Transcript Active Filters

Active Filters
This is not in the lab manual
• While the circuit that will be used is very
similar to the one described in the lab manual,
the steps in the lab procedure are significantly
different.
– Please follow the laboratory procedure posted in
the Week 9 module.
Passive Bandpass Filter
• Cascading a RC
(or RL) low pass
filter with a RC
(or RL) high
pass filter
• LC filter
Active Bandpass Filter
• While an active bandpass filter can be
constructed by cascading an active low pass
filter with an active high pass filter, there are
designs that use only one op amp.
Active vs. Passive Filters
• The maximum magnitude of the voltage transfer
function of a passive filter is at best equal to 1.
– Parasitic resistances and capacitances in RL can make
this value considerably lower.
• Active filters incorporate amplifiers with the filter
elements so that the filter has gain.
– Maximum of the voltage transfer function can be
greater than 1.
– fo limited by frequency of the operational amplifier
The transfer function for the active bandpass filter is
where
Goal of Experiment
• Use active filter to remove specified Fourier
components from a square wave voltage
source.
1
 2 2n  1 t 
Vsquare(t ) 
sin 


 n1 2n  1
T


4

T is the period of the square wave
Summation of Fourier Components
Bandwidth
• The difference between the frequencies (in
Hz) where the power is ½ of the maximum
output power
– Maximum power to output occurs at fo
Order of the Filter
• The magnitude of a voltage transfer function
of filter will decreases at a rate of -20dB per
decade times n, where n is the order of the
filter.
PSpice
• There is a DB
macro or you
can put in an
equation into
the Trace
Expression to
calculate -20
log (Vo/Vin)
MATLAB
• Note that you need to specify the frequency
range for the bode plot.
– The default is radians/s for the bode plot
– Help on bode plots explains how to define the
frequency range over which the plot is performed.
Velleman Spectrum Analyzer
Voltage/Frequency Measurement