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What do the capacitors in this ducker circuit do?

M

MRW

Jan 1, 1970
0
Hello, about this picture:
http://i14.tinypic.com/4pudydx.jpg

I understand that OA1 is a threshold circuit using an opamp precision
half-wave rectifier and that OA2 is a polarity inverter and a gain
control circuit combined into an inverting mixer configuration.

What I don't get is the purpose of capacitors C8 and C5. Can any one
explain please? Gracias!
 
E

Eeyore

Jan 1, 1970
0
MRW said:
Hello, about this picture:
http://i14.tinypic.com/4pudydx.jpg

I understand that OA1 is a threshold circuit using an opamp precision
half-wave rectifier and that OA2 is a polarity inverter and a gain
control circuit combined into an inverting mixer configuration.

What I don't get is the purpose of capacitors C8 and C5. Can any one
explain please? Gracias!

C5 with R14 is creating a lossy integrator to provide a DC control voltage.

C8 looks like a bodge to me !

Graham
 
J

John Popelish

Jan 1, 1970
0
MRW said:
Hello, about this picture:
http://i14.tinypic.com/4pudydx.jpg

I understand that OA1 is a threshold circuit using an opamp precision
half-wave rectifier and that OA2 is a polarity inverter and a gain
control circuit combined into an inverting mixer configuration.

What I don't get is the purpose of capacitors C8 and C5. Can any one
explain please? Gracias!

Thanks for high lighting the capacitors, so I didn't have to
look all over for them. :)

C5 is easiest. It is connected as negative feedback across
the feedback resistor, turning the amplifier into a low pass
filter. Since the capacitor impedance has the same
magnitude as the 12400 ohm feedback resistor at about 270
Hz, all frequencies higher than that are progressively
rolled off.

C8 is somewhat similarly connected, but in a nonlinear
situation. It may be there to reduce the overshoot of the
output as one diode turns off, and the output slews, open
loop, (going as fast as it can) to the voltage where the
other diode turns on, reclosing the feedback loop. It has a
very much higher frequency effect, because it is normally
across either the impedance of a turned on D2, or across the
turned on impedance of D1 in series with R2, except for the
brief moments I mentioned, first. Both those paths are
pretty low impedance, so the capacitor current is
significant only when both diodes are turned off.
 
M

MRW

Jan 1, 1970
0
Eeyore said:
C5 with R14 is creating a lossy integrator to provide a DC control
voltage.

C8 looks like a bodge to me !

Graham


Hey Graham, Thanks! What is a bodge?


--
 
M

MRW

Jan 1, 1970
0
John said:
Thanks for high lighting the capacitors, so I didn't have to look all
over for them. :)

C5 is easiest. It is connected as negative feedback across the
feedback resistor, turning the amplifier into a low pass filter.
Since the capacitor impedance has the same magnitude as the 12400 ohm
feedback resistor at about 270 Hz, all frequencies higher than that
are progressively rolled off.

C8 is somewhat similarly connected, but in a nonlinear situation. It
may be there to reduce the overshoot of the output as one diode turns
off, and the output slews, open loop, (going as fast as it can) to
the voltage where the other diode turns on, reclosing the feedback
loop. It has a very much higher frequency effect, because it is
normally across either the impedance of a turned on D2, or across the
turned on impedance of D1 in series with R2, except for the brief
moments I mentioned, first. Both those paths are pretty low
impedance, so the capacitor current is significant only when both
diodes are turned off.



Hi John, Thanks! I'm curious about the low pass filter characteristics.
I'm having a hard time understanding because I'm seeing the output of
the RMS Detector 4301P us strictly DC. How does the low pass filter
play in this DC environment? Thanks again!
 
E

Eeyore

Jan 1, 1970
0
MRW said:
Hey Graham, Thanks! What is a bodge?

Like kluge/kludge.

I've never used a cap there myself in a FW rectifier for audio. Maybe there was
something not quite right with the performance and that fixed it.

Graham
 
J

John Popelish

Jan 1, 1970
0
MRW said:
Hi John, Thanks! I'm curious about the low pass filter characteristics.
I'm having a hard time understanding because I'm seeing the output of
the RMS Detector 4301P us strictly DC. How does the low pass filter
play in this DC environment? Thanks again!

The output of the RMS detector must not be pure DC, but
varying, unidirectional voltage. The low pass filter
smooths the bumps. Anything that changes faster than the RC
time constant of the feedback pair is smoothed
(12400*47n=.00058 seconds), and the faster it changes, the
more it is smoothed.

A better question might be:
If the RMS detector outputs a DC signal, why is it followed
by a rectifier?
 
M

MRW

Jan 1, 1970
0
John said:
A better question might be:
If the RMS detector outputs a DC signal, why is it followed by a
rectifier?


Hi John, I was looking at the rectifier as circuit that categorizes the
voltage level. If the output of the RMS detector is above a certain
value, then the output of the rectifier follows the detector. If the
RMS detector output is below threshold, then the rectifier stays at 0V.
Did I look at this correctly?

Thanks!
 
J

John Popelish

Jan 1, 1970
0
MRW said:
Hi John, I was looking at the rectifier as circuit that categorizes the
voltage level. If the output of the RMS detector is above a certain
value, then the output of the rectifier follows the detector. If the
RMS detector output is below threshold, then the rectifier stays at 0V.
Did I look at this correctly?

I haven't studied how the RMS converter works.
But after some digging, I did find the data sheet:
http://www.thatcorp.com/datashts/4301data.pdf

The RMS converter does not look very well defined, but it
does include a low pass filter (time constant = .026*Ct/It).
But it also shows a bidirectional output current of +- 100 uA.

So you may be exactly right, that the negative values for
below threshold levels, are clipped off by the ideal diode.
 
A

Anthony Fremont

Jan 1, 1970
0
Eeyore said:
C5 with R14 is creating a lossy integrator to provide a DC control
voltage.

C8 looks like a bodge to me !

Ridiculous. I'm not that clever with this stuff, but I recognize high
frequency roll-off when I see it.
 
E

Eeyore

Jan 1, 1970
0
Anthony said:
Ridiculous. I'm not that clever with this stuff, but I recognize high
frequency roll-off when I see it.

That IS NOT high frequiency roll-off.

Graham
 
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