Maker Pro
Maker Pro

Help - simple filter maths please

W

WAYNEL

Jan 1, 1970
0
Hi All

I seem to be having a bad day.
I am trying to calc to freq. resp. (-3db) of the following circuit.

Vin
¦
--
¦ ¦
¦ ¦ 2K R1
¦ ¦
--
¦
¦--------------- Vout
-- ¦
¦ ¦ ___¦__
¦ ¦ 20K R2 C1 100nF
¦ ¦ ______
-- ¦
¦ ¦
----------------
¦
___
_

Cheers


WayneL
 
P

Paul Burke

Jan 1, 1970
0
WAYNEL said:
I seem to be having a bad day.
I am trying to calc to freq. resp. (-3db) of the following circuit.

You're not kidding it's a bad day, it's just the AC version of Ohm's Law:

Calculate R2||Xc as though Xc were a resistance - hint Xc = 1/wC. Call it Z.

Calculate the output of Vo = Vi Z/(R+Z).

Sort out to get the form G* 1/(1+wT) where G is some resistive
combination and T is some sort of RC. Set wT = 1, solve for w hence f0.

You'll learn more if you DIY.

Paul Burke
 
G

Genome

Jan 1, 1970
0
Hi All

I seem to be having a bad day.
I am trying to calc to freq. resp. (-3db) of the following circuit.

Vin
¦
--
¦ ¦
¦ ¦ 2K R1
¦ ¦
--
¦
¦--------------- Vout
-- ¦
¦ ¦ ___¦__
¦ ¦ 20K R2 C1 100nF
¦ ¦ ______
-- ¦
¦ ¦
----------------
¦
___
_

Cheers


WayneL

If you are looking for when VOUT is 3dB down from VOUT at DC then it's when
the capacitors impedance is equal to the parallel impedance of the two
resistors.

DNA
 
W

WAYNEL

Jan 1, 1970
0
Tried ohms law varient, the day is not that bad.
It is this G* 1/(1+wT) that I am struggling on?

Anymore taker please.

WayneL
 
G

Genome

Jan 1, 1970
0
WAYNEL said:
Tried ohms law varient, the day is not that bad.
It is this G* 1/(1+wT) that I am struggling on?

Anymore taker please.

WayneL

Oh, you wanted an analytical answer. Can't be arsed.

By virtue of beer, anti-depressants, an argument with the girlfreud and
general introspection...


I am right.

QED

Unless I am wrong....

DNA
 
L

Larry Brasfield

Jan 1, 1970
0
WAYNEL said:
Tried ohms law varient, the day is not that bad.
It is this G* 1/(1+wT) that I am struggling on?

Anymore taker please.


Mr. Burke's advice is mostly good. You should show what you have
done and where it has led you. 'G' will be simply the low-frequency
voltage gain of that circuit. 'T' will be the product of the Thevenin
equivalent resistance driving C1 and C1's value. To get the frequency
response, evaluate G / (1 + j * w * T) for whatever range of angular
frequencies you are interested in. The magnitude of the response is
sqrt( re(G / (1 + j * w * T))^2 + im(G / (1 + j * w * T))^2)
while the phase of the response is
atan2( im(G / (1 + j * w * T)) + re(G / (1 + j * w * T)))
where re() and im() are the real and imaginary extraction functions.
Of course, 'j' is the unit imaginary number, sqrt(-1) and 'w' is the
angular frequency, equal to 2 * Pi * frequency_in_Hertz.

You'll need to be more specific about your puzzlement if this is
not clear. People here generally refrain from doing homework
for students. You can get plenty of help, but you have to make
some effort yourself and show some of it.
 
J

Jim Thompson

Jan 1, 1970
0
Oh, you wanted an analytical answer. Can't be arsed.

By virtue of beer, anti-depressants, an argument with the girlfreud and
general introspection...


I am right.

QED

Unless I am wrong....

DNA

Genome, You sure do seem to have a lot of "girlfreud" problems.

Bwahahahahaha! ROTFLMAO!

...Jim Thompson
 
J

John Larkin

Jan 1, 1970
0
Oh, you wanted an analytical answer. Can't be arsed.

By virtue of beer, anti-depressants, an argument with the girlfreud and
general introspection...


I am right.

QED

Unless I am wrong....

DNA

The GF is always right.

John
 
J

John Larkin

Jan 1, 1970
0
Hi All

I seem to be having a bad day.
I am trying to calc to freq. resp. (-3db) of the following circuit.

Vin
¦
--
¦ ¦
¦ ¦ 2K R1
¦ ¦
--
¦
¦--------------- Vout
-- ¦
¦ ¦ ___¦__
¦ ¦ 20K R2 C1 100nF
¦ ¦ ______
-- ¦
¦ ¦
----------------
¦
___
_

Cheers


WayneL


Low frequency gain is 20/22.

That drops off at high freqs. The 3dB corner freq is

F = 1/(2*pi*r*c)

where r = 2k || 20k

which is a tad under 1 KHz.

John
 
J

Jim Thompson

Jan 1, 1970
0
The GF is always right.

John

Only two phrases are required for a long relationship, "Yes, Dear" and
"I'm sorry" ;-)

...Jim Thompson
 
Very good explanation.

Please could you detail the real and imaginary extraction functions, OR
preferrably how to get them.

Thanks
Barry
 
J

Jon

Jan 1, 1970
0
WayneL:

The resistance that the Capacitor "sees" is R1 || R2 =
` R1R2/(R1+R2) = 2X20/(2+20) = 1.818181......K
` Time constant (Tau) = RC = 1.8181818...E3 X 1E-7 =
` 1.81818E-4.
f3db = 1/(2 X Pi X Tau) = 1/(2 X Pi X 1.81818E-4) =
' 8.7535E2 = 875.35 Hz
 
L

Larry Brasfield

Jan 1, 1970
0
Very good explanation.

We aim to please, Barry.
Please could you detail the real and imaginary extraction functions, OR
preferrably how to get them.

Get the real part of something by penetrating the veil
of sensation to derive the alleged underlying reality.

Get the imaginary part by other means, perhaps aided
by drugs, sleep deprivation, or wishful thinking.
 
L

Larry Brasfield

Jan 1, 1970
0
Very good explanation.

We aim to please, Barry.
Please could you detail the real and imaginary extraction functions, OR
preferrably how to get them.

Get the real part of something by penetrating the veil
of sensation to derive the alleged underlying reality.

Get the imaginary part by other means, perhaps aided
by drugs, sleep deprivation, or wishful thinking.
 
T

Tim Wescott

Jan 1, 1970
0
John said:
Low frequency gain is 20/22.

That drops off at high freqs. The 3dB corner freq is

F = 1/(2*pi*r*c)

where r = 2k || 20k

which is a tad under 1 KHz.

John
Unless your problem is a difference between the above prediction and an
actual real live measument -- in which the first thing I would look at
would be the impedance of the source, and the second would be the actual
(not nominal) value of the components, particularly the cap.
 
K

Keith Williams

Jan 1, 1970
0
Only two phrases are required for a long relationship, "Yes, Dear" and
"I'm sorry" ;-)

Got this one from a female friend today:

HER DIARY:
Saturday night I thought he was acting weird. We had made plans to
meet at a bar to have a drink. I was shopping with my friend all day
long, so I thought he was upset at the fact that I was a bit late, but
he made no comment. Conversation wasn't flowing so I suggested that we
go somewhere quiet so we could talk, he agreed but he kept quiet and
absent. I asked him what was wrong he said nothing. I asked him if it
was my fault that he was upset. He said it had nothing to do with me
and not to worry.

On the way home I told him that I loved him, he simply smiled and kept
driving. I can't explain his behavior; I don't know why he didn't say I
love you too.


When we got home I felt as if I had lost him, as if he wanted nothing
to do with me anymore. He just sat there and watched T.V. He seemed
distant and absent. Finally I decided to go to bed, about 10 minutes
later he came to bed and to my surprise he responded to my caress and
we made love, but I still felt that he was distracted and his thoughts
were somewhere else. I decided that I could not take it anymore so I
decided to confront him with the situation but he had fallen asleep. I
started crying and cried until I too fell asleep. I don't know what to
do. I'm almost sure that his thoughts are with someone else. My life is
a disaster.


HIS DIARY:
Played a horrible game of golf today! Shot a 95. Can't putt for shit.
Got laid though.
 
J

Jim Thompson

Jan 1, 1970
0
[snip]
Got this one from a female friend today:

HER DIARY:
Saturday night I thought he was acting weird. We had made plans to
meet at a bar to have a drink. I was shopping with my friend all day
long, so I thought he was upset at the fact that I was a bit late, but
he made no comment. Conversation wasn't flowing so I suggested that we
go somewhere quiet so we could talk, he agreed but he kept quiet and
absent. I asked him what was wrong he said nothing. I asked him if it
was my fault that he was upset. He said it had nothing to do with me
and not to worry.

On the way home I told him that I loved him, he simply smiled and kept
driving. I can't explain his behavior; I don't know why he didn't say I
love you too.


When we got home I felt as if I had lost him, as if he wanted nothing
to do with me anymore. He just sat there and watched T.V. He seemed
distant and absent. Finally I decided to go to bed, about 10 minutes
later he came to bed and to my surprise he responded to my caress and
we made love, but I still felt that he was distracted and his thoughts
were somewhere else. I decided that I could not take it anymore so I
decided to confront him with the situation but he had fallen asleep. I
started crying and cried until I too fell asleep. I don't know what to
do. I'm almost sure that his thoughts are with someone else. My life is
a disaster.


HIS DIARY:
Played a horrible game of golf today! Shot a 95. Can't putt for shit.
Got laid though.

Sno-o-o-o-rt! ROTFLMAO!

...Jim Thompson
 
J

John Larkin

Jan 1, 1970
0
Unless your problem is a difference between the above prediction and an
actual real live measument -- in which the first thing I would look at
would be the impedance of the source, and the second would be the actual
(not nominal) value of the components, particularly the cap.


2k || 20k isn't much different from 2k || infinity.

John
 
Top