# How to bootstrap

Started by March 28, 2013
```I see the term bootstrapping, used when the designer wants a high
impedance and usually low capacitance. I know it involves feedback
but that's all I know.

I want to learn enough to build a bootstrapped input with 10Meg/2pf
impedance. Those are ballpark numbers, the end use would be used
to measure voltage on a high Q coil and NOT load it. Frequency 100khz up

I would prefer a transistor circuit, that way I'll learn something,
but if there is an obvious IC circuit, I would like to know.
Mikek

```
```"amdx"
>
>I see the term bootstrapping, used when the designer wants a high impedance
>and usually low capacitance. I know it involves feedback
> but that's all I know.
>
> I want to learn enough to build a bootstrapped input with 10Meg/2pf
> impedance. Those are ballpark numbers, the end use would be used
> to measure voltage on a high Q coil and NOT load it. Frequency 100khz up
> to 2Mhz, but 10 Mhz adds more uses. (crystal radio stuff)
>
> I would prefer a transistor circuit, that way I'll learn something,
> but if there is an obvious IC circuit, I would like to know.

** I think you have been told this already  -  but a single JFET ( wired as
a source follower ) is ideal.

BTW:  a 2pF cap has an impedance of 8000 ohms at 10MHz.

...  Phil

```
```On Thu, 28 Mar 2013 17:28:51 -0500, amdx <amdx@knologynotthis.net>
wrote:

>I see the term bootstrapping, used when the designer wants a high
>impedance and usually low capacitance. I know it involves feedback
>but that's all I know.
>
>I want to learn enough to build a bootstrapped input with 10Meg/2pf
>impedance. Those are ballpark numbers, the end use would be used
>to measure voltage on a high Q coil and NOT load it. Frequency 100khz up
>
>  I would prefer a transistor circuit, that way I'll learn something,
>but if there is an obvious IC circuit, I would like to know.
>                                 Mikek

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.

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```
```"Jim Thompson"
>
> 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.

** Another totally pedantic WANK.

"  Jim Thompson = Autistic  Pig "

...  Phil

```
```On 3/28/2013 5:40 PM, Phil Allison wrote:
> "amdx"
>>
>> I see the term bootstrapping, used when the designer wants a high impedance
>> and usually low capacitance. I know it involves feedback
>> but that's all I know.
>>
>> I want to learn enough to build a bootstrapped input with 10Meg/2pf
>> impedance. Those are ballpark numbers, the end use would be used
>> to measure voltage on a high Q coil and NOT load it. Frequency 100khz up
>> to 2Mhz, but 10 Mhz adds more uses. (crystal radio stuff)
>>
>> I would prefer a transistor circuit, that way I'll learn something,
>> but if there is an obvious IC circuit, I would like to know.
>
>
> ** I think you have been told this already  -  but a single JFET ( wired as
> a source follower ) is ideal.
>
> BTW:  a 2pF cap has an impedance of 8000 ohms at 10MHz.
>
>
> ...  Phil
>
>
Don't recall being told about the JFET source follower.
With your reminder, I do recall the circuit I built before
had 0.3pf.  So forget the 2pf, need 0.3pf or less.

Can you give me incite to this;

I have a High Q LC circuit, I put my bootstrapped measurement device
in parallel, the C of the circuit adds to the C I used to
resonate the L. So does the C of my measurement device load the circuit?
Or just change the resonant frequency?
Assuming a high Q C in my measurement circuit.
Mikek
```
```On 3/28/2013 5:52 PM, Jim Thompson wrote:
> On Thu, 28 Mar 2013 17:28:51 -0500, amdx <amdx@knologynotthis.net>
> wrote:
>
>> I see the term bootstrapping, used when the designer wants a high
>> impedance and usually low capacitance. I know it involves feedback
>> but that's all I know.
>>
>> I want to learn enough to build a bootstrapped input with 10Meg/2pf
>> impedance. Those are ballpark numbers, the end use would be used
>> to measure voltage on a high Q coil and NOT load it. Frequency 100khz up
>> to 2Mhz, but 10 Mhz adds more uses. (crystal radio stuff)
>>
>>   I would prefer a transistor circuit, that way I'll learn something,
>> but if there is an obvious IC circuit, I would like to know.
>>                                  Mikek
>
> 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.
>
>                                          ...Jim Thompson
>
Now Jim, if I could do that I wouldn't be asking such a question.
Mikek
```
```"amdx"
> Phil Allison wrote:
>> "amdx"
>>>
>>> I see the term bootstrapping, used when the designer wants a high
>>> impedance
>>> and usually low capacitance. I know it involves feedback
>>> but that's all I know.
>>>
>>> I want to learn enough to build a bootstrapped input with 10Meg/2pf
>>> impedance. Those are ballpark numbers, the end use would be used
>>> to measure voltage on a high Q coil and NOT load it. Frequency 100khz up
>>> to 2Mhz, but 10 Mhz adds more uses. (crystal radio stuff)
>>>
>>> I would prefer a transistor circuit, that way I'll learn something,
>>> but if there is an obvious IC circuit, I would like to know.
>>
>>
>> ** I think you have been told this already  -  but a single JFET ( wired
>> as
>> a source follower ) is ideal.
>>
>> BTW:  a 2pF cap has an impedance of 8000 ohms at 10MHz.
>>
>>
>  Don't recall being told about the JFET source follower.
> With your reminder, I do recall the circuit I built before
> had 0.3pf.  So forget the 2pf, need 0.3pf or less.
>

** Don't invent silly specs.

It's the mark of an utter fuckwit to do that.

> I have a High Q LC circuit, I put my bootstrapped measurement device
> in parallel,

** FET follower OK.

No bootstrapping needed.

> So does the C of my measurement device load the circuit?
> Or just change the resonant frequency?

**  It will probably  do neither.

Cos you circuit is a fantasy wank anyhow.

Piss off.

```
```"amdx"
>>
>> 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.
>>
>>                                          ...Jim Thompson
>>
>  Now Jim, if I could do that I wouldn't be asking such a question.

**  What the smug with delusions of grandeur is attempting to point out is
that if  YOU  organise things so that the same voltage appears at both ends
of a resistor  -  it passes no current.

Makes an input bias resistor virtually disappear.

....  Phil

```
```On Mar 28, 6:28=A0pm, amdx <a...@knologynotthis.net> wrote:
> I see the term bootstrapping, used when the designer wants a high
> impedance and usually low capacitance. I know it involves feedback
> but that's all I know.
>
> I want to learn enough to build a bootstrapped input with 10Meg/2pf
> impedance. Those are ballpark numbers, the end use would be used
> to measure voltage on a high Q coil and NOT load it. Frequency 100khz up
> to 2Mhz, but 10 Mhz adds more uses. (crystal radio stuff)
>
> =A0 I would prefer a transistor circuit, that way I'll learn something,
> but if there is an obvious IC circuit, I would like to know.
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Mikek

The easiest way (for me) to make a bootstrap is an opamp unity gain
buffer the output drives a shield 'capacitance'.  But as Phil said the
jfet follower is a classic.  A bootstrap in a High Q LC circuit is
hard to understand.  What are you doing?

George H.
```
```On Thu, 28 Mar 2013 18:33:14 -0500, amdx <amdx@knologynotthis.net>
wrote:

>On 3/28/2013 5:52 PM, Jim Thompson wrote:
>> On Thu, 28 Mar 2013 17:28:51 -0500, amdx <amdx@knologynotthis.net>
>> wrote:
>>
>>> I see the term bootstrapping, used when the designer wants a high
>>> impedance and usually low capacitance. I know it involves feedback
>>> but that's all I know.
>>>
>>> I want to learn enough to build a bootstrapped input with 10Meg/2pf
>>> impedance. Those are ballpark numbers, the end use would be used
>>> to measure voltage on a high Q coil and NOT load it. Frequency 100khz up
>>> to 2Mhz, but 10 Mhz adds more uses. (crystal radio stuff)
>>>
>>>   I would prefer a transistor circuit, that way I'll learn something,
>>> but if there is an obvious IC circuit, I would like to know.
>>>                                  Mikek
>>
>> 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.
>>
>>                                          ...Jim Thompson
>>
>  Now Jim, if I could do that I wouldn't be asking such a question.
>                                  Mikek

I hear that delta is a babe >:-}

comprehension... what does the problem say?

The bootstrap...

http://www.analog-innovations.com/SED/SimplifiedBootstrapCalculation.pdf

In my late 20's I taught Algebra to disadvantaged youth... otherwise
known as South Phoenix thugs.

I hired my best student as my technician and he stayed with me for 27
years.  (Jim Foster, the JF initials seen on many of my ancient
hand-drawn schematics.)

...Jim Thompson
--
| James E.Thompson                                 |    mens     |