# Interesting negative capacitance circuit

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bitrex
Guest

Wed Jan 11, 2017 6:31 am

On 01/10/2017 02:19 PM, Jim Thompson wrote:
Quote:
On Tue, 10 Jan 2017 11:49:01 -0500, bitrex

Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents.

[snip]

Can you show us the math behind your conclusion, "It looks like...." ?

...Jim Thompson

Sure, when is my homework due?

bitrex
Guest

Wed Jan 11, 2017 6:32 am

On 01/10/2017 06:31 PM, bitrex wrote:
Quote:
On 01/10/2017 02:19 PM, Jim Thompson wrote:
On Tue, 10 Jan 2017 11:49:01 -0500, bitrex

Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents.

[snip]

Can you show us the math behind your conclusion, "It looks like...." ?

...Jim Thompson

Sure, when is my homework due?

Also the fact that it's claimed in the text, though they don't give the
derivation there.

Jim Thompson
Guest

Wed Jan 11, 2017 7:59 am

On Tue, 10 Jan 2017 11:49:01 -0500, bitrex

Quote:
Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

[snip]

We're still waiting... Can you show us the math behind your
conclusion, "It looks like...." ?

...Jim Thompson
--
| James E.Thompson | mens |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

bitrex
Guest

Wed Jan 11, 2017 8:30 am

On 01/10/2017 03:11 PM, Phil Hobbs wrote:
Quote:
On 01/10/2017 02:09 PM, John Larkin wrote:
On Tue, 10 Jan 2017 11:49:01 -0500, bitrex

Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

Is anyone aware of any similar topologies for grounded negative
capacitances? I've seen some other topologies using a couple transistors
but they all seemed to require transformers on the input port.

It might be useful to have a circuit like that where the negative
capacitance was proportional to a control voltage.

That is pretty cool, a potential replacement for a varicap. Varicaps
can be a real pain.

Reactance modulators have been around for a long time. They're
generally crap because the amplifier is too slow and noisy to do a good
job at it.

If you replace the output transistor with a diff pair, you could make a
negative-capacitance varactor, which would be neat, I agree. You could
even make its bias stable by running the diff pair as an exponentiator,
so that the extra DC gets dumped to ground but the rest of the circuit
sees a constant value.

Cheers

Phil Hobbs

Wait until they see my patent for a negative capacitance tank circuit to
generate infinite power! Just it it with a sine wave and the amplitude
grows exponentially forever. They all laughed and said it couldn't be
done, but I found the secret: bootstrap the active devices in the
negative cap circuit off the tank output voltage. Hah!

Guest

Wed Jan 11, 2017 10:31 am

Quote:
Wait until they see my patent for a negative capacitance tank circuit to
generate infinite power! Just it it with a sine wave and the amplitude
grows exponentially forever. They all laughed and said it couldn't be
done, but I found the secret: bootstrap the active devices in the
negative cap circuit off the tank output voltage. Hah!

Mr. Hendershot, time for your medication.

bitrex
Guest

Wed Jan 11, 2017 2:32 pm

On 01/10/2017 07:59 PM, Jim Thompson wrote:
Quote:
On Tue, 10 Jan 2017 11:49:01 -0500, bitrex

Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

[snip]

We're still waiting... Can you show us the math behind your
conclusion, "It looks like...." ?

...Jim Thompson

This is the first part of my attempt at an analysis; if I don't notice
anything horribly wrong I'll continue in the morning:

http://imgur.com/a/1wUS4

bitrex
Guest

Wed Jan 11, 2017 10:50 pm

On 01/11/2017 03:31 AM, jurb6006_at_gmail.com wrote:
Quote:

Wait until they see my patent for a negative capacitance tank circuit to
generate infinite power! Just it it with a sine wave and the amplitude
grows exponentially forever. They all laughed and said it couldn't be
done, but I found the secret: bootstrap the active devices in the
negative cap circuit off the tank output voltage. Hah!

Mr. Hendershot, time for your medication.

I think I was around 7 years old when I first thought "Hmm, what would
happen if I connected a lamp up to a solar cell, and then shined the
lamp on a solar cell and used it to power the lamp?"

Then I thought "No, that's stupid." Then I went and watched cartoons

Jim Thompson
Guest

Wed Jan 11, 2017 11:48 pm

On Tue, 10 Jan 2017 11:49:01 -0500, bitrex

Quote:
Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

Is anyone aware of any similar topologies for grounded negative
capacitances? I've seen some other topologies using a couple transistors
but they all seemed to require transformers on the input port.

It might be useful to have a circuit like that where the negative
capacitance was proportional to a control voltage.

See "NegativeCapacitor.pdf" on the S.E.D/Schematics Page of my
website.

...Jim Thompson
--
| James E.Thompson | mens |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Jim Thompson
Guest

Thu Jan 12, 2017 2:04 am

On Wed, 11 Jan 2017 09:48:15 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon_at_On-My-Web-Site.com> wrote:

Quote:
On Tue, 10 Jan 2017 11:49:01 -0500, bitrex

Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

Is anyone aware of any similar topologies for grounded negative
capacitances? I've seen some other topologies using a couple transistors
but they all seemed to require transformers on the input port.

It might be useful to have a circuit like that where the negative
capacitance was proportional to a control voltage.

See "NegativeCapacitor.pdf" on the S.E.D/Schematics Page of my
website.

...Jim Thompson

"CapacitorMultiplier_Variable.pdf"

...Jim Thompson
--
| James E.Thompson | mens |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Jim Thompson
Guest

Thu Jan 12, 2017 2:31 am

On Tue, 10 Jan 2017 11:56:47 -0500, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
On 01/10/2017 11:49 AM, bitrex wrote:
Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

Is anyone aware of any similar topologies for grounded negative
capacitances? I've seen some other topologies using a couple transistors
but they all seemed to require transformers on the input port.

It might be useful to have a circuit like that where the negative
capacitance was proportional to a control voltage.

I'm surprised that you could patent that in 2009.

The usual negative capacitor is a noninverting amp with a gain slightly
larger than unity, and a cap connected from output to + input.

They're very rarely useful, because any phase lag gives the input a
negative conductance as well as susceptance, so unless the amp is way
faster than what you're connecting it to, it'll oscillate before the
total capacitance goes away.

(I did use one last year, but that's almost a first.)

Cheers

Phil Hobbs

Digging thru my past designs I find a capacitance multiplier based on
the AD835... good upwards of 250MHz.

I haven't tried, but I'd guess that one could twist it around to be a
negative capacitance... though I'd expect oscillation

...Jim Thompson
--
| James E.Thompson | mens |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Phil Hobbs
Guest

Thu Jan 12, 2017 3:33 am

On 01/11/2017 02:31 PM, Jim Thompson wrote:
Quote:
On Tue, 10 Jan 2017 11:56:47 -0500, Phil Hobbs
pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 01/10/2017 11:49 AM, bitrex wrote:
Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

Is anyone aware of any similar topologies for grounded negative
capacitances? I've seen some other topologies using a couple transistors
but they all seemed to require transformers on the input port.

It might be useful to have a circuit like that where the negative
capacitance was proportional to a control voltage.

I'm surprised that you could patent that in 2009.

The usual negative capacitor is a noninverting amp with a gain slightly
larger than unity, and a cap connected from output to + input.

They're very rarely useful, because any phase lag gives the input a
negative conductance as well as susceptance, so unless the amp is way
faster than what you're connecting it to, it'll oscillate before the
total capacitance goes away.

(I did use one last year, but that's almost a first.)

Cheers

Phil Hobbs

Digging thru my past designs I find a capacitance multiplier based on
the AD835... good upwards of 250MHz.

I haven't tried, but I'd guess that one could twist it around to be a
negative capacitance... though I'd expect oscillation

...Jim Thompson

Sure, my negative capacitor was based on an ADA4817 (1.4 GHz). That
gave me enough space to roll off the gain before the phase shift got too
ugly.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

Jim Thompson
Guest

Thu Jan 12, 2017 3:45 am

On Wed, 11 Jan 2017 15:33:44 -0500, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
On 01/11/2017 02:31 PM, Jim Thompson wrote:
On Tue, 10 Jan 2017 11:56:47 -0500, Phil Hobbs
pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 01/10/2017 11:49 AM, bitrex wrote:
Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

Is anyone aware of any similar topologies for grounded negative
capacitances? I've seen some other topologies using a couple transistors
but they all seemed to require transformers on the input port.

It might be useful to have a circuit like that where the negative
capacitance was proportional to a control voltage.

I'm surprised that you could patent that in 2009.

The usual negative capacitor is a noninverting amp with a gain slightly
larger than unity, and a cap connected from output to + input.

They're very rarely useful, because any phase lag gives the input a
negative conductance as well as susceptance, so unless the amp is way
faster than what you're connecting it to, it'll oscillate before the
total capacitance goes away.

(I did use one last year, but that's almost a first.)

Cheers

Phil Hobbs

Digging thru my past designs I find a capacitance multiplier based on
the AD835... good upwards of 250MHz.

I haven't tried, but I'd guess that one could twist it around to be a
negative capacitance... though I'd expect oscillation

...Jim Thompson

Sure, my negative capacitor was based on an ADA4817 (1.4 GHz). That
gave me enough space to roll off the gain before the phase shift got too
ugly.

Cheers

Phil Hobbs

That'll cover the audio range >:-}

...Jim Thompson
--
| James E.Thompson | mens |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

bitrex
Guest

Thu Jan 12, 2017 6:33 am

On 01/10/2017 03:11 PM, Phil Hobbs wrote:
Quote:
On 01/10/2017 02:09 PM, John Larkin wrote:
On Tue, 10 Jan 2017 11:49:01 -0500, bitrex

Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

Is anyone aware of any similar topologies for grounded negative
capacitances? I've seen some other topologies using a couple transistors
but they all seemed to require transformers on the input port.

It might be useful to have a circuit like that where the negative
capacitance was proportional to a control voltage.

That is pretty cool, a potential replacement for a varicap. Varicaps
can be a real pain.

Reactance modulators have been around for a long time. They're
generally crap because the amplifier is too slow and noisy to do a good
job at it.

If you replace the output transistor with a diff pair, you could make a
negative-capacitance varactor, which would be neat, I agree. You could
even make its bias stable by running the diff pair as an exponentiator,
so that the extra DC gets dumped to ground but the rest of the circuit
sees a constant value.

Cheers

Phil Hobbs

Some issues I have with the circuit analysis given by the patent (and JT):

First thing is that it's not unstable for all component values; if
they're appropriately selected it "does what it says on the tin."

Second issue I'll get to by a bit of rhetoric: the circuit in the
diagram is composed entirely of ideal components with no negative
reactances anywhere that I can see. So, where is the "magic" happening
that makes the impedance looking into the input port negative? Yeah it
has positive feedback but positive feedback doesn't make magic.

Hypothetical question: suppose I had a huge compliance and replaced the
rightmost current source with a large, but finite resistor. Would the
circuit work, for any component values or signal levels? All real world
current sources have finite impedances, so...

Phil Hobbs
Guest

Thu Jan 12, 2017 7:52 am

On 01/11/2017 03:45 PM, Jim Thompson wrote:
Quote:
On Wed, 11 Jan 2017 15:33:44 -0500, Phil Hobbs
pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 01/11/2017 02:31 PM, Jim Thompson wrote:
On Tue, 10 Jan 2017 11:56:47 -0500, Phil Hobbs
pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 01/10/2017 11:49 AM, bitrex wrote:
Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

Is anyone aware of any similar topologies for grounded negative
capacitances? I've seen some other topologies using a couple transistors
but they all seemed to require transformers on the input port.

It might be useful to have a circuit like that where the negative
capacitance was proportional to a control voltage.

I'm surprised that you could patent that in 2009.

The usual negative capacitor is a noninverting amp with a gain slightly
larger than unity, and a cap connected from output to + input.

They're very rarely useful, because any phase lag gives the input a
negative conductance as well as susceptance, so unless the amp is way
faster than what you're connecting it to, it'll oscillate before the
total capacitance goes away.

(I did use one last year, but that's almost a first.)

Cheers

Phil Hobbs

Digging thru my past designs I find a capacitance multiplier based on
the AD835... good upwards of 250MHz.

I haven't tried, but I'd guess that one could twist it around to be a
negative capacitance... though I'd expect oscillation

...Jim Thompson

Sure, my negative capacitor was based on an ADA4817 (1.4 GHz). That
gave me enough space to roll off the gain before the phase shift got too
ugly.

Cheers

Phil Hobbs

That'll cover the audio range >:-}

...Jim Thompson

Well, sometimes a bigger hammer is better.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

Phil Hobbs
Guest

Thu Jan 12, 2017 7:55 am

On 01/11/2017 06:33 PM, bitrex wrote:
Quote:
On 01/10/2017 03:11 PM, Phil Hobbs wrote:
On 01/10/2017 02:09 PM, John Larkin wrote:
On Tue, 10 Jan 2017 11:49:01 -0500, bitrex

Saw this interesting circuit while trolling around on patent sites
looking for inspiration:

It looks like the negative capacitance is proportional to the
transconductance of "Qn2", i.e. proportional to the bias currents. In
the patent they seem to be using it to null the gate/source capacitance
on the MOSFET output stage of power amps.

Is anyone aware of any similar topologies for grounded negative
capacitances? I've seen some other topologies using a couple
transistors
but they all seemed to require transformers on the input port.

It might be useful to have a circuit like that where the negative
capacitance was proportional to a control voltage.

That is pretty cool, a potential replacement for a varicap. Varicaps
can be a real pain.

Reactance modulators have been around for a long time. They're
generally crap because the amplifier is too slow and noisy to do a good
job at it.

If you replace the output transistor with a diff pair, you could make a
negative-capacitance varactor, which would be neat, I agree. You could
even make its bias stable by running the diff pair as an exponentiator,
so that the extra DC gets dumped to ground but the rest of the circuit
sees a constant value.

Cheers

Phil Hobbs

Some issues I have with the circuit analysis given by the patent (and JT):

First thing is that it's not unstable for all component values; if
they're appropriately selected it "does what it says on the tin."

Second issue I'll get to by a bit of rhetoric: the circuit in the
diagram is composed entirely of ideal components with no negative
reactances anywhere that I can see. So, where is the "magic" happening
that makes the impedance looking into the input port negative? Yeah it
has positive feedback but positive feedback doesn't make magic.

It's pretty simple. When the input goes up, the follower dumps current
into the emitter of the output transistor, which _subtracts_ current
from the collector circuit. This works because both transistors have
current-source biasing.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics