Even the cascode stage would benefit from an added b-e cap. It (should) kill any tendency to oscillate, and it allows the base to be stiff, without a quenching resistor, which will kill Miller effects.
With a big b-e cap on the cascode PNP, the upper current source can be a pokey mosfet, with lots of capacitance but no beta error.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
It's kind of a variation on Phil's active inductor. That defeats all the oscillation modes I can think of so far. (c-b, e-b, c-e)(Which means little, since circuits are so ingenious inventing new ones.)
You could use the beta-cancellation trick. But, stray and parasitic capacitance will be big factors compared to a 10pF integration cap.
-- Cheers, James Arthur
Yup. But I'd still add a base resistor, for the very reasons Gerhard Hoffmann brought up... the output Z of the OpAmp is a big unknown (*).
(*) Which brings to mind... maybe add some DC load to the OpAmp to keep it out of the class-B region. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | 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.
The base and emitter are shorted at AC. The opamp Zout doesn't matter any more.
LM7301 is a r-r output design. It isn't an ancient class-B like an LM324.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Whatever. I assume no responsibility for any dumb stunts you attempt.
It can still have dead-band. Though my own rail-to-rail designs go class-A at the crossover ;-)
My design done exactly one year ago does +/-100mA rail-to-rail from
+3.3V supply, yet avoids dead-band... but I _am_ the greatest ;-) ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | 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.
Or, apparently, for making sense.
So you keep telling us, without evidence.
Greatest asshole, actually.
-- 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 can't divulge proprietary information, which, if you had half a brain, you'd understand.
However, anyone who doubts my competency, feel free to call TLSI (631-470-8880, on Long Island), ask for Dr. Zhigang Ma, or Nicolas Salamina, and ask them what they think of my work. Ask about the power OpAmp. Then call me if you'd like something similar on _your_ custom chip :-)
Larkin's opinion matters not... with his head all the way up his posterior orifice he sees nothing but his own turd-laden ideas.
Nope. You hold that title. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | 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.
he
nLooks like the same thing; BAD ASCII ART FOLLOWS
+---- 47k-----+ (Iout) | | | | |\ | | Vref>--1k----+---| \ | |/ | >---+--+---47ohm----+--| +---|-/ | | | | |/ =3D | |\ +----------+---+--47k------+ V | | +--1k------------+ | (1ohm) | GNDThe use of a cascode can be folded into this scheme, by (1) using matched transistors for the cascode (2) doubling the feedback (to compensate two base currents, while only sensing one). A 1.2V reference as well as 2.5V = cascode base bias can be had from a LV431 style programmable zener; set it for 2.5V and the sense node gives you a stable 1.25V..
I agree with both those points, and meant to mention them. I don't trust the op-amp to behave nicely to impulses, or its output to be stiff. I'd expect the output to be hi-z on this timescale.
If anything I'd be tempted to bypass the bjt's base to the supply rail. The BJT then handles the fast stuff. Isolate that capacitive load from the op amp's output with a series resistor, and stabilize the op amp loop itself with a feed-forward cap. Standard stuff.
Does it all matter? Unclamping the integration cap will feed-thru a small impulse through to the bjt / CCS. After that, it's pretty clean. So, the op-amp's output impedance might not matter, but I suspect it will.
I invented loading LM324 outputs all by myself waayyyy back as a punk kid, to stiffen them up and kill the crossover THD. Of course everyone else on the planet thought of it too. :-)
-- Cheers, James Arthur
Yep, that's a good point. If the op amp can drive that load all by itself, it seems okay. That load being 1nF in series with 120ohms, it should be a snap.
I love LM324As. Yeah, I know you hate 'em.
-- Cheers, James Arthur
I thought you wanted it to switch?
RL
I don't trust a BJT current mirror to be that well-behaved under transient conditions. My inclination would be to have the BJT "charge" an inductor that takes all the abuse. The OpAmp + PNP is likely to fart.
YOU invented that? I was doing that in the mid '70's so I could use LM324's in active filters. I even banned Motorola as a supplier to OmniComp/GenRad of LM324's because of excessive dead-band. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | 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.
CMOS R-R OpAmps inherently have load-dependent gain and phase, so I avoid them when I can. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | 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.
81 r .
But John rightly points out that in the ramp timescale, Vbe is held constant by the 1nF cap. So, the transistor's fine.
In the longer term a load-step on the op amp's output may make it overshoot or ring. That would happen opening and closing the shorting switch. Probably not a big deal, AFAICT.
I was initially concerned the op amp is effectively absent on the 1nS response level, so there's no base drive, but John's right, that behavior is set by the 1nF b-e cap. IOW, it doesn't matter.
-- Cheers, James Arthur
The opamp is slowly servoing Vbe to get the average current right. The ramp is so fast that the transistor is still running in constant Vbe mode during the ramp. The cap could actually be much bigger... doesn't matter much.
It looks very weird. I like that.
-- 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
Me too! And I also invented loading the charge pump phase detector in the 4046.
I also invented the successive-detection log video detector, and the dual-slope ADC. I did those at times when I could have patented them. I'd be smoking seegars on the Riviera.
-- 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
DLVAs have been around since at least the war: e.g. US Patent 2577506 to Belleville, filed 1945.
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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Oh well. But I did invent the dual-slope some years before Fairchild (?) patented it.
-- 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
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5VYes. As cute as i(b) cancellation is, Phil's pseudo-darlington has two advantages-- 1. by reducing i(b), the i(b) error becomes less important, and 2. it should be more linear, since it removes Early's effect.
-- Cheers, James Arthur
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You can cancel the d.c. base current error (and use the faster transistor without fear) if you tack on a bunch of resistors--I worked up a PNP version of the technique whit3rd's citing--but the Phil- lington is cuter and mostly eliminates the need.
-- Cheers, James Arthur
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