Strange Behaviour of Filter

[George K]

I have a circuit which takes the output of a one-shot and filter it.
There are two stages. First the signal goes to two parallel capacitors
between which there is a resistor, and one end of the resistor is
connected to the negative input of the amplifier and the other to the
output of the amplifier. The output of the amplifier through a
resistance goes to a similar network with a second op-amp which output
is the final output. The positive inputs of both op-amps are connected
through a voltage divider to Vcc/2.
To make it more clear I will number the components and I will describe
one stage of the filter as the second is the same. C1 1,p2 - C2:
p1,p3 - R1: p2, p3 - Op.amp. + : Vcc/2 - Op-amp - : p3 -
Op.amp out : p2.

R2 = resistor between the two stages : p2, p1' (=p1 of the second
stage)
And now what is going on and I cannot understand. I have two almost
identical prototypes of this circuit. In one of them the points p1',
p2', p3' have a Vcc/2=2.5V voltage and in the other circuit for the
same points there is a Dc component of Vcc/2 but there is a sinwave of
2V p-p added to the DC voltage. In the circuit with the aforementioned
AC voltage, the voltage before the resistor which is placed between the
two stages is only the DC component.

 

[Dan Hollands]

I have a circuit which takes the output of a one-shot and filter it.
There are two stages. First the signal goes to two parallel capacitors
between which there is a resistor, and one end of the resistor is
connected to the negative input of the amplifier and the other to the
output of the amplifier. The output of the amplifier through a
resistance goes to a similar network with a second op-amp which output
is the final output. The positive inputs of both op-amps are connected
through a voltage divider to Vcc/2.
To make it more clear I will number the components and I will describe
one stage of the filter as the second is the same. C1 1,p2 - C2:
p1,p3 - R1: p2, p3 - Op.amp. + : Vcc/2 - Op-amp - : p3 -
Op.amp out : p2.

R2 = resistor between the two stages : p2, p1' (=p1 of the second
stage)
And now what is going on and I cannot understand. I have two almost
identical prototypes of this circuit. In one of them the points p1',
p2', p3' have a Vcc/2=2.5V voltage and in the other circuit for the
same points there is a Dc component of Vcc/2 but there is a sinwave of
2V p-p added to the DC voltage. In the circuit with the aforementioned
AC voltage, the voltage before the resistor which is placed between the
two stages is only the DC component.

I have no idea how your circuit works but if you have two identical circuits
and they act differently, they probably aren't identical.

Dan

--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com

 

[Jon]

It would be helpful to know what you are trying to accomplish with your
circuit. I see 2 potential problems:
~
Both stages have capacitors between the inverting inputs and ground,
and between the outputs and ground. If the amplifiers you are
describing are op-amps, both of these conditions are invitations to
oscillation, depending on what the gain-bandwidth product of the
op-amps happen to be.
~
A question: Is there also a resistor on the input side of the first
stage?
Regards,
Jon