I've never studied or used a cap multiplier so this may well be
completely unusable, but at worst I'll learn a bit more about them :-).
What determines the frequency response of a cap multiplier? If it
could be made large enough, could a cap multiplier be used with a
varactor in a frequency tuning application? For a given tuning range
this would narrow the required capacitance range of the varactor which
should improve the linearity and perhaps the stability of the overall
circuit.

The applications I've seen have all been for power supply stabilization
where the frequency response did not need to be very high and the net
capacitance and current was "large". I'm hoping that with the tiny
capacitance of a varactor and a good small-signal transistor the
frequency range could be extended into the normal range where varactors
are used.

--
Regards,
Carl

On Wednesday, June 23, 2021 at 1:52:31 PM UTC-4, Carl wrote:
> I've never studied or used a cap multiplier so this may well be
> completely unusable, but at worst I'll learn a bit more about them :-).
> What determines the frequency response of a cap multiplier? If it
> could be made large enough, could a cap multiplier be used with a
> varactor in a frequency tuning application? For a given tuning range
> this would narrow the required capacitance range of the varactor which
> should improve the linearity and perhaps the stability of the overall
> circuit.
>
> The applications I've seen have all been for power supply stabilization
> where the frequency response did not need to be very high and the net
> capacitance and current was "large". I'm hoping that with the tiny
> capacitance of a varactor and a good small-signal transistor the
> frequency range could be extended into the normal range where varactors
> are used.
>
> --
> Regards,
> Carl

It's easier to manipulate the frequency with multipliers and mixers than fool around with tuning.

On 6/23/2021 2:47 PM, Fred Bloggs wrote:
> On Wednesday, June 23, 2021 at 1:52:31 PM UTC-4, Carl wrote:
>> I've never studied or used a cap multiplier so this may well be
>> completely unusable, but at worst I'll learn a bit more about them :-).
>> What determines the frequency response of a cap multiplier? If it
>> could be made large enough, could a cap multiplier be used with a
>> varactor in a frequency tuning application? For a given tuning range
>> this would narrow the required capacitance range of the varactor which
>> should improve the linearity and perhaps the stability of the overall
>> circuit.
>>
>> The applications I've seen have all been for power supply stabilization
>> where the frequency response did not need to be very high and the net
>> capacitance and current was "large". I'm hoping that with the tiny
>> capacitance of a varactor and a good small-signal transistor the
>> frequency range could be extended into the normal range where varactors
>> are used.
>>
>> --
>> Regards,
>> Carl
>
> It's easier to manipulate the frequency with multipliers and mixers than fool around with tuning.
>

This kind of circuit has srs probs for small capacitances and/or high
frequencies. Sometimes it's not even stable for low ones, either:

<http://www.seekic.com/circuit_diagram/index1434.html>

On 6/23/2021 1:51 PM, Carl wrote:
> I've never studied or used a cap multiplier so this may well be
> completely unusable, but at worst I'll learn a bit more about them :-).
>  What determines the frequency response of a cap multiplier?  If it
> could be made large enough, could a cap multiplier be used with a
> varactor in a frequency tuning application?  For a given tuning range
> this would narrow the required capacitance range of the varactor which
> should improve the linearity and perhaps the stability of the overall
> circuit.
>
> The applications I've seen have all been for power supply stabilization
> where the frequency response did not need to be very high and the net
> capacitance and current was "large".  I'm hoping that with the tiny
> capacitance of a varactor and a good small-signal transistor the
> frequency range could be extended into the normal range where varactors
> are used.
>

Muck with high frequency voltage-feedback cap multipliers at the risk of
your sanity; unlike inductance gyrators which tend to be fairly
well-behaved all that voltage-feedback cap multipliers for applications
other than supply filtering/low frequency really wanna do is oscillate,
latch up, or generally jump off.

Don't assume a transistor is just a DC-to-daylight current multiplier or
an op amp is an ideal op amp to analyze them, at least.

Basically the simple ones suck for signal processing. There are good
high-frequency cap multipliers used in ICs but I think they tend to use
topologies you can't do easily with discretes for high frequency like
high-performance OTAs and current conveyers.

On Wednesday, June 23, 2021 at 10:52:31 AM UTC-7, Carl wrote:
> I've never studied or used a cap multiplier so this may well be
> completely unusable, but at worst I'll learn a bit more about them :-).
> What determines the frequency response of a cap multiplier? If it
> could be made large enough, could a cap multiplier be used with a
> varactor in a frequency tuning application?

The issue is not frequency response (at least in a power filtering operation,
the transistor is as good as the electrolytic filter) but impedance. The
emitter resistance of an ideal bipolar transistor, at low currents (1 mA) is
about 0.025V/I_emitter, so that's gonna be 25 ohms.
It gets better if you're supplying more current (200 mA, closer to 0.13 ohms) so
it's good for a power supply, but that 25 ohms series resistance isn't
optimal for high-ish frequencies if you want a pure capacitance.

MOSFETs, while touting low ON resistance, are worse for linear operation
than bipolars in this respect.

On Wednesday, June 23, 2021 at 3:11:01 PM UTC-4, bitrex wrote:
> On 6/23/2021 2:47 PM, Fred Bloggs wrote:
> > On Wednesday, June 23, 2021 at 1:52:31 PM UTC-4, Carl wrote:
> >> I've never studied or used a cap multiplier so this may well be
> >> completely unusable, but at worst I'll learn a bit more about them :-).
> >> What determines the frequency response of a cap multiplier? If it
> >> could be made large enough, could a cap multiplier be used with a
> >> varactor in a frequency tuning application? For a given tuning range
> >> this would narrow the required capacitance range of the varactor which
> >> should improve the linearity and perhaps the stability of the overall
> >> circuit.
> >>
> >> The applications I've seen have all been for power supply stabilization
> >> where the frequency response did not need to be very high and the net
> >> capacitance and current was "large". I'm hoping that with the tiny
> >> capacitance of a varactor and a good small-signal transistor the
> >> frequency range could be extended into the normal range where varactors
> >> are used.
> >>
> >> --
> >> Regards,
> >> Carl
> >
> > It's easier to manipulate the frequency with multipliers and mixers than fool around with tuning.
> >
> This kind of circuit has srs probs for small capacitances and/or high
> frequencies. Sometimes it's not even stable for low ones, either:
>
> <http://www.seekic.com/circuit_diagram/index1434.html>

That's a low frequency application.

On 6/23/2021 4:09 PM, Fred Bloggs wrote:
> On Wednesday, June 23, 2021 at 3:11:01 PM UTC-4, bitrex wrote:
>> On 6/23/2021 2:47 PM, Fred Bloggs wrote:
>>> On Wednesday, June 23, 2021 at 1:52:31 PM UTC-4, Carl wrote:
>>>> I've never studied or used a cap multiplier so this may well be
>>>> completely unusable, but at worst I'll learn a bit more about them :-).
>>>> What determines the frequency response of a cap multiplier? If it
>>>> could be made large enough, could a cap multiplier be used with a
>>>> varactor in a frequency tuning application? For a given tuning range
>>>> this would narrow the required capacitance range of the varactor which
>>>> should improve the linearity and perhaps the stability of the overall
>>>> circuit.
>>>>
>>>> The applications I've seen have all been for power supply stabilization
>>>> where the frequency response did not need to be very high and the net
>>>> capacitance and current was "large". I'm hoping that with the tiny
>>>> capacitance of a varactor and a good small-signal transistor the
>>>> frequency range could be extended into the normal range where varactors
>>>> are used.
>>>>
>>>> --
>>>> Regards,
>>>> Carl
>>>
>>> It's easier to manipulate the frequency with multipliers and mixers than fool around with tuning.
>>>
>> This kind of circuit has srs probs for small capacitances and/or high
>> frequencies. Sometimes it's not even stable for low ones, either:
>>
>> <http://www.seekic.com/circuit_diagram/index1434.html>
>
> That's a low frequency application.
>

It is, what I mean is that embodiment doesn't translate too well to
other applications (even with an op amp that isn't as old as I am)

On Wed, 23 Jun 2021 13:51:54 -0400, Carl <carl.ijamesXX@YYverizon.net>
wrote:

>I've never studied or used a cap multiplier so this may well be
>completely unusable, but at worst I'll learn a bit more about them :-).
> What determines the frequency response of a cap multiplier? If it
>could be made large enough, could a cap multiplier be used with a
>varactor in a frequency tuning application? For a given tuning range
>this would narrow the required capacitance range of the varactor which
>should improve the linearity and perhaps the stability of the overall
>circuit.
>
>The applications I've seen have all been for power supply stabilization
>where the frequency response did not need to be very high and the net
>capacitance and current was "large". I'm hoping that with the tiny
>capacitance of a varactor and a good small-signal transistor the
>frequency range could be extended into the normal range where varactors
>are used.

What do you mean by cap multiplier? Got a sketch?

Tube-type FM transmitters sometimes used a "reactance tube"
oscillator, which was in fact a cap multiplier.

Carl wrote:
> I've never studied or used a cap multiplier so this may well be
> completely unusable, but at worst I'll learn a bit more about them
> :-). What determines the frequency response of a cap multiplier? If
> it could be made large enough, could a cap multiplier be used with a
> varactor in a frequency tuning application?

> For a given tuning range this would narrow the required capacitance
> range of the varactor which should improve the linearity and perhaps
> the stability of the overall circuit.
>
> The applications I've seen have all been for power supply
> stabilization where the frequency response did not need to be very
> high

A cap multiplier works better with a faster transistor, because it
reduces the C-B feedthrough.

The main difference is that for a power supply you don't care much about
the Q of the capacitance, whereas for tuning you care very much. Since
a power supply C-mult is an RC lowpass with an emitter follower hung
on it, it's completely unsuitable for tuning.

> and the net capacitance and current was "large".

If you take a 1-pole cap multiplier and move the cold end of the cap
from ground to the emitter,

0--*---------* *---------*---0
| \ A |
| ----- |
| | | | |
*---RRRR-----*------| |---*
| |

you get a simulated (gyrated) inductor. Using one or more varactors for
the capacitor, you could make a tuned circuit. You have to watch out
for biasing issues, of course.

> I'm hoping that with the tiny capacitance of a varactor and a good
> small-signal transistor the frequency range could be extended into
> the normal range where varactors are used.

There's not a lot of point in using varactor tuning for AM radios
nowadays, so the bigger ones such as the MVAM series are long gone.

There are still highish-capacitance varactors around, e.g. the BB201,
which work fine for signal-level applications above a few megahertz.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 6/23/21 5:48 PM, Phil Hobbs wrote:
>
> Carl wrote:
>> I've never studied or used a cap multiplier so this may well be
>> completely unusable, but at worst I'll learn a bit more about them
>> :-). What determines the frequency response of a cap multiplier?  If
>>  it could be made large enough, could a cap multiplier be used with a
>>  varactor in a frequency tuning application?
>
>> For a given tuning range this would narrow the required capacitance
>> range of the varactor which should improve the linearity and perhaps
>> the stability of the overall circuit.
>>
>> The applications I've seen have all been for power supply
>> stabilization where the frequency response did not need to be very high
>
> A cap multiplier works better with a faster transistor, because it
> reduces the C-B feedthrough.
>
> The main difference is that for a power supply you don't care much about
> the Q of the capacitance, whereas for tuning you care very much.  Since
>  a power supply C-mult is an RC lowpass with an emitter follower hung
> on it, it's completely unsuitable for tuning.
>
>> and the net capacitance and current was "large".
>
> If you take a 1-pole cap multiplier and move the cold end of the cap
> from ground to the emitter,
>
> 0--*---------*     *---------*---0
>    |          \   A          |
>    |          -----          |
>    |            |      | |   |
>    *---RRRR-----*------| |---*
>                        | |
>
> you get a simulated (gyrated) inductor.  Using one or more varactors for
> the capacitor, you could make a tuned circuit.  You have to watch out
> for biasing issues, of course.
>
>> I'm hoping that with the tiny capacitance of a varactor and a good
>> small-signal transistor the frequency range could be extended into the
>> normal range where varactors are used.
>
> There's not a lot of point in using varactor tuning for AM radios
> nowadays, so the bigger ones such as the MVAM series are long gone.
>
> There are still highish-capacitance varactors around, e.g. the BB201,
> which work fine for signal-level applications above a few megahertz.
>
> Cheers
>
> Phil Hobbs
>

Thanks to all who commented. I was pretty sure that if this could be
useful it would have been done long ago.

--
Regards,
Carl

Carl wrote:
> On 6/23/21 5:48 PM, Phil Hobbs wrote:
>>
>> Carl wrote:
>>> I've never studied or used a cap multiplier so this may well be
>>> completely unusable, but at worst I'll learn a bit more about them
>>> :-). What determines the frequency response of a cap multiplier?  If
>>>  it could be made large enough, could a cap multiplier be used with a
>>>  varactor in a frequency tuning application?
>>
>>> For a given tuning range this would narrow the required capacitance
>>> range of the varactor which should improve the linearity and perhaps
>>> the stability of the overall circuit.
>>>
>>> The applications I've seen have all been for power supply
>>> stabilization where the frequency response did not need to be very high
>>
>> A cap multiplier works better with a faster transistor, because it
>> reduces the C-B feedthrough.
>>
>> The main difference is that for a power supply you don't care much about
>> the Q of the capacitance, whereas for tuning you care very much.
>> Since   a power supply C-mult is an RC lowpass with an emitter
>> follower hung on it, it's completely unsuitable for tuning.
>>
>>> and the net capacitance and current was "large".
>>
>> If you take a 1-pole cap multiplier and move the cold end of the cap
>> from ground to the emitter,
>>
>> 0--*---------*     *---------*---0
>>     |          \   A          |
>>     |          -----          |
>>     |            |      | |   |
>>     *---RRRR-----*------| |---*
>>                         | |
>>
>> you get a simulated (gyrated) inductor.  Using one or more varactors
>> for the capacitor, you could make a tuned circuit.  You have to watch
>> out for biasing issues, of course.
>>
>>> I'm hoping that with the tiny capacitance of a varactor and a good
>>> small-signal transistor the frequency range could be extended into
>>> the normal range where varactors are used.
>>
>> There's not a lot of point in using varactor tuning for AM radios
>> nowadays, so the bigger ones such as the MVAM series are long gone.
>>
>> There are still highish-capacitance varactors around, e.g. the BB201,
>> which work fine for signal-level applications above a few megahertz.
>>
>> Cheers
>>
>> Phil Hobbs
>>
>
> Thanks to all who commented.  I was pretty sure that if this could be
> useful it would have been done long ago.
>

I'd be perfectly prepared to try it for the right application.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 6/23/2021 5:42 PM, John Larkin wrote:
> On Wed, 23 Jun 2021 13:51:54 -0400, Carl <carl.ijamesXX@YYverizon.net>
> wrote:
>
>> I've never studied or used a cap multiplier so this may well be
>> completely unusable, but at worst I'll learn a bit more about them :-).
>> What determines the frequency response of a cap multiplier? If it
>> could be made large enough, could a cap multiplier be used with a
>> varactor in a frequency tuning application? For a given tuning range
>> this would narrow the required capacitance range of the varactor which
>> should improve the linearity and perhaps the stability of the overall
>> circuit.
>>
>> The applications I've seen have all been for power supply stabilization
>> where the frequency response did not need to be very high and the net
>> capacitance and current was "large". I'm hoping that with the tiny
>> capacitance of a varactor and a good small-signal transistor the
>> frequency range could be extended into the normal range where varactors
>> are used.
>
> What do you mean by cap multiplier? Got a sketch?
>
> Tube-type FM transmitters sometimes used a "reactance tube"
> oscillator, which was in fact a cap multiplier.
>

Ya, it helps if the circuit is expected to be oscillating to begin with
which cap multiplier circuits tend to want to do.

A two-pole circuit coupling the plate to grid can make the admittance
looking into the plate of the tube appear negative, and can exploit the
Miller effect to make either the capacitance or inductance from grid to
plate appear bigger. then since the Miller-multiplication is a function
of gain you can modulate the frequency with control grid bias.

Pentagrid converter tube works well for a one-tube low-level frequency
modulator like this, with feedback on one signal grid and AF on the other.

On 6/23/2021 7:00 PM, Phil Hobbs wrote:
> Carl wrote:
>> On 6/23/21 5:48 PM, Phil Hobbs wrote:
>>>
>>> Carl wrote:
>>>> I've never studied or used a cap multiplier so this may well be
>>>> completely unusable, but at worst I'll learn a bit more about them
>>>> :-). What determines the frequency response of a cap multiplier?  If
>>>>  it could be made large enough, could a cap multiplier be used with a
>>>>  varactor in a frequency tuning application?
>>>
>>>> For a given tuning range this would narrow the required capacitance
>>>> range of the varactor which should improve the linearity and perhaps
>>>> the stability of the overall circuit.
>>>>
>>>> The applications I've seen have all been for power supply
>>>> stabilization where the frequency response did not need to be very high
>>>
>>> A cap multiplier works better with a faster transistor, because it
>>> reduces the C-B feedthrough.
>>>
>>> The main difference is that for a power supply you don't care much about
>>> the Q of the capacitance, whereas for tuning you care very much.
>>> Since   a power supply C-mult is an RC lowpass with an emitter
>>> follower hung on it, it's completely unsuitable for tuning.
>>>
>>>> and the net capacitance and current was "large".
>>>
>>> If you take a 1-pole cap multiplier and move the cold end of the cap
>>> from ground to the emitter,
>>>
>>> 0--*---------*     *---------*---0
>>>     |          \   A          |
>>>     |          -----          |
>>>     |            |      | |   |
>>>     *---RRRR-----*------| |---*
>>>                         | |
>>>
>>> you get a simulated (gyrated) inductor.  Using one or more varactors
>>> for the capacitor, you could make a tuned circuit.  You have to watch
>>> out for biasing issues, of course.
>>>
>>>> I'm hoping that with the tiny capacitance of a varactor and a good
>>>> small-signal transistor the frequency range could be extended into
>>>> the normal range where varactors are used.
>>>
>>> There's not a lot of point in using varactor tuning for AM radios
>>> nowadays, so the bigger ones such as the MVAM series are long gone.
>>>
>>> There are still highish-capacitance varactors around, e.g. the BB201,
>>> which work fine for signal-level applications above a few megahertz.
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>>>
>>
>> Thanks to all who commented.  I was pretty sure that if this could be
>> useful it would have been done long ago.
>>
>
> I'd be perfectly prepared to try it for the right application.
>
> Cheers
>
> Phil Hobbs
>

The current-mode c-mulitplier seems more suited to high frequency and
sounds simple in theory, sense current through the capacitor, multiply
sensed current by a gain > 1, apply it back to the sense node with a
current-controlled current source.

On 24/6/21 7:48 am, Phil Hobbs wrote:
>
> Carl wrote:
>> I've never studied or used a cap multiplier so this may well be
>> completely unusable, but at worst I'll learn a bit more about them
>> :-). What determines the frequency response of a cap multiplier?  If
>>  it could be made large enough, could a cap multiplier be used with a
>>  varactor in a frequency tuning application?
>
>> For a given tuning range this would narrow the required capacitance
>> range of the varactor which should improve the linearity and perhaps
>> the stability of the overall circuit.
>>
>> The applications I've seen have all been for power supply
>> stabilization where the frequency response did not need to be very high
>
> A cap multiplier works better with a faster transistor, because it
> reduces the C-B feedthrough.
>
> The main difference is that for a power supply you don't care much about
> the Q of the capacitance, whereas for tuning you care very much.  Since
>  a power supply C-mult is an RC lowpass with an emitter follower hung
> on it, it's completely unsuitable for tuning.
>
>> and the net capacitance and current was "large".
>
> If you take a 1-pole cap multiplier and move the cold end of the cap
> from ground to the emitter,
>
> 0--*---------*     *---------*---0
>    |          \   A          |
>    |          -----          |
>    |            |      | |   |
>    *---RRRR-----*------| |---*
>                        | |
>
> you get a simulated (gyrated) inductor.  Using one or more varactors for
> the capacitor, you could make a tuned circuit.  You have to watch out
> for biasing issues, of course.

Does this become better-behaved if you add emitter resistance? Or does
that kill the Q?

I haven't used one at high frequency, but they're commonly used in telco
applications to separate signalling from DC power. For that, I added 3
ohms of Re and it seems to work fairly well.

CH

Clifford Heath wrote:
> On 24/6/21 7:48 am, Phil Hobbs wrote:
>>
>> Carl wrote:
>>> I've never studied or used a cap multiplier so this may well be
>>> completely unusable, but at worst I'll learn a bit more about them
>>> :-). What determines the frequency response of a cap multiplier?  If
>>>  it could be made large enough, could a cap multiplier be used with a
>>>  varactor in a frequency tuning application?
>>
>>> For a given tuning range this would narrow the required capacitance
>>> range of the varactor which should improve the linearity and perhaps
>>> the stability of the overall circuit.
>>>
>>> The applications I've seen have all been for power supply
>>> stabilization where the frequency response did not need to be very high
>>
>> A cap multiplier works better with a faster transistor, because it
>> reduces the C-B feedthrough.
>>
>> The main difference is that for a power supply you don't care much about
>> the Q of the capacitance, whereas for tuning you care very much.
>> Since   a power supply C-mult is an RC lowpass with an emitter
>> follower hung on it, it's completely unsuitable for tuning.
>>
>>> and the net capacitance and current was "large".
>>
>> If you take a 1-pole cap multiplier and move the cold end of the cap
>> from ground to the emitter,
>>
>> 0--*---------*     *---------*---0
>>     |          \   A          |
>>     |          -----          |
>>     |            |      | |   |
>>     *---RRRR-----*------| |---*
>>                         | |
>>
>> you get a simulated (gyrated) inductor.  Using one or more varactors
>> for the capacitor, you could make a tuned circuit.  You have to watch
>> out for biasing issues, of course.
>
> Does this become better-behaved if you add emitter resistance? Or does
> that kill the Q?
>
> I haven't used one at high frequency, but they're commonly used in telco
> applications to separate signalling from DC power. For that, I added 3
> ohms of Re and it seems to work fairly well.
>
> CH

I haven't gone through the math lately, but I expect that adding emitter
resistance will reduces the loop gain, and so will reduce the Q at high
frequency. Same sort of deal as emitter-degenerating a current
mirror--the matching improves at DC but the high-frequency performance
tanks.

The main issue, as with most all simulated inductors, is that it's up to
the designer to make sure there's enough supply headroom for the
simulated inductive kick when the current changes rapidly. With a
high-beta transistor and the right choice of R and C, it can work fine
for lots of things. (One probably woouldn't do it in the RF amp of a
communications receiver, of course.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 24/6/21 1:50 pm, Phil Hobbs wrote:
> Clifford Heath wrote:
>> On 24/6/21 7:48 am, Phil Hobbs wrote:
>>>
>>> Carl wrote:
>>>> I've never studied or used a cap multiplier so this may well be
>>>> completely unusable, but at worst I'll learn a bit more about them
>>>> :-). What determines the frequency response of a cap multiplier?  If
>>>>  it could be made large enough, could a cap multiplier be used with a
>>>>  varactor in a frequency tuning application?
>>>
>>>> For a given tuning range this would narrow the required capacitance
>>>> range of the varactor which should improve the linearity and perhaps
>>>> the stability of the overall circuit.
>>>>
>>>> The applications I've seen have all been for power supply
>>>> stabilization where the frequency response did not need to be very high
>>>
>>> A cap multiplier works better with a faster transistor, because it
>>> reduces the C-B feedthrough.
>>>
>>> The main difference is that for a power supply you don't care much about
>>> the Q of the capacitance, whereas for tuning you care very much.
>>> Since   a power supply C-mult is an RC lowpass with an emitter
>>> follower hung on it, it's completely unsuitable for tuning.
>>>
>>>> and the net capacitance and current was "large".
>>>
>>> If you take a 1-pole cap multiplier and move the cold end of the cap
>>> from ground to the emitter,
>>>
>>> 0--*---------*     *---------*---0
>>>     |          \   A          |
>>>     |          -----          |
>>>     |            |      | |   |
>>>     *---RRRR-----*------| |---*
>>>                         | |
>>>
>>> you get a simulated (gyrated) inductor.  Using one or more varactors
>>> for the capacitor, you could make a tuned circuit.  You have to watch
>>> out for biasing issues, of course.
>>
>> Does this become better-behaved if you add emitter resistance? Or does
>> that kill the Q?
>>
>> I haven't used one at high frequency, but they're commonly used in
>> telco applications to separate signalling from DC power. For that, I
>> added 3 ohms of Re and it seems to work fairly well.
>>
>> CH
>
> I haven't gone through the math lately, but I expect that adding emitter
> resistance will reduces the loop gain, and so will reduce the Q at high
> frequency.  Same sort of deal as emitter-degenerating a current
> mirror--the matching improves at DC but the high-frequency performance
> tanks.

See I don't understand that. The gyrator transistor Re ought to be
reduced by its current gain. As long as you aren't pushing Ft it should
be ok, just servo-ing the output current?

> The main issue, as with most all simulated inductors, is that it's up to
> the designer to make sure there's enough supply headroom for the
> simulated inductive kick when the current changes rapidly.  With a
> high-beta transistor and the right choice of R and C, it can work fine
> for lots of things.  (One probably woouldn't do it in the RF amp of a
> communications receiver, of course.)

No, but the question is whether it would work in a tuned circuit...
perhaps with a varactor tuning the inductance as well as another to
resonate with.

Clifford Heath

On 24/06/2021 00:00, Phil Hobbs wrote:
> Carl wrote:
>> On 6/23/21 5:48 PM, Phil Hobbs wrote:
>>>
>>> Carl wrote:
>>>> I've never studied or used a cap multiplier so this may well be
>>>> completely unusable, but at worst I'll learn a bit more about them
>>>> :-). What determines the frequency response of a cap multiplier?  If
>>>>  it could be made large enough, could a cap multiplier be used with a
>>>>  varactor in a frequency tuning application?
>>>
>>>> For a given tuning range this would narrow the required capacitance
>>>> range of the varactor which should improve the linearity and perhaps
>>>> the stability of the overall circuit.
>>>>
>>>> The applications I've seen have all been for power supply
>>>> stabilization where the frequency response did not need to be very high
>>>
>>> A cap multiplier works better with a faster transistor, because it
>>> reduces the C-B feedthrough.
>>>
>>> The main difference is that for a power supply you don't care much about
>>> the Q of the capacitance, whereas for tuning you care very much.
>>> Since   a power supply C-mult is an RC lowpass with an emitter
>>> follower hung on it, it's completely unsuitable for tuning.
>>>
>>>> and the net capacitance and current was "large".
>>>
>>> If you take a 1-pole cap multiplier and move the cold end of the cap
>>> from ground to the emitter,
>>>
>>> 0--*---------*     *---------*---0
>>>     |          \   A          |
>>>     |          -----          |
>>>     |            |      | |   |
>>>     *---RRRR-----*------| |---*
>>>                         | |
>>>
>>> you get a simulated (gyrated) inductor.  Using one or more varactors
>>> for the capacitor, you could make a tuned circuit.  You have to watch
>>> out for biasing issues, of course.
>>>
>>>> I'm hoping that with the tiny capacitance of a varactor and a good
>>>> small-signal transistor the frequency range could be extended into
>>>> the normal range where varactors are used.
>>>
>>> There's not a lot of point in using varactor tuning for AM radios
>>> nowadays, so the bigger ones such as the MVAM series are long gone.
>>>
>>> There are still highish-capacitance varactors around, e.g. the BB201,
>>> which work fine for signal-level applications above a few megahertz.
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>>>
>>
>> Thanks to all who commented.  I was pretty sure that if this could be
>> useful it would have been done long ago.
>>
>
> I'd be perfectly prepared to try it for the right application.

Gyrators were once all the rage for cheap mass produced high Q bandpass
filters in the days before digital signal processing was possible.

These days software defined radios are quite common and more easily
implemented. Several TV tuner dongles can be subverted this way.

--
Regards,
Martin Brown

"Carl" wrote in message news:savsbq02o23@news1.newsguy.com...

>I've never studied or used a cap multiplier so this may well be completely
>unusable, but at worst I'll learn a bit more about them :-). What
>determines the frequency response of a cap multiplier? If it could be made
>large enough, could a cap multiplier be used with a varactor in a frequency
>tuning application? For a given tuning range this would narrow the
>required capacitance range of the varactor which should improve the
>linearity and perhaps the stability of the overall circuit.

>The applications I've seen have all been for power supply stabilization
>where the frequency response did not need to be very high and the net
>capacitance and current was "large". I'm hoping that with the tiny
>capacitance of a varactor and a good small-signal transistor the frequency
>range could be extended into the normal range where varactors are used.

No chance of working for any *quality* *HF* oscillator.

Capacitor multipliers can only work because they have gain at HF to generate
the multiplication. This is a major problem even at 10 MHz.

Secondly, it will generate huge amounts of Phase Noise.

There is no such thing as a free lunch...

-- Kevin Aylward
http://www.anasoft.co.uk/ SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

On Wed, 23 Jun 2021 13:51:54 -0400, Carl <carl.ijamesXX@YYverizon.net>
wrote:

>I've never studied or used a cap multiplier so this may well be
>completely unusable, but at worst I'll learn a bit more about them :-).
> What determines the frequency response of a cap multiplier? If it
>could be made large enough, could a cap multiplier be used with a
>varactor in a frequency tuning application? For a given tuning range
>this would narrow the required capacitance range of the varactor which
>should improve the linearity and perhaps the stability of the overall
>circuit.
>
>The applications I've seen have all been for power supply stabilization
>where the frequency response did not need to be very high and the net
>capacitance and current was "large". I'm hoping that with the tiny
>capacitance of a varactor and a good small-signal transistor the
>frequency range could be extended into the normal range where varactors
>are used.

We have a couple of products that use a digital capacitor (like
NCD2100TTR) for coarse oscillator tuning and then a varicap for fine
tune.

At powerup, we center the varicap voltage and step the digital cap to
find the code that's closest to our target frequency.

These Ixys parts are deliberately non-monotonic, so we sweep through a
lot of steps and then pick the best one. Can't binary search.