istence element in an automatic gain control type circuit? Power involved i s a bare whisper; worst case voltage is probably 30 volts. There may well b e a better remidy than an mosfet, but it looks good at this point.
Lotsa nice ideas already posted. How much gain control do you need? and how much distortion can you tolerate? I did these Wein bridge oscillators a few years ago, and clipped off the ti ppy tops of the sine wave with just some diodes. I'd have to check but IIR C the third harmonic was down 50-60dB.
At one time, I took care of some variable gain amplifiers that used a MOSFET as a variable resistance in the emitter of a CE amplifier stage. Not only was the gain vs. Vg curve very non- linear and unpredictable, but at some settings, the gain from Vg to output was so high that the FET's noise was the greatest noise contributor. (From the point of view of the FET, this was a cascode circuit after all.) I'm glad to be rid of them. I re-designed the amplifiers using an AD630 as the VGA stage.
An anecdote about customs: A colleague of mine tried to ship a "Base-Band Q" measurement system to Fermilab. He referred to it as "BBQ" in the paperwork, which made US customs think it contained meat. Oops. It took a month to sort that one out.
(In the particle accelerator world, the Q of a synchrotron is the number of oscillations the beam makes around its central orbit in one revolution. It's an important parameter for beam stability.)
Turn the input into a current, then multiply. Then restore to voltage. Maybe I can spin one up using a CA30xx ;-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 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.
Years ago I heard a story of someone trying to import some electric motors into a 'developing' country (they called them 'backwards' in those days). The customs declaration read: " 10 x electric motors, 5 horsepower, 415 volts, 50 cycles, 3-phase".
The reply from the customs office was as follows:
"The electric motors may be imported upon payment of the appropriate duty. No horses may be brought into the country unless accompanied by the appropriate veterinary certificates. The volts and phases are not listed as permitted items and will be impounded awaiting further investigation. The cycles may not be imported as these are now being manufactured in this country."
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
I mistakedly referred to a graphic printed item as an "overlay", which apparently has some meaning in the textile trade (a highly non-free segment of trade with all kinds of ugly tariffs and quotas on a country-by-country basis). Ended up costing some extra money.
Drain capacitance is a fraction of a pF, so it's good for fast stuff. More like 30 millivolts than 30 volts, though.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
Le Wed, 11 Dec 2013 10:08:52 -0500, Phil Hobbs a écrit:
I recall you already saying that.
In fact, for the usual circuits conditions a JFET is not a MOSFET (indeed :-)
The main difference is that, at ordinary low level currents, you use the JFET in its quadratic region, while you use the MOSFET in its subthreshold region. And that makes for all the difference...
See, for a JFET, in the triode region:
ID = k ((vgs-vt) vds - 1/2 vds^2) when substituting vgs for vc+1/2*vds (vc for control voltage) you get ID = k vds (vc - 2 vt) which indeed is the mark of a resistor.
Now for a MOSFET in the subthreshold region the drain current is diffusion driven and you have: ID = is W/L Exp[vgs/(n uT)] (1 - Exp[-vds/uT] + vds/va)
with va being the equivalent of BJT early voltage, and n an "ideality factor" above 1, ordinarily between 3 and 5 for power MOSFETS. (the 2N7002 is a power MOSFET)
In the low level triode region vds/va 4*uT (roughly 100mV) there's no more ID dependency on vds and the MOSFET behaves like a super FET with constant 100mV triode to saturation "vds knee voltage".
For a constant vgs, the (1 - Exp[-vds/uT]) term makes for a drain current barely linear up to the 25mV uT
If now you set vgs=vc+k1*vds by mean of external circuitery you get:
ID = is W/L Exp[(vc+k1*vds)/(n uT)] (1 - Exp[-vds/uT]) (Eq2)
You factor out a beta=W/L is Exp[vc/(n uT)] term which depends only on physics and control voltage, then you Taylor expand ID at vds=0 to 3rd order (get it simple :-) and you get:
ID = beta [vds/uT + (k/n-1/2)(vds/uT)^2 + (3 k^2 - 3 k n + n^2)/(6 n^2) (vds/uT)^3)] (Eq3)
You can null the 2nd order term by setting k=n/2, (1.5 to 2.5, depending on the ideality factor value). The 3rd order term then simplifies to 1/24.
Note that this is kind of optimal for low distortion at vanishingly low drain voltage.
If you want a usable vds range up to say 100mV, maybe 200mV, at cost of a somewhat reduced linearity you just lower k1. For a 100mV vds range, a pretty good linearization occurs for k1=n/3 (=1.6 for a small power MOS with n=5) which roughly tallies with your results.
With k1=n/3 the expanded ID becomes: ID = beta [vds/uT - 1/6 (vds/uT)^2 + 1/18 (vds/uT)^3)]
and that gives a resistance variation of only 1.5% up to 100mV vds.
Note that this is totally independant from the resistor value as vc doesn't enter into the equation.
We've had great luck with the NEC parts, and some bigger SOT-89's.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
I've never got into the deep theoretical details of FETs, because all the theory I've seen has these fudge factors with huge ranges like your factor of 3 to 5. Bipolars are doing badly if they're off by a few percent, once you include the extrinsic E, B, and C resistances. Those are real resistances, are reasonably linear, drop actual voltages, and have Johnson noise of exactly the magnitude you'd expect. The only real fudge factor in BJTs is the Early voltage.
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
hobbs at electrooptical dot net
http://electrooptical.net
Hmmm, seems you did not read my reply via comp.arch.embedded.
Does the 2N7000 still look like your best bet after you checked how much its capacitance D-G is.
JFET-s have been used as variable resistors for ages, depending on the bandwidth you are after there can be different choices. Up to a few (tens of) MHz your best option is the 2N4391 (or the 2N4856, same thing practically). In my reply via comp.arch.embedded I already told you about what sort of linearity, temperature drift etc. to expect.
2N7000 is an _enhancement_ device, with abrupt conduction, not the best thing to use for AGC. Use a JFET. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 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.
As you say, the enhancement types aren't a good fit. The hooker is that they do fit and the depletion devices don't - no room for a negative supply or control mechanism. The board is 2 by 1/2 inches and it's already filled and bursting.
Jfets are horrible for repeatability. Idss specs can be 3:1 if you're lucky, 10:1 on some parts. Enhancement mosfets won't be worse that that. But any fet-based open-loop attenuator is going to be bad. If you close a loop around it, an enhancement device, run at low voltage swings, will work about as well as a jfet.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
If you don't care about the noise (times if not orders of magnitude higher) then a 2N7002 or sort of might do.
Repeatability is not that bad, with the 2N4391 you can rely on it going down to around 30 Ohm at 0V GS over temp (some will go well below 20 Ohm at 25C). I know this because I have done variable gain using FETs 15+ years ago; I had to pair them (no paired 4391, only 4393 as 2N5566 IIRC) for temperature compensation, to eeprom calibrate against temperature drift (which was still too high for a 13 bit ADC), and to digitally linerarize the conversion results (12 bit INL was OK but with FETs you go into a few % INL,
1% is the best case)... (
formatting link
- photo taken with the 2N5566, before the paired 4391 were bulkier and uglier (soldered cans,
formatting link
- the companion board,
formatting link
- the servo driven rotary switches, caps etc. not yet soldered when photo was taken,
formatting link
- boards & servo put together,
formatting link
- the still kept historic reference to the entire mid-90s designed madness).
But since he cannot afford the -10V headroom a 4391 would take this is a non-issue, either the 2N7002 will do or not.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.