Low voltage drop capacitance multiplier

[John Devereux]

Tube guys don't care...
http://www.hammondmfg.com/193.htm

I don't think that solution would be acceptable, considering my *whole
circuit* is currently on a PCB about 0.5 inch square

 

[Jim Thompson]

Winfield Hill wrote...

Phil Hobbs wrote...

Winfield Hill wrote:

. ,---||----------,
. | |
. Vin ---+------ | ---- C E ---+------ out
. | | B
. | | |
. '-/\/\--+--/\/\--+---||--- gnd

The Sallen-Key trick gives you a much sharper knee, but because of
the sneak path through the cap to the output, it limits your ultimate
attenuation. It's usually better just to split R, as you did, but
return both caps to ground.

. Vin ---+------------- C E ------- out
. | B
. | |
. '-/\/\--+--/\/\--+---||--- gnd
. |
. '----||--- gnd

[ snip ]
Above 10kHz the two-caps-to-ground form won by an impressive 35 dB.

In summary, the Sallen-Key appears superior for hum reduction tasks
if properly designed, and two-caps-to-ground appears superior for
high-frequency noise reduction. And either form dramatically beats
using the same total capacitance with one twice-value resistor,
typically by 20dB in the critical line-frequency-harmonics region.

Substituting a MOSFET for the BJT (admittedly with a poor model),
using the same values (430 ohms and 22uF), the region of Sallen-Key
superiority shrinks, and doesn't occur at a useful frequency unless
the values are carefully chosen. The two-caps-to-ground form wins
by 35 to 45dB above 1kHz -- a nice result. But of course, when using
a MOSFET one wouldn't use small resistors and big electrolytics. If
the values are scaled by 10x the Sallen-Key form doesn't look so bad.

If the values are scaled by ~200x to 220k and 0.1uF (makes sense to
me), the filter's performance is significantly improved, e.g. -50dB
at 120Hz. Surprisingly, the Sallen-Key and two-caps-to-ground forms
are nearly identical with a 500-ohm load (both have nice 86dB dip at
360Hz - can we believe that?), and are better than -67dB above 1kHz.
With a light 5k load the two-caps-to-ground form pulls ahead by a
modest 6dB, above 2kHz. But with the 67 to 86 attenuation results
shown by the Spice analysis, I'd want to spend some time fixing the
2n7000 FET's subthreshold model before taking them too seriously.

. 2n7000
. Vin ---+------------- D S ----- out
. | G * use a 10V gate-source
. | 220k 220k | 0.1 zener if Vin > 20 volts
. '-/\/\--+--/\/\--+---||--- gnd
. | 0.1
. '----||--- gnd

Before leaving this investigation, in which I used resistive loads,
the issue of capacitive loading should be taken into account. For
example, if a three-terminal regulator follows the noise filter, a
separate input capacitor is good for the regulator's stability.
Or if other "ordinary" linear circuitry follows, that'll certainly
need bypass caps.

Note, if a BJT is used, as shown below, a small base resistor or
ferrite bead might be wise to dampen RF oscillation, viz,

. ___
. Vin ---+-------------- C E ---+-------|___|--- out
. | B | |
. | | '----||---+-- gnd
. '-/\/-+-/\/-+-/\/-'
. | '----||--- gnd
. '--||-- gnd

A quick spice look with the MOSFET: adding load capacitors didn't
show much difference, more HF attenuation (78dB), and both plots
coming together above 20kHz. However, in these high-attenuation
regions, poor capacitor esr modeling can create significant errors.

REAL MEN don't count on followers having unity gain ;-)

...Jim Thompson

 

[John Larkin]

Winfield Hill wrote...

Phil Hobbs wrote...

Winfield Hill wrote:

. ,---||----------,
. | |
. Vin ---+------ | ---- C E ---+------ out
. | | B
. | | |
. '-/\/\--+--/\/\--+---||--- gnd

The Sallen-Key trick gives you a much sharper knee, but because of
the sneak path through the cap to the output, it limits your ultimate
attenuation. It's usually better just to split R, as you did, but
return both caps to ground.

. Vin ---+------------- C E ------- out
. | B
. | |
. '-/\/\--+--/\/\--+---||--- gnd
. |
. '----||--- gnd

[ snip ]
Above 10kHz the two-caps-to-ground form won by an impressive 35 dB.

In summary, the Sallen-Key appears superior for hum reduction tasks
if properly designed, and two-caps-to-ground appears superior for
high-frequency noise reduction. And either form dramatically beats
using the same total capacitance with one twice-value resistor,
typically by 20dB in the critical line-frequency-harmonics region.

Substituting a MOSFET for the BJT (admittedly with a poor model),
using the same values (430 ohms and 22uF), the region of Sallen-Key
superiority shrinks, and doesn't occur at a useful frequency unless
the values are carefully chosen. The two-caps-to-ground form wins
by 35 to 45dB above 1kHz -- a nice result. But of course, when using
a MOSFET one wouldn't use small resistors and big electrolytics. If
the values are scaled by 10x the Sallen-Key form doesn't look so bad.

If the values are scaled by ~200x to 220k and 0.1uF (makes sense to
me), the filter's performance is significantly improved, e.g. -50dB
at 120Hz. Surprisingly, the Sallen-Key and two-caps-to-ground forms
are nearly identical with a 500-ohm load (both have nice 86dB dip at
360Hz - can we believe that?), and are better than -67dB above 1kHz.
With a light 5k load the two-caps-to-ground form pulls ahead by a
modest 6dB, above 2kHz. But with the 67 to 86 attenuation results
shown by the Spice analysis, I'd want to spend some time fixing the
2n7000 FET's subthreshold model before taking them too seriously.

. 2n7000
. Vin ---+------------- D S ----- out
. | G * use a 10V gate-source
. | 220k 220k | 0.1 zener if Vin > 20 volts
. '-/\/\--+--/\/\--+---||--- gnd
. | 0.1
. '----||--- gnd

Before leaving this investigation, in which I used resistive loads,
the issue of capacitive loading should be taken into account. For
example, if a three-terminal regulator follows the noise filter, a
separate input capacitor is good for the regulator's stability.
Or if other "ordinary" linear circuitry follows, that'll certainly
need bypass caps.

Note, if a BJT is used, as shown below, a small base resistor or
ferrite bead might be wise to dampen RF oscillation, viz,

. ___
. Vin ---+-------------- C E ---+-------|___|--- out
. | B | |
. | | '----||---+-- gnd
. '-/\/-+-/\/-+-/\/-'
. | '----||--- gnd
. '--||-- gnd

A quick spice look with the MOSFET: adding load capacitors didn't
show much difference, more HF attenuation (78dB), and both plots
coming together above 20kHz. However, in these high-attenuation
regions, poor capacitor esr modeling can create significant errors.

Which sort of suggests using a couple of LM1117s or something...


+---------------------------------+
| |
adj |
--------in out--+--------in out------------+--------
| adj |
| | |
| +-----+-------r-------+
| | | |
| | | |
c r c c
| | | |
| | | |
+-----+-----+---------------+--gnd


John

 

[Jim Thompson]

Winfield Hill wrote...

Phil Hobbs wrote...

Winfield Hill wrote:

. ,---||----------,
. | |
. Vin ---+------ | ---- C E ---+------ out
. | | B
. | | |
. '-/\/\--+--/\/\--+---||--- gnd

The Sallen-Key trick gives you a much sharper knee, but because of
the sneak path through the cap to the output, it limits your ultimate
attenuation. It's usually better just to split R, as you did, but
return both caps to ground.

. Vin ---+------------- C E ------- out
. | B
. | |
. '-/\/\--+--/\/\--+---||--- gnd
. |
. '----||--- gnd

[ snip ]
Above 10kHz the two-caps-to-ground form won by an impressive 35 dB.

In summary, the Sallen-Key appears superior for hum reduction tasks
if properly designed, and two-caps-to-ground appears superior for
high-frequency noise reduction. And either form dramatically beats
using the same total capacitance with one twice-value resistor,
typically by 20dB in the critical line-frequency-harmonics region.

Substituting a MOSFET for the BJT (admittedly with a poor model),
using the same values (430 ohms and 22uF), the region of Sallen-Key
superiority shrinks, and doesn't occur at a useful frequency unless
the values are carefully chosen. The two-caps-to-ground form wins
by 35 to 45dB above 1kHz -- a nice result. But of course, when using
a MOSFET one wouldn't use small resistors and big electrolytics. If
the values are scaled by 10x the Sallen-Key form doesn't look so bad.

If the values are scaled by ~200x to 220k and 0.1uF (makes sense to
me), the filter's performance is significantly improved, e.g. -50dB
at 120Hz. Surprisingly, the Sallen-Key and two-caps-to-ground forms
are nearly identical with a 500-ohm load (both have nice 86dB dip at
360Hz - can we believe that?), and are better than -67dB above 1kHz.
With a light 5k load the two-caps-to-ground form pulls ahead by a
modest 6dB, above 2kHz. But with the 67 to 86 attenuation results
shown by the Spice analysis, I'd want to spend some time fixing the
2n7000 FET's subthreshold model before taking them too seriously.

. 2n7000
. Vin ---+------------- D S ----- out
. | G * use a 10V gate-source
. | 220k 220k | 0.1 zener if Vin > 20 volts
. '-/\/\--+--/\/\--+---||--- gnd
. | 0.1
. '----||--- gnd

Before leaving this investigation, in which I used resistive loads,
the issue of capacitive loading should be taken into account. For
example, if a three-terminal regulator follows the noise filter, a
separate input capacitor is good for the regulator's stability.
Or if other "ordinary" linear circuitry follows, that'll certainly
need bypass caps.

Note, if a BJT is used, as shown below, a small base resistor or
ferrite bead might be wise to dampen RF oscillation, viz,

. ___
. Vin ---+-------------- C E ---+-------|___|--- out
. | B | |
. | | '----||---+-- gnd
. '-/\/-+-/\/-+-/\/-'
. | '----||--- gnd
. '--||-- gnd

A quick spice look with the MOSFET: adding load capacitors didn't
show much difference, more HF attenuation (78dB), and both plots
coming together above 20kHz. However, in these high-attenuation
regions, poor capacitor esr modeling can create significant errors.

Which sort of suggests using a couple of LM1117s or something...


+---------------------------------+
| |
adj |
--------in out--+--------in out------------+--------
| adj |
| | |
| +-----+-------r-------+
| | | |
| | | |
c r c c
| | | |
| | | |
+-----+-----+---------------+--gnd


John

John, Does that configuration meet the I/O minimum drop requirement?

...Jim Thompson

 

[John Larkin]

John, Does that configuration meet the I/O minimum drop requirement?

...Jim Thompson

This looks OK to me up to at least a few hundred mA... has a full
quarter of a volt of headroom!

John

 

[Ken Smith]

Vin _______ _________ Vout
| \_^
R |
'-------|
C
|
--------------------

BTW: This circuit is also sometimes called a psudo-inductor. When seen
from the Vin side, the impedance looks inductive. It is used where power
and signal both travel on the same wire.


If you need more HFE you can use a "White Darlington"

__ ______
! V_/ !
! ! !
! ! !

Vin +__R_+_ ___+_____ Vout
| \_^
R |
'-------|
C
|
--------------------

The NPN ends up with a quite small Vce but it still makes useful gain.

 

[Winfield Hill]

Jim Thompson wrote...

REAL MEN don't count on followers having unity gain ;-)

What's your point? Neither does Spice. Our goal was to
see what can be accomplished with a few parts, and to see
how well it does or doesn't perform.

 

[Fred Bloggs]

BTW: This circuit is also sometimes called a psudo-inductor. When seen
from the Vin side, the impedance looks inductive. It is used where power
and signal both travel on the same wire.


If you need more HFE you can use a "White Darlington"

__ ______
! V_/ !
! ! !
! ! !



The NPN ends up with a quite small Vce but it still makes useful gain.

I wish you would use Tech-Chat or something, the circuits are excellent
but the drawing is hellacious.

 

[John Larkin]

BTW: This circuit is also sometimes called a psudo-inductor. When seen
from the Vin side, the impedance looks inductive. It is used where power
and signal both travel on the same wire.

Right! I "invented" this when I was just a sprout, for the Winch
Control Intercommunications Subsystem (catchy title, huh) for the C-5A
transport. The handheld comm/control boxes derived filtered DC from
the shared party-line twisted pair, and also injected audio and
supersonic control tones (to control the cargo winch) into the line
(by wiggling the base here.) I did a third-order version and
accidentally discovered that a passive R-C network can have voltage
gain... it oscillated until I added a resistor to kill the Q. All this
was done in a fairly theory-free environment.

John

 

[John Devereux]

BTW: This circuit is also sometimes called a psudo-inductor. When seen
from the Vin side, the impedance looks inductive. It is used where power
and signal both travel on the same wire.

I had not thought of it that way... nice.

If you need more HFE you can use a "White Darlington"

__ ______
! V_/ !
! ! !
! ! !

The NPN ends up with a quite small Vce but it still makes useful gain.

OK.

 

[Fred Bartoli]

BTW: This circuit is also sometimes called a psudo-inductor. When seen
from the Vin side, the impedance looks inductive. It is used where power
and signal both travel on the same wire.


If you need more HFE you can use a "White Darlington"

__ ______
! V_/ !
! ! !
! ! !

The NPN ends up with a quite small Vce but it still makes useful gain.

Unfortunatly this becomes beta dependent for its bias point and either the
NPN gets saturated or the bias point will vary with the NPN's beta and
temperature.

A simple cure would be either a diode drop or a divider to bias the base.


.------- --------.
| v / |
| --- |
| ___ | |
--+-|___|--+--- ---+-----+-
| \ ^ |
| --- |
| ___ | |
'->|---|___|--+----. |
D | | |
--- .-. ---
--- | | ---
either R or D | |R| |
| '-' |
| | |
=== === ===
GND GND GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

 

[Winfield Hill]

Phil Hobbs wrote...

Yes, that's more or less what I've found too. I'm usually much more
worried about tens-of-kilohertz crap from SMPS ripple.

Ah, a post SMPS filter, that's a good application.

 

[Phil Hobbs]

Phil Hobbs wrote...



Ah, a post SMPS filter, that's a good application.

Twenty years ago, we were all nodding our heads and agreeing that you needed
a linear regulator for high-sensitivity analogue circuits. Nobody has that
luxury any more, and cap multipliers fix the problem.

Although their voltage regulation and low-frequency ripple suppression aren't
that great, op amps have wonderful supply rejection down there. Up in the
tens of kilohertz, where op amp supply rejection is poorer and all the SMPS
junk lives, the cap multiplier is like a brick wall. Not bad for ten cents!

And I _have_ to be able to use that pseudo-inductor idea for something.
What, I wonder? (I've been doing device work for too long, and need a good
instrument to design.)

Cheers,

Phil Hobbs

 

[Ken Smith]

BTW: This circuit is also sometimes called a psudo-inductor. When seen
from the Vin side, the impedance looks inductive. It is used where power
and signal both travel on the same wire.

Right! I "invented" this when I was just a sprout, for the Winch
Control Intercommunications Subsystem (catchy title, huh) for the C-5A
transport.[/QUOTE]

I think a lot of people have invented this circuit or the version where
the capacitor ties to the emitter.

I knew someone that got a patent on "Psudosilence". It is a very catchy
name but you have to speak clearly when you say it.

 

[Ken Smith]

"Ken Smith" <[email protected]> a écrit dans le message de


Unfortunatly this becomes beta dependent for its bias point and either the
NPN gets saturated or the bias point will vary with the NPN's beta and
temperature.

"Beta dependant beta shmependant"

The goal was a low drop out voltage. The circuit does this to varying
degrees for various transistors. Yes it is sloppy but it does work.

A simple cure would be either a diode drop or a divider to bias the base.


.------- --------.
| v / |
| --- |
| ___ | |
--+-|___|--+--- ---+-----+-
| \ ^ |
| --- |
| ___ | |
'->|---|___|--+----. |
D | | |
--- .-. ---
--- | | ---
either R or D | |R| |
| '-' |
| | |
=== === ===
GND GND GND

If you are going to do that, why not make that diode another NPN
transistor and get even more HFE out of it?