Page 307 - Complete Wireless Design

Filter Design

306 Chapter Six

Figure 6.55 An active bandpass filter.

An active audio bandpass filter (Fig. 6.55).
1. Select the filter’s desired voltage gain (A ).
v
2. Select the filter’s desired center frequency (f ) and bandwidth (BW).
c
2
3. Calculate the required Q of the filter by Q f /BW. (Note: 2Q must be
c
greater than A , or the filter design will not function as expected.)
v
4. C C 0.1
f
1 2
Q
5. R
1
A 6.28 f C
v c 1
Q
6. R
2 2
(2Q A ) 6.28 f C
v c 1
Q
7. R
3 6.28 f C C

2
c
1
C C
1 2
These active filters can be cascaded for more selectivity. However, the band-
width will become increasingly more narrow as more active sections are
added. The problem can be neutralized by anticipating the reaction, and
designing the initial filter section wider than we would if employing just a sin-
gle filter. As a very rough guide, we should double the bandwidth (BW) in the
above equations for each section added in cascade. For example, if we require
a three-stage bandpass filter with a bandwidth that will pass 500 Hz, then we
would design the initial single stage to have a bandwidth of 1500 Hz, and then
simply add two more of these duplicate stages. The three-stage cascaded audio
filter will then end up with a final bandwidth of approximately 500 Hz.
An active audio low-pass filter (Fig. 6.53)

1. A 1
v
2. C C 0.022
F
1 2

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