R
rickman
Guest
On 3/21/2017 12:24 PM, amdx wrote:
If the design is sensitive to external noise, it might be useful to use
triaxial cable with the inner shield driven as a guard and the outer
shield grounded to the case.
With the input impedance this circuit has I don't see how it wouldn't be
sensitive to noise. But with such a short run of wire I suppose noise
is not much of an issue.
--
Rick C
On 3/21/2017 9:28 AM, dagmargoodboat@yahoo.com wrote:
On Tuesday, March 21, 2017 at 8:27:51 AM UTC-4, amdx wrote:
On 3/21/2017 2:19 AM, dagmargoodboat@yahoo.com wrote:
On Monday, March 20, 2017 at 9:49:55 PM UTC-4, dagmarg...@yahoo.com
wrote:
On Monday, March 20, 2017 at 6:15:37 PM UTC-4, amdx wrote:
Can I tell this is working if my 1X gain increases?
The 17 to 1 divider of the input cap and the gate capacitance and
the 17
times gain of the amplifier equals 1X.
Say I get 80% T1 gate cancellation (by moving the 20Meg), now we have
effectively 1pf.
1pf/0.3pf = 3.33 and the amp gain 17 / 3.33 = 5.1
So I would think my total circuit gain would increase to 5.1.
Or do I not get it?
You've got it perfectly. I don't expect a very large improvement from
bootstrapping the 20M alone though--a resistor's capacitance is pretty
low already, and two in series, even lower.
Your author's figures are inconsistent. He starts saying the input
capacitance is 1.4pF and the input coupling cap is 0.3pF, but then he
says the 0.3pF and FET T1's capacitances form a 17:1 divider. That
can't
all be true--0.3pF should form a 5.7:1 divider with a 1.4pF input, not
17:1.
When I guesstimate a 5x improvement, I'm banking on the 17:1 being
true,
c.in(eff) being 5pF, and getting that down to 1pF, roughly, with the
circuit I sketched.
If you're already really at 1.4pF the improvement will only be
1.0pF/1.4pF,
and not 1.0pF/5pF.
As I said before, a better buffer could do better--you could tweak the
bootstrap to perfect null--but then chances are you'd have an
oscillator.
What I posted seemed like a reasonable compromise for a first try.
I thought about this a bit and came up with an improved follower.
The main limitation of the previous circuit was the FET's poor
performance
as a voltage-follower. Unaided, the T1 has a gain of about 0.6.
That hits
our bootstrapping from all sides. First, c(gs) (the largest
capacitance)
is only bootstrapped by 60%, leaving 40% of the BC547C's ~5pF c(gs).
Next,
we use that voltage to drive our less-than-unity Q2, which drives
less-than-
unity Q1. This all adds up.
Changing T1's load to a current sink makes T1 into a much better
follower,
increasing voltage gain from 0.6 to about 0.95. The better 'follower'
action now bootstraps away nearly all of c(gs) (T1's largest
capacitance),
and gives us a better signal to drive the drain bootstrap as well.
Good,
good, and good. And not terribly much trouble to do, either.
Vdd Vdd
-+- -+-
| |
| [22k] R5
Q1 \| |
BC547B |---+-------.
.<| | |
| [47k] R6 |
(shield) T1 |--' | |
------ BF256C | === |
----------+----->|--. |
---+-- | | Vdd --- C2
| | | -+- ---100n
| | | | |
| R1 [10M] | |/ Q2 |
| | +---| BC547B |
| | | |>. |
| | R3 [470] | |
| | | | | C3
| | | | | 100n
| +----||---+-----+-------+-----||---> to ampl.
| | C1 | |
| R2 [10M] 100pF R4 [470] --- C4
| | | --- 100n
| === === |
| |
'------------------------------+
|
Cin ~200fF [2.2k] R7
|
===
Cheers,
James Arthur
Thanks for the time.
As it is now constructed the enclosure is the shield.
Is that good or bad? ie. Should the enclosure be isolated from the
shield?
The enclosure should be grounded! Let's not confuse that with
bootstrapping the input coax's shield (which you'll only do _if_
you use coax).
So yes, the enclosure should be isolated from the driven shield that is
shown in the schematic.
OK, so I ground the enclosure to (battery) ground and use an isolated
input connector. I have some isolated BNC connectors that will work great.
If the design is sensitive to external noise, it might be useful to use
triaxial cable with the inner shield driven as a guard and the outer
shield grounded to the case.
With the input impedance this circuit has I don't see how it wouldn't be
sensitive to noise. But with such a short run of wire I suppose noise
is not much of an issue.
(My shield-driver is pretty wimpy, only suitable for a very short, low-
capacitance run. Might need beefing up.)
My plan is to build this into a setup that has my highest Q tuning cap
and sitting just below the cap. So a short run. Hope to rectify* and
drive a voltmeter with some type of attenuator.
*Maybe even a peak detector.
--
Rick C