How to bootstrap

Phil Hobbs · Mar 30, 2013

On 3/29/2013 9:04 PM, Phil Allison wrote:

"Phil Hobbs"

As Phil A. says, the idea of bootstrapping is that if the input admittance
of your circuit has zero swing across it, it draws zero current.

(snip)

This is not a free lunch, because the SNR stays more or less the same, but
it does give you a much nicer frequency response in general.

** One issue with bootstrapping input resistors is that it can dramatically
increase the noise compared to simply using a large value resistor.

30 odd years ago I attempted to build a JFET pre-amp for a condenser mic
capsule and not having any 1Gohm resistors handy tried bootstrapping a
10Mohm one. The pre-amp tested fine, with an effective input resistance
close to 1Gohm - ie response was flat across the audio band when driven
via a 22pF cap simulating the capsule.

When the capsule was tried, the background noise ( hiss) was about 20dB more
than with a commercial pre-amp and quite unacceptable for studio work.

Thing is, with a 1Gohm ( gate bias) resistor, 22pF is enough to shunt nearly
all the audio frequency ( Johnson ) noise away - not so with 10 Mohms and a
bunch of positive feedback in place.

Yup. Bootstrapping doesn't help the SNR, it just flattens out the
frequency response. It's the same with photodiodes--the noise floor
due to the amplifier's voltage noise rises steadily with frequency.

Cheers

Phil Hobbs

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

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

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

Tom Del Rosso · Mar 30, 2013

Jim Thompson wrote:

On Fri, 29 Mar 2013 19:53:44 -0400, "Tom Del Rosso"
tomd_u1@verizon.net.invalid> wrote:

Jim Thompson wrote:

Just analyze an ideal amplifier of gain +(1-delta) where delta is
small, but non-zero Apply a feedback resistor from output to
input, then calculate input impedance.

What delta - just a change on the input? If gain is a function of
that, even with the minus sign, I'd think it was an exponential
amplifier rather than 'ideal'.

GAIN = (1-delta) , delta small, but non-zero; so say that GAIN = 0.99,
for example.

Draw yourself a picture/schematic of what I wrote in words.

Ok I can solve for that, if I treat delta like a constant of, say, 0.01.

But why do you call it delta? What change does it refer to?

--

Reply in group, but if emailing add one more
zero, and remove the last word.

Phil Allison · Mar 30, 2013

"Tom Del Rosso"

But why do you call it delta? What change does it refer to?

** One is reminded of Humpty Dumpty's declaration to Alice:

" When I use a word ... it means just what I want it to mean, neither more
nor less"

.... Phil

Jim Thompson · Mar 30, 2013

On Fri, 29 Mar 2013 22:45:03 -0400, "Tom Del Rosso"
<tomd_u1@verizon.net.invalid> wrote:

Jim Thompson wrote:
On Fri, 29 Mar 2013 19:53:44 -0400, "Tom Del Rosso"
tomd_u1@verizon.net.invalid> wrote:

Jim Thompson wrote:

Just analyze an ideal amplifier of gain +(1-delta) where delta is
small, but non-zero Apply a feedback resistor from output to
input, then calculate input impedance.

What delta - just a change on the input? If gain is a function of
that, even with the minus sign, I'd think it was an exponential
amplifier rather than 'ideal'.

GAIN = (1-delta) , delta small, but non-zero; so say that GAIN = 0.99,
for example.

Draw yourself a picture/schematic of what I wrote in words.

Ok I can solve for that, if I treat delta like a constant of, say, 0.01.

But why do you call it delta? What change does it refer to?

Phil Allison · Mar 30, 2013

"Phil Allison"

** One issue with bootstrapping input resistors is that it can
dramatically increase the noise compared to simply using a large value
resistor.

30 odd years ago I attempted to build a JFET pre-amp for a condenser mic
capsule and not having any 1Gohm resistors handy tried bootstrapping a
10Mohm one. The pre-amp tested fine, with an effective input resistance
close to 1Gohm - ie response was flat across the audio band when driven
via a 22pF cap simulating the capsule.

When the capsule was tried, the background noise ( hiss) was about 20dB
more than with a commercial pre-amp and quite unacceptable for studio
work.

Thing is, with a 1Gohm ( gate bias) resistor, 22pF is enough to shunt
nearly all the audio frequency ( Johnson ) noise away - not so with 10
Mohms and a bunch of positive feedback in place.

** Another sort of bootstrapping involves the output stage of an audio power
amplifier.

See low budget Germanium output stage typical of the late 1960s:

http://1.bp.blogspot.com/_7q93wN0Pq1k/TRcwk1PjNZI/AAAAAAAAAA0/BvJcYYW0-FA/s1600/Picture+012.jpg

The 180ohm collector load of the BFX88 is bootstrapped direct to the
loudspeaker. Doing this improves both drive and drive linearity - as the
speaker signal swings to 5 volts below the ground rail.

Note how if the ( 3 ohm) speaker is disconnected, the OP stage is disabled.

There is a double dose of bootstrapping going on at the input too.

..... Phil

Jon Kirwan · Mar 30, 2013

On Fri, 29 Mar 2013 12:02:51 -0700, I wrote:

And the DC loading can be
increased because, although R1 and R2 still have their
Thevenin equivalent shunting effect, now you can add R3
straight away to that, so that the DC loading is much lighter
than before despite a stiff bias pair.

Very bad wording. What I should have written (and intended to
write) was:

"And the DC load impedance as seen by the input increases..."

I think it's clear if you take the whole context in... but I
wrote too abruptly and it could have changed the meaning.
Sorry about that.

Jon

Phil Allison · Mar 30, 2013

"Phil Allison"

See low budget Germanium output stage typical of the late 1960s:

http://1.bp.blogspot.com/_7q93wN0Pq1k/TRcwk1PjNZI/AAAAAAAAAA0/BvJcYYW0-FA/s1600/Picture+012.jpg

** Seems there are a couple of drawing errors in the above schem:

R6 should be 1.5kohms & R5 ought not be shorted out.

.... Phil

Jon Kirwan · Mar 31, 2013

Left me worried, mikek. Was my explanation too poor to get
the idea across?

Jon

Jon Kirwan · Apr 2, 2013

On Sun, 31 Mar 2013 14:15:02 -0700, I wrote:

Left me worried, mikek. Was my explanation too poor to get
the idea across?

I guess so. Is there anything you may want to ask about,
then?

Jon

amdx · Apr 3, 2013

On 4/2/2013 2:04 PM, Jon Kirwan wrote:

On Sun, 31 Mar 2013 14:15:02 -0700, I wrote:

Left me worried, mikek. Was my explanation too poor to get
the idea across?

I guess so. Is there anything you may want to ask about,
then?

Jon

Things for your info, I printed it out, it's on my bench waiting for
me. I'm playing catch up at work, we're starting our busy season.
One thing I'm pondering, the circuit I have is close to Version 2
about 1/2 way down this page. As you will note, the input wiring will
have capacitance to the case, how can I best eliminate that?

I think I need to build the second unit, so I can use KLeijer's
method. Check Q with the first amplifier, connect the second amp with
first still connected then retest* Q see if it lowers the Q.

Mikek

*using measurement data from first amp.

amdx · Apr 3, 2013

On 4/3/2013 10:53 AM, amdx wrote:

On 4/2/2013 2:04 PM, Jon Kirwan wrote:
On Sun, 31 Mar 2013 14:15:02 -0700, I wrote:

Left me worried, mikek. Was my explanation too poor to get
the idea across?

I guess so. Is there anything you may want to ask about,
then?

Jon

Things for your info, I printed it out, it's on my bench waiting for
me. I'm playing catch up at work, we're starting our busy season.
One thing I'm pondering, the circuit I have is close to Version 2
about 1/2 way down this page. As you will note, the input wiring will
have capacitance to the case, how can I best eliminate that?

I think I need to build the second unit, so I can use KLeijer's
method. Check Q with the first amplifier, connect the second amp with
first still connected then retest* Q see if it lowers the Q.

Mikek

*using measurement data from first amp.

Forgot to add link, http://www.crystal-radio.eu/index.html

Jon Kirwan · Apr 3, 2013

On Wed, 03 Apr 2013 10:53:24 -0500, amdx <amdx@knology.net>
wrote:

On 4/2/2013 2:04 PM, Jon Kirwan wrote:
On Sun, 31 Mar 2013 14:15:02 -0700, I wrote:

Left me worried, mikek. Was my explanation too poor to get
the idea across?

I guess so. Is there anything you may want to ask about,
then?

Jon

Things for your info, I printed it out, it's on my bench waiting for
me. I'm playing catch up at work, we're starting our busy season.
One thing I'm pondering, the circuit I have is close to Version 2
about 1/2 way down this page. As you will note, the input wiring will
have capacitance to the case, how can I best eliminate that?

I think I need to build the second unit, so I can use KLeijer's
method. Check Q with the first amplifier, connect the second amp with
first still connected then retest* Q see if it lowers the Q.

Mikek

*using measurement data from first amp.

Are you talking about the FET amplifier fro measuring LC
circuits? (I'm sorry it's not clear to me which page to look
at.)

By the way, this page:
http://www.crystal-radio.eu/enlctest2.htm

Shows some coil forms that would be trivial to make with my
3D printer. In fact, I could probably make quite a variety of
coil formers of various kinds. That's a service I never even
considered before, but would be very easy to offer and
deliver on. Hmm.

Jon

amdx · Apr 4, 2013

On 4/3/2013 12:54 PM, Jon Kirwan wrote:

On Wed, 03 Apr 2013 10:53:24 -0500, amdx <amdx@knology.net
wrote:

On 4/2/2013 2:04 PM, Jon Kirwan wrote:
On Sun, 31 Mar 2013 14:15:02 -0700, I wrote:

Left me worried, mikek. Was my explanation too poor to get
the idea across?

I guess so. Is there anything you may want to ask about,
then?

Jon

Things for your info, I printed it out, it's on my bench waiting for
me. I'm playing catch up at work, we're starting our busy season.
One thing I'm pondering, the circuit I have is close to Version 2
about 1/2 way down this page. As you will note, the input wiring will
have capacitance to the case, how can I best eliminate that?

I think I need to build the second unit, so I can use KLeijer's
method. Check Q with the first amplifier, connect the second amp with
first still connected then retest* Q see if it lowers the Q.

Mikek

*using measurement data from first amp.

Are you talking about the FET amplifier fro measuring LC
circuits? (I'm sorry it's not clear to me which page to look
at.)

By the way, this page:
http://www.crystal-radio.eu/enlctest2.htm

Shows some coil forms that would be trivial to make with my
3D printer. In fact, I could probably make quite a variety of
coil formers of various kinds. That's a service I never even
considered before, but would be very easy to offer and
deliver on. Hmm.

Jon

Here's the amp that he uses. http://www.crystal-radio.eu/enfetamp.htm

As a test, he adds a second amp to see how it affects the measurement of
the first amp.

http://www.crystal-radio.eu/enfetamp.htm

Can you print polypropylene? That is one of the better low loss materials.
An entrepreneur is born.

Here's a 3D coil I built with polystyrene. This was first iteration,
then I lost access to my mill. The next was planned to be 600/46 litz,
Now I've lost interest in the 3D unit. This was based on Kleijers design.

Mikek

amdx · Apr 4, 2013

On 4/4/2013 7:27 AM, amdx wrote:

On 4/3/2013 12:54 PM, Jon Kirwan wrote:
On Wed, 03 Apr 2013 10:53:24 -0500, amdx <amdx@knology.net
wrote:

On 4/2/2013 2:04 PM, Jon Kirwan wrote:
On Sun, 31 Mar 2013 14:15:02 -0700, I wrote:

Left me worried, mikek. Was my explanation too poor to get
the idea across?

I guess so. Is there anything you may want to ask about,
then?

Jon

Things for your info, I printed it out, it's on my bench waiting for
me. I'm playing catch up at work, we're starting our busy season.
One thing I'm pondering, the circuit I have is close to Version 2
about 1/2 way down this page. As you will note, the input wiring will
have capacitance to the case, how can I best eliminate that?

I think I need to build the second unit, so I can use KLeijer's
method. Check Q with the first amplifier, connect the second amp with
first still connected then retest* Q see if it lowers the Q.

Mikek

*using measurement data from first amp.

Are you talking about the FET amplifier fro measuring LC
circuits? (I'm sorry it's not clear to me which page to look
at.)

By the way, this page:
http://www.crystal-radio.eu/enlctest2.htm

Shows some coil forms that would be trivial to make with my
3D printer. In fact, I could probably make quite a variety of
coil formers of various kinds. That's a service I never even
considered before, but would be very easy to offer and
deliver on. Hmm.

Jon

Here's the amp that he uses. http://www.crystal-radio.eu/enfetamp.htm

As a test, he adds a second amp to see how it affects the measurement of
the first amp.

http://www.crystal-radio.eu/enfetamp.htm

Can you print polypropylene? That is one of the better low loss materials.
An entrepreneur is born.

Here's a 3D coil I built with polystyrene. This was first iteration,
then I lost access to my mill. The next was planned to be 600/46 litz,
Now I've lost interest in the 3D unit. This was based on Kleijers design.

Mikek

Dang, keep forgetting my links.
http://i395.photobucket.com/albums/pp37/Qmavam/3DPolyHepwith24wire.jpg

Jon Kirwan · Apr 4, 2013

On Thu, 04 Apr 2013 07:27:17 -0500, amdx
<amdx@knologynotthis.net> wrote:

On 4/3/2013 12:54 PM, Jon Kirwan wrote:
On Wed, 03 Apr 2013 10:53:24 -0500, amdx <amdx@knology.net
wrote:

On 4/2/2013 2:04 PM, Jon Kirwan wrote:
On Sun, 31 Mar 2013 14:15:02 -0700, I wrote:

Left me worried, mikek. Was my explanation too poor to get
the idea across?

I guess so. Is there anything you may want to ask about,
then?

Jon

Things for your info, I printed it out, it's on my bench waiting for
me. I'm playing catch up at work, we're starting our busy season.
One thing I'm pondering, the circuit I have is close to Version 2
about 1/2 way down this page. As you will note, the input wiring will
have capacitance to the case, how can I best eliminate that?

I think I need to build the second unit, so I can use KLeijer's
method. Check Q with the first amplifier, connect the second amp with
first still connected then retest* Q see if it lowers the Q.

Mikek

*using measurement data from first amp.

Are you talking about the FET amplifier fro measuring LC
circuits? (I'm sorry it's not clear to me which page to look
at.)

By the way, this page:
http://www.crystal-radio.eu/enlctest2.htm

Shows some coil forms that would be trivial to make with my
3D printer. In fact, I could probably make quite a variety of
coil formers of various kinds. That's a service I never even
considered before, but would be very easy to offer and
deliver on. Hmm.

Jon

Here's the amp that he uses. http://www.crystal-radio.eu/enfetamp.htm

As a test, he adds a second amp to see how it affects the measurement of
the first amp.

http://www.crystal-radio.eu/enfetamp.htm

I'd looked at it. Were you looking for an analysis of the BJT
stage? (In effect, the 1k2 looks like a very high impedance
to AC. But not to DC. So start there.)

Can you print polypropylene? That is one of the better low loss materials.
An entrepreneur is born.

Probably could. Supposedly, according to wiki, a "perfectly
isotactic PP" has a melting point of 171C. Which is well
within range of my 3D printer. All the rest of the types melt
even lower still, so even easier. What I'd need is filament
wire of it at 1.75mm diameter, though.

Here's a 3D coil I built with polystyrene. This was first iteration,
then I lost access to my mill. The next was planned to be 600/46 litz,
Now I've lost interest in the 3D unit. This was based on Kleijers design.

http://i395.photobucket.com/albums/pp37/Qmavam/3DPolyHepwith24wire.jpg

Yeah, I could build that thing.

Jon

amdx · Apr 4, 2013

On 4/4/2013 1:25 PM, Jon Kirwan wrote:

On Thu, 04 Apr 2013 07:27:17 -0500, amdx
amdx@knologynotthis.net> wrote:

On 4/3/2013 12:54 PM, Jon Kirwan wrote:
On Wed, 03 Apr 2013 10:53:24 -0500, amdx <amdx@knology.net
wrote:

On 4/2/2013 2:04 PM, Jon Kirwan wrote:
On Sun, 31 Mar 2013 14:15:02 -0700, I wrote:

Left me worried, mikek. Was my explanation too poor to get
the idea across?

I guess so. Is there anything you may want to ask about,
then?

Jon

Things for your info, I printed it out, it's on my bench waiting for
me. I'm playing catch up at work, we're starting our busy season.
One thing I'm pondering, the circuit I have is close to Version 2
about 1/2 way down this page. As you will note, the input wiring will
have capacitance to the case, how can I best eliminate that?

I think I need to build the second unit, so I can use KLeijer's
method. Check Q with the first amplifier, connect the second amp with
first still connected then retest* Q see if it lowers the Q.

Mikek

*using measurement data from first amp.

Are you talking about the FET amplifier fro measuring LC
circuits? (I'm sorry it's not clear to me which page to look
at.)

By the way, this page:
http://www.crystal-radio.eu/enlctest2.htm

Shows some coil forms that would be trivial to make with my
3D printer. In fact, I could probably make quite a variety of
coil formers of various kinds. That's a service I never even
considered before, but would be very easy to offer and
deliver on. Hmm.

Jon

Here's the amp that he uses. http://www.crystal-radio.eu/enfetamp.htm

As a test, he adds a second amp to see how it affects the measurement of
the first amp.

http://www.crystal-radio.eu/enfetamp.htm

I'd looked at it. Were you looking for an analysis of the BJT
stage? (In effect, the 1k2 looks like a very high impedance
to AC. But not to DC. So start there.)

No, the FET input stage, want to connect it to a high Q circuit and
have it look as if nothing is connected.

Can you print polypropylene? That is one of the better low loss materials.
An entrepreneur is born.

Probably could. Supposedly, according to wiki, a "perfectly
isotactic PP" has a melting point of 171C. Which is well
within range of my 3D printer. All the rest of the types melt
even lower still, so even easier. What I'd need is filament
wire of it at 1.75mm diameter, though.

Here's a 3D coil I built with polystyrene. This was first iteration,
then I lost access to my mill. The next was planned to be 600/46 litz,
Now I've lost interest in the 3D unit. This was based on Kleijers design.

http://i395.photobucket.com/albums/pp37/Qmavam/3DPolyHepwith24wire.jpg

Yeah, I could build that thing.

Jon

Jon Kirwan · Apr 4, 2013

On Thu, 04 Apr 2013 13:43:32 -0500, amdx
<amdx@knologynotthis.net> wrote:

snip
No, the FET input stage, want to connect it to a high Q circuit and
have it look as if nothing is connected.
snip

Oh. Well, I'm weak at JFET design -- especially 1st stage.
But I'll think about it. The main reason that jumps out at me
for why the JFET is used here is about noise. The only
current noise in a JFET comes from the shot noise due to
leakage currents, which are very low. The voltage noise is
Johnson noise due to the channel resistance. Only at very
high source impedances (which is the case on that page) does
a good JFET beat out a good BJT.

But as I said I'm not experienced with first stage JFET
design issues and I need to think a bit about that part of
the circuit before responding.

Jon

whit3rd · Apr 30, 2013

On Thursday, April 4, 2013 11:43:32 AM UTC-7, amdx wrote:

No, the FET input stage, want to connect it to a high Q circuit and
have it look as if nothing is connected.

It's possible to position a small probe coil (with a low-Z preamplifier) so that
it intercepts some very small flux from an RF coil. If the coupling is
kept small, it looks like nothing is connected. Because, nothing
is connected. Gain is cheap.

How to bootstrap

Phil Hobbs

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Tom Del Rosso

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Phil Allison

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Jim Thompson

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Phil Allison

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Jon Kirwan

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Jon Kirwan

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