Design:

2nd-order high-pass filter

with a specified:

Cut-off frequency
Gain
Q

SPICE verification

Compare:
Calculated response vs simulated response

MODULE 8 — Band-Pass Filters

8.1 What is a Band-Pass Filter?

Lower cut-off
Upper cut-off
Centre frequency
Bandwidth

8.2 Active Band-Pass Topologies

Cascaded HP + LP
Multiple-feedback band-pass
State-variable band-pass

Parameters

Centre frequency
Bandwidth
Q
Gain

Online design project

Design a filter passing:

900 Hz–1.1 kHz

while attenuating frequencies outside the desired band.

MODULE 9 — Band-Stop & Notch Filters

Topics

Band-stop operation
Notch filters
Centre frequency
Notch depth
Bandwidth

Applications

Mains-frequency rejection
Interference removal
Sensor systems
Audio
Instrumentation

Practical project

50/60 Hz Notch Filter

Students design a notch filter to remove mains interference from a simulated sensor signal.

They investigate:

Notch frequency
Notch depth
Q
Component tolerance

MODULE 10 — Filter Approximation & Response Types

This is where the course moves from basic circuit design into professional filter design.

Butterworth

Maximally flat magnitude response
Passband behaviour
Applications

Chebyshev Type I

Passband ripple
Sharper transition

Chebyshev Type II

Stopband ripple
Passband characteristics

Bessel

Phase characteristics
Group delay
Waveform preservation

Elliptic

Passband ripple
Stopband ripple
Very sharp transition

Comparison

Students investigate:

Filter	Key characteristic
Butterworth	Flat magnitude response
Chebyshev I	Faster transition, passband ripple
Chebyshev II	Stopband ripple
Bessel	Good phase/group-delay behaviour
Elliptic	Very sharp transition

MODULE 11 — Filter Order & Cascading

Why increase filter order?

Sharper roll-off
Better stopband attenuation

Topics

First-order
Second-order
Fourth-order
Sixth-order
Higher-order filters

Cascading stages

Example:

2nd-order stage + 2nd-order stage = 4th-order filter

Practical considerations

Gain distribution
Q distribution
Component interaction
Op-amp limitations

Design exercise

Design a:

4th-order Butterworth low-pass filter

and verify its attenuation.

MODULE 12 — Sallen-Key Filters

One of the most important practical active-filter topologies.

Topics

Sallen-Key architecture
Unity-gain Sallen-Key
Non-inverting Sallen-Key
Low-pass
High-pass

Design parameters

R values
C values
Gain
Q
Cut-off frequency

Advantages

Simple design
Low component count
Easy implementation

Limitations

Q limitations
Component sensitivity
Op-amp limitations

Online simulation

Students design several Sallen-Key filters and compare their responses.

MODULE 13 — Multiple-Feedback Filters

Topics

Multiple-feedback topology
Low-pass
High-pass
Band-pass

Advantages

High-Q applications
Compact implementation
Useful band-pass designs

Limitations

Component sensitivity
Gain dependence
Design complexity

Design exercise

Create a:

High-Q band-pass filter

for a specified centre frequency.

MODULE 14 — State-Variable Filters

Topics

State-variable architecture
Integrators
Summing amplifier
Multiple outputs

Simultaneous outputs

Low-pass
Band-pass
High-pass

Advantages

Independent tuning
Multiple outputs
Good control of Q

Applications

Instrumentation
Audio
Signal processing

Virtual experiment

Students adjust one parameter and observe how:

Centre frequency
Q
Gain

change.

MODULE 15 — Twin-T & Active Notch Filters

Topics

Twin-T network
Passive notch
Active notch
Q improvement
Notch depth

Applications

Mains rejection
Sensor interference
Audio systems
Instrumentation

Practical simulation

Inject:

50 Hz + 1 kHz signal

and demonstrate removal of the 50 Hz component.

MODULE 16 — High-Q Active Filters

Topics

What is Q?
High-Q response
Resonance
Stability concerns
Component sensitivity

Applications

Communications
Audio
Instrumentation
Frequency-selective systems

Design exercise

Design a filter with:

High Q + narrow bandwidth

and investigate its sensitivity to component variation.

MODULE 17 — Op-Amp Selection for Active Filters

This is essential for practical filter design.

Students learn that an ideal op-amp model isn't enough for a real filter.

Op-amp parameters

Gain-bandwidth product
Slew rate
Input offset voltage
Input bias current
Input/output voltage range
Output current
Noise
Common-mode range

GBW Requirement

Students learn why the op-amp must have sufficient bandwidth relative to the filter's operating frequency.

Slew Rate

Understand how insufficient slew rate causes waveform distortion.

Online simulation

Compare:

Ideal op-amp vs limited-bandwidth op-amp

inside the same filter.

MODULE 18 — Stability & Phase

Topics

Phase shift
Feedback
Stability
Phase margin introduction
Oscillation

Active-filter stability

High-Q filters
Op-amp phase response
Feedback interaction

Practical investigation

Students increase filter Q and observe how the real op-amp affects stability.

MODULE 19 — Component Selection & Tolerances

Real components are not exact.

Topics

Resistor tolerance
Capacitor tolerance
Temperature coefficient
Component availability

Sensitivity

Frequency sensitivity
Q sensitivity
Gain sensitivity

Monte Carlo simulation

Students simulate:

100 component variations

and observe the spread in:

Cut-off frequency
Gain
Q

This is particularly valuable in an online engineering course.

MODULE 20 — Noise in Active Filters

Noise sources

Resistor thermal noise
Op-amp voltage noise
Op-amp current noise
Sensor noise
Supply noise

Filter vs noise

Filtering unwanted frequencies
Noise bandwidth
In-band noise

Practical exercise

Students compare:
Noisy signal → Filter → Output

and calculate/observe the improvement.

MODULE 21 — Loading & Impedance Effects

Topics

Input impedance
Output impedance
Loading
Buffering

Why active filters help

Isolation between stages
Gain
High input impedance
Low output impedance

Practical simulation

Compare:

Passive filter → Load

Buffered active filter → Load

and observe the difference.

MODULE 22 — Filter Design for ADC Inputs

A very important bridge between analogue electronics and embedded systems.

Topics

ADC input requirements
Anti-aliasing
Noise reduction
Signal bandwidth
Sampling frequency

Design flow

Sensor → Active Filter → ADC → MCU

Example

Given:

ADC sampling rate = 10 kHz

students design an appropriate analogue low-pass filter.

Design considerations

Cut-off frequency
Filter order
Stopband attenuation
Op-amp bandwidth
Noise

MODULE 23 — Active Filters for Sensor Signal Conditioning

Applications

Temperature
Pressure
Current
Voltage
Accelerometers
Strain gauges

Signal-conditioning chain

Sensor → Gain → Filter → Level shift → ADC

Practical project

Design an active filter for a simulated temperature sensor.

MODULE 24 — Active Filters in Power Electronics

This makes the course particularly relevant to advanced electronics students.

Applications

Current sensing
Voltage sensing
DC-link measurement
Feedback loops
Switching-noise attenuation
Gate-driver sensing
Power-converter control

Topics

Switching-frequency noise
Low-pass sensing filters
Control-loop filtering
Differential sensing

Example

Design an analogue filter to extract the average value from a switching waveform.

MODULE 25 — Filter Design Using SPICE

This should be a major practical module.

Students learn

Creating schematics
Selecting op-amps
AC sweep
Transient analysis
Parameter sweep
Monte Carlo analysis
FFT
Bode plots

Simulations

Students simulate:

Low-pass
High-pass
Band-pass
Notch
4th-order filter
High-Q filter

MODULE 26 — Time-Domain vs Frequency-Domain Analysis

Frequency domain

Bode plot
Gain
Phase
Bandwidth

Time domain

Step response
Impulse response
Transient response

Relationship between them

Students investigate how filter frequency response affects a time-domain waveform.

Practical exercise

Apply:

Square wave
Sine wave
Pulse

to the same filter and observe the outputs.

MODULE 27 — FFT & Spectrum Analysis

What is FFT?

Time domain
Frequency domain
Spectrum

Topics

Fundamental frequency
Harmonics
Noise
Sampling
Aliasing introduction

Online simulation

Students apply a distorted waveform to a filter and use FFT to identify unwanted frequency components.

MODULE 28 — Filter Troubleshooting

Students receive intentionally faulty filter designs.

Fault examples

Wrong resistor value
Wrong capacitor value
Incorrect op-amp connection
Incorrect gain
Insufficient GBW
Saturated output
Excessive Q
Loading
Component tolerance

Troubleshooting method

Specification → Simulation → Measurement → Compare → Diagnose → Correct

MODULE 29 — Practical Active Filter Design Workflow

Students learn a professional design process.

Step 1

Define the specification.

Step 2

Select filter type.

Step 3

Determine required order.

Step 4

Select topology.

Step 5

Calculate component values.

Step 6

Select the op-amp.

Step 7

Run SPICE simulation.

Step 8

Perform tolerance analysis.

Step 9

Check noise.

Step 10

Check transient response.

Step 11

Optimise components.

Step 12

Document the design.

MODULE 30 — Final Design Projects

Project 1 — Audio Low-Pass Filter

Design a:

4th-order active low-pass filter

for an audio application.

Students investigate:

Cut-off
Roll-off
Phase
Distortion
Op-amp selection

Project 2 — 50 Hz Notch Filter

Design:

Active notch filter

to remove mains interference from a sensor signal.

Students investigate:

Notch depth
Q
Component tolerance
Op-amp limitations

Project 3 — Sensor Anti-Aliasing Filter

Design:

Active low-pass filter → ADC

with a defined sampling frequency and stopband requirement.

This project connects the course directly to MCU/embedded-system design.

Project 4 — Power Electronics Measurement Filter

Design an analogue filter for a simulated:

Switching converter current/voltage measurement

Students must remove switching-frequency components while retaining the required control/measurement bandwidth.

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