Forums

How to bootstrap

Started by amdx 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 
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

"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 >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 -- | James E.Thompson | mens | | Analog Innovations | et | | 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. I'll Google delta later. 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. > I'll Google delta later.
** 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. >I'll Google delta later. > Mikek
I hear that delta is a babe >:-} Math is your friend. It's mostly really all about reading 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 had about a 90% success rate. 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 | | Analog Innovations | et | | 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.