1. Op amp 2 Active Filters

2. Voltage subtraction - I

3. Volt Subtraction - II
Using superposition theorem,

4. Voltage Buffer

6. Filter Active filter Passive filter
Analog filter - An analog filter processes continuous-time signals
Digital filter - A digital filter processes discrete-time signals
Software
Hardware

7. What if filter?
Purpose of filter?
Types of filters?
What is cut-off frequency?
Draw frequency responses of 4 filters?
What is pass-band?

8. Types Low-pass filter High-pass filter Band-pass filter
Band-rejection / elimination filter
Notch filter
Ideal vs. real filter
Cut-off frequency
Pass-band

9. What is active filter?
What passive filter?

10. EE3110 Active Filter (Part 1)
Passive Filters
made up of passive components - resistors, capacitors and inductors
no amplifying elements (- transistors, op-amps, etc)
no signal gain
1st order - design is simple (just use standard equations to find resonant frequency of the circuit)
2nd order - complex equations
require no power supplies
not restricted by the bandwidth limitations of the op-amps
can be used at very high frequencies
can handle larger current or voltage levels than active devices
They are particularly non-ideal (lossy)
They are bulky and expensive
Ref:080222HKN
EE3110 Active Filter (Part 1)

11. EE3110 Active Filter (Part 1)
Passive LPF
Ref:080222HKN
EE3110 Active Filter (Part 1)

12. Passive elements : Inductor BIG PROBLEM!
high accuracy (1% or 2%), small physical size, or large inductance values are required??
standard values of inductors are not very closely spaced
difficult to find an off-the-shelf inductor within 10 percent of any arbitrary value
adjustable inductors are used
tuning such inductors to the required values is time-consuming and expensive for larger quantities of filters
inductors are often prohibitively expensive
EE3110 Active Filter (Part 1)

13. EE3110 Active Filter (Part 1)
no inductors
made up of op-amps, resistors and capacitors
provides virtually any arbitrary gain
generally easier to design
high input impedance prevents excessive loading of the driving source
low output impedance prevents the filter from being affected by the load
at high frequencies is limited by the gain-bandwidth of the op-amps
easy to adjust over a wide frequency range without altering the desired response
Ref:080222HKN
EE3110 Active Filter (Part 1)

14. Active filter using OpAmp
A filter circuit can be constructed using passive components: resistors and capacitors.
An active filter additionally uses an amplifier to provide voltage amplification and signal isolation or buffering.

15. Op Amp Advantages Advantages of active RC filters include:
reduced size and weight, and therefore parasitics
increased reliability and improved performance
simpler design than for passive filters and can realize a wider range of functions as well as providing voltage gain
in large quantities, the cost of an IC is less than its passive counterpart

17. Analog Filter Responses
H(f)
H(f) f f fc fc
Ideal “brick wall” filter
Practical filter

18. Ideal Filters Lowpass Filter Highpass Filter Bandstop Filter
M(w)
Stopband
Passband
Passband
Stopband w c w w c w
Bandstop Filter
Bandpass Filter
M(w)
Passband
Stopband
Passband
Stopband
Passband
Stopband w c 1 w c 2 w w c 1 w c 2 w

19. EE3110 Active Filter (Part 1)
Categories of Filters
Low Pass Filters:
pass all frequencies from dc up to the upper cutoff frequency.
High Pass Filters:
pass all frequencies that are above its lower cutoff frequency
Low-pass response
High-pass response
Ref:080222HKN
EE3110 Active Filter (Part 1)

20. EE3110 Active Filter (Part 1)
Categories of Filters
Band Pass Filters:
pass only the frequencies that fall between its values of the lower and upper cutoff frequencies.
Band Stop (Notch) Filters:
eliminate all signals within the stop band while passing all frequencies outside this band.
Band Pass Response
Band Stop Response
Ref:080222HKN
EE3110 Active Filter (Part 1)

21. EE3110 Active Filter (Part 1)
By Definition:
Decibel (dB)
(1) Power Gain in dB :
(2) Voltage Gain in dB: (P=V2/R)
Ref:080222HKN
EE3110 Active Filter (Part 1)

22. EE3110 Active Filter (Part 1)
Cascaded System
Ref:080222HKN
EE3110 Active Filter (Part 1)

23. Filter Response Characteristics
Ref:080222HKN
EE3110 Active Filter (Part 1)

24. Bessel Characteristic
Flat response in the passband.
Role-off rate less than 20dB/decade/pole.
Phase response is linear.
Used for filtering pulse waveforms without distorting the shape of the waveform.
Ref:080222HKN
EE3110 Active Filter (Part 1)

25. Butterworth Characteristic
Very flat amplitude, Av(dB) , response in the passband.
Role-off rate is 20dB/decade/pole.
Phase response is not linear.
Used when all frequencies in the passband must have the same gain.
Often referred to as a maximally flat response.
Ref:080222HKN
EE3110 Active Filter (Part 1)

26. Chebyshev Characteristic
Overshoot or ripples in the passband.
Role-off rate greater than 20dB/decade/pole.
Phase response is not linear - worse than Butterworth.
Used when a rapid roll-off is required.
Ref:080222HKN
EE3110 Active Filter (Part 1)

27. LPF or HPF?

29. Better response? 2nd order?

31. HPF

32. 2nd order HPF

33. Single-Pole Low/High-Pass Filter
Low Pass Filter
High Pass Filter
Ref:080222HKN
EE3110 Active Filter (Part 1)

34. Two-Pole (Sallen-Key) Filters
Low Pass Filter
High Pass Filter
Ref:080222HKN
EE3110 Active Filter (Part 1)

35. Three-Pole Low-Pass Filter
Ref:080222HKN
EE3110 Active Filter (Part 1)

36. Two-Stage Band-Pass Filter
BW = f2 – f1
Q = f0 / BW
Ref:080222HKN
EE3110 Active Filter (Part 1)

37. Band-Stop (Notch) Filter
The notch filter is designed to block all frequencies that fall within its bandwidth. The circuit is made up of a high pass filter, a low-pass filter and a summing amplifier. The summing amplifier will have an output that is equal to the sum of the filter output voltages.
Block diagram
Frequency response
Ref:080222HKN
EE3110 Active Filter (Part 1)

38. EE3110 Active Filter (Part 1)
Notch filter
Ref:080222HKN
EE3110 Active Filter (Part 1)