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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
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| 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.
What Glenn and Jim said plus -- didn't he say so in the text? I would have expected him to justify every part in the circuit, given how few of them there are.
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Nah, there's nothing in the text about how it works. But I sorta get the Q2 as current source/ offset adjust. I could sometimes use a low capacitance probe.
He does claim that it only has 0.29 pF of input C. I wonder how that works. The jfet's look to have at least a few pF
It's a source follower. The Miller Effect works for you in this case: the capacitance is equal to Cgs * (1 - Av) + Cgd (if I have my math right). Since it's a source follower, Av is almost 1, which means that Cgs gets multiplied by a teeny number.
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
OK thanks for the "hint". I know of the Miller effect, but I don't really "know it", if you know what I mean. I thought it was Cgd that got "Millered". (I'm mostly analogizing to the base-collector capacitance in a bjt.) I'll got read some.
Phemts would be good for really fast, low capacitance probing, instead of klunky old jfets.
Take a look at the schematic of a Tektronix 7A13 plugin. It has hundred-volt overload capability and instant recovery at 100 MHz. The bootstraps and tweaks are amazing.
The old W and Z plugins were impressive, too.
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John Larkin Highland Technology, Inc
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
Worst case tolerance on the "cancellation" seems to be +/-200mV - the data sheet Vbe's at 10mA are both 0.65V to 0.85V. You'd expect both to be mid-ra nge, but the distribution is a trifle unpredictable. If they meet the spec by design you'd expect 200mv to be something like 6 standard deviations, an d the most likely offset would be down to about 50mV, while if they test an d discard parts that don't meet the spec it would be more.
You forgot that I live on the trailing edge of technology. If it's old and klunky and soon to disappear, I should learn about it as soon as possible. :^)
Seriously, too fast can be a problem. I mostly use an old 60MHz TEK DSO* so the bandwidth is fine. Even 1pF Cin would be over an order of magnitude improvement.
George H.
*one good thing is that it's less noisy than other (cheap) DSO's
a sheet Vbe's at 10mA are both 0.65V to 0.85V. You'd expect both to be mid- range, but the distribution is a trifle unpredictable. If they meet the spe c by design you'd expect 200mv to be something like 6 standard deviations, and the most likely offset would be down to about 50mV, while if they test and discard parts that don't meet the spec it would be more.
OK thanks, I guess I should order some jfets and make it. (The 100 ohm pot is a bit of a nuisance... can I run it a bit leaner? higher resistance.) You can tweak the offset, so the next big thing is the tempco. (as Glenn and Jim said.) Lots of times I don't care about DC..
That's the classic Miller capacitance, Cgp * (1 + gain) gp = grid:plate
Cgs can be reduced by having the source follow the gate. That's sort of an anti-Miller effect. At a follower gain of 1, Cgs disappears.
Cgd can be reduced by bootstrapping the drain from the source voltage, which itself is following the gate. So gate capacitance can be driven almost to zero.
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John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Yeah.. I got it, thanks. (I was reading in AoE last night.... the FET chapter even has the jfet follower as scope vertical amplifier input... so many circuits that I've looked at.. but unless you try 'em they are not really "yours".)
I would expect the variant for a particular source nowadays to be quite a bit less for discretes. Modem process are much tighter controlled than 35 years ago, from whence the data sheet probably originated for the
2n3904/2n3906. Its reasonable to expect a Vbe to be within +/-10mv to
+/-50mv over all processes corners for ics. I have yet to see one specified larger than 50mv for some time. 25mv seems an average, but 10mv for the better processes.
It is possible to cascode couple an emitter/source follower:
See fig. 7b:
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Quote: "... Besides eliminating voltage caused nonlinearities, cascode operation can yield an additional benefit in increased bandwidth. Because the collector-base voltage is held constant there is minimal charging of the collector-base junction capacitance in the transistor. Eliminating the effects of this internal lag capacitance allows higher frequency response, thus cascode circuitry is commonly found in ultra-high frequency amplifiers and wide bandwidth oscilloscopes where response is required beyond 100 megaHertz. ... [Is this correct? [subjective "ease"]:] While the distortion characteristics of a fully cascode amplifier are not equivalent to those obtained through class-A operation, the lack of signal compression produces a subjective '`ease" to the reproduced sound that closely approximates that of the smooth nonlinearities which characterize class-A operation and are achieved without the cost penalties attendant to a class-A output stage. ..."
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