I need to build an amplifier very similar to this one i found in a 
book
(Webster, 1995)
http://img111.imagevenue.com/img.php?loc=loc256&image=c55_webster.jpg

Mine needs to have a BW (-3dB) from 0.05 to 500Hz.

According to this book design, the 1uF and 3,3M resistor forms a high 
pass
filter at 0.05Hz, but 1/RC = 0.3Hz.

/ He says "passes all fequencies above 0.05 Hz". "Passes" is a rather 
loose term. A single pole filter has a "roll off" of 6 dB per octave 
and 0.33/0.05 ~=6 or about 2^2.5 so loss at 0.05 hz would be *about* 
15 dB (voltage).
/

Also, the low pass filter after that (150K and 0.01uF) is said to be 
at
100Hz, but 1/RC = 667Hz.

/
At the top end he seems to have hedged his bets in the opposite 
direction and has placed the cutoff frequency a factor of 667/100 ~=6 
times outside the passband.
/

Am I getting something wrong?

/
Probably not - I suspect his words and his circuit don't match. He may 
be an order of magnitude out (or two) in his words.

Question: WHY do you meed 0.05 Hz lower frequency? - that's 20 seconds 
per cycle.
His original circuit is in the context of an ECG amplifier. It seems 
very unlikel;y that any 1/20th of a Hz components would be significant 
there.

What is you application.

It's easy enough to build the circuit shown and then experiment.

I must say [IMHO of course :-) ] that that's a nasty looking webpage 
hoster you've got there :-(.



        RM


-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist