CHAPTER 12: Filter Circuits ppsx

A filter is a circuit designed to separate signals from each other based on their frequency.. There are four basic types: • Low-Pass Filter LPF: passes signals below some frequency • H

CHAPTER 12

Filter Circuits

Describe and Analyze:

• Filter types: LPF, HPF, BPF, BSF

• Passive filters

• Active filters

• LC tuned amplifiers

• Other filter topics

• Troubleshooting

A filter is a circuit designed to separate signals from each other

based on their frequency There are four basic types:

• Low-Pass Filter (LPF): passes signals below some

frequency

• High-Pass Filter (HPF): passes signals above some

frequency

• Bandpass Filter (BPF): passes signals between two

frequencies

• Bandstop Filter (BSF): blocks signals between two

frequencies

Frequency Response

Bands are measured to the 3dB points

Passive RC Filters

• All four types of filter can be made with just resistors and capacitors They are not “high performance”, but they work

RC Filters

The frequency f = 1/(2RC) has several names:

• Break frequency

• Corner frequency

• Cutoff frequency

• Roll-off frequency

• 3dB frequency (or -3dB frequency)

• 0.707 frequency

• f 0 (“eff-zero”)

• f B (“eff-bee”)

RC Filters

Bode plot for a”single-stage” LPF (one R and one C)

RC LPF Example

What is the cutoff frequency for a LPF with

R = 1.59 k and C = 0.01 F ?

Time constant  = RC = (1.59 k)  (0.01 F) = 15.9 s

f 0 = 1 / (2 ) = 10 kHz

Bode Plots

Note that 6 dB /octave is equal to 20 dB /decade

• The term order used to describe filters tells us how fast the Bode plot rolls off A first-order filter, such as

an RC filter made with one capacitor, has a roll-off

of 20 dB/decade, a second-order filter has a roll-off

of 40 dB/decade, and so on

• The roll-off is N  20 dB/decade where N is the

order of the filter

• You will see the word pole used to mean the same

thing: a 1-pole filter is a first-order filter, a 2-pole

filter is a second-order filter, and so on

<insert figure 12-10 here>

The Bode plot says it all

Active Filters

• When you make a filter, you want its Bode plot to have a shape appropriate to the application While a second-order filter can

be made with two resistors and two capacitors, its Bode plot will not have a “clean” break-point That’s where active filters come

in

• Active filters use an amp together with Rs and Cs The

op-amp’s feedback loop allows you to control the shape of the

Bode plot

• Feedback RC filters are often called “Sallen & Key” filters after

the two men who first described them in the 1950s Of course, they used vacuum tubes!

Active Filters

(a) is a LPF, (b) is a HPF

Active Filters

<insert figure 12-16 here>

Active Filters

Switched Capacitor Filters

Break-point is controlled by clock rate

LC Tuned Amplifier

) π

/(

• A tuned circuit amplifier is essentially a bandpass

filter with a very narrow pass band The parameter Q

(stands for “Quality”) measures the narrowness of the pass band How high is high depends on the

application, but usually Q = 10 or more is high Q The

width of the pass band is the center frequency

divided by Q

• Q = R EQ /(2 f o L) where R EQ is the equivalent

resistance across the parallel LC circuit

) π

/(

Piezoelectric Filters

Piezoelectric crystals and ceramics act like tuned

circuits

• Determine the filter type you are working on

• Use an oscilloscope to look at inputs and outputs to check for correct filter response

• If necessary, inject a sine wave and vary the

frequency to test the filter response

• Tuned-circuits sometimes “drift”, and may have a small trimmer capacitor to make adjustments

• Active filters either work or the op-amp is dead

• Crystals and ceramics do not drift, but they can

crack from rough handling or too much current