Discrete Opamp Open Design

[bcarso]

Originally Posted by scott wurcer

It does contain the most common mis-conception about CFA's. I rewrote Black's basic equation for VFA's and CFA's to create equivalent terms and it is clear that the CURRENT into the inverting input is the feedback signal.

Or take the simplest case, a follower, in a VFA and instantaneous step appears at the input and the voltage across the inputs determines the current delivered to the gain node, in a CFA the instantaneous current is determined by the output voltage across the feedback resistor or the current into the inverting input.

Good point.

[scott wurcer]

Originally Posted by grhughes

Scott: How is the breadboard build along? What parts do you want to change? Did you use the 50 ohm pot between the diff amp? I won't proceed with mine until you freeze the design. Ray

I plan to spend this weekend at it. Customers like Phillips and GE do have a habit of coming first.

[stinius]

Originally Posted by scott wurcer

It does contain the most common mis-conception about CFA's. I rewrote Black's basic equation for VFA's and CFA's to create equivalent terms and it is clear that the CURRENT into the inverting input is the feedback signal.

Or take the simplest case, a follower, in a VFA and instantaneous step appears at the input and the voltage across the inputs determines the current delivered to the gain node, in a CFA the instantaneous current is determined by the output voltage across the feedback resistor or the current into the inverting input.

Absolutely correct.

BTW: I have designed two new discrete op amps (or building blocks), one CFA and one VFA. They both have (to quote Brad from the BH thread) "precisely equivalent positive and negative slewing/settling behavior"

I have also designed two new preamps using these building blocks, the simulated results are very promising. Have to build them to confirm the simulated results.
One has balanced differential input, balanced "singel ended drive differential" output. The other has balanced differential input, balanced differential output. Both can of course also be used as se input, se output, se input, diff output and diff input, se output.

Attached some simulated results:
Preamp 1: (10V pp sinus out into 600ohm (less than 0.1ppm THD))
Preamp 1: 20k Square se out
Preamp 2: 20k square diff out

Both are using BF862 with BF545C as cascode


Cheers
Stein

[bcarso]

On this general subject of current versus voltage as variables, I was reminded of some help I gave to a visiting Bell Labs physicist at UCLA whose specialization was plasma physics. It was a very strategic bit of benevolence on my part although I didn't know it at the time.

He was lamenting, as I loaned him my copy of the RCA photomultiplier tube handbook that was missing from the engineering library, that he just couldn't get his equipment up and running for long, if at all. So I paid a visit to his lab and looked over some of the setup. It was difficult to suppress a few chuckles, as there were so many things effectively booby-trapped. For openers, the high voltage power supply was a huge and quite lethal thing capable of iirc a few 100mA up to several kilovolts Yet it was to be used to drive dynode voltage divider strings for the PM tubes, which drew at most a mA or so.

Then, the tube base he had turned out to have a 50 ohm termination resistor buried inside, anticipating pulse-counting photometry, but he was doing low-frequency "d.c." photometry. He initially didn't believe me when I said I measured 50 ohms. We got rid of that, or found a tube base that didn't have one.

But the funniest part: all of the connectors in the system were BNC. High voltage, opamp supply voltages, signal inputs, signal outputs . So of course the grad students were frying everything on a daily basis.

Anyway, I said Let me make something up for you that's a bit more bulletproof. I wired up an opamp (I think it was an ADI module) and used a feedback R and a little feedback C, tailored to the cable capacitance. I found an incompatible connector for the HV, and reserved normal BNCs for input and output. And I put a little safety resistor in series with the input, along with some clamp diodes to common ---I think it was 1k. If someone still managed to get HV to it, it would fuse before the opamp blew.

The problem was, that the physicist could not wrap his head around the appearance of the schematic, which, with the input protection resistor, looked like a voltage-input inverting amplifier with tons of gain (I think the feedback R may have been 1Mohm). I explained repeatedly that a PM tube had about the highest intrinsic output impedance of anything in routine use, and in that sense looked very close to an ideal current source. But even so, if he wanted to persist in viewing the amp as having a voltage gain of -1000, that still "worked": the, say, -10uA of phototube current would develop about -10mV across the 1k, and the gain of -1000 would then cause the output to be about +10V (given adequate opamp open-loop gain). It wasn't the most felicitous way to view things, but that explanation, and the fact that everything worked and had good signal-to-noise ratio, seemed to satisfy him.

[linuxguru]

Originally Posted by luvdunhill

... I could volunteer to look at higher output bias version. As you can see in my prototype, I'm using DIP14/16 heatsinks to bolster the heat dissipation of a SOT223 device. Does anyone have a recommended complementary SOT223 device? I have tried using BCP53-16 and BCP56-16 from On ...

Marc, I hope you haven't frozen your design. As I posted earlier, the NXP BCX53/56 are available in 0.5 W SOT-89 packages.

I added a few circuit elaborations to the basic folded Kaneda, and the performance improvement is dramatic.

1. Added a Class-AB push-pull buffer to reduce loading on the totem-pole VAS.
2. Added a Hawksford Cascode to the PNP driving the upper NPN of the totem pole - this is the game changer for the VAS.
3. Cascoded the LTP JFETs - contributes a few dB in THD20 reduction.

It's now at 13 actives, but all the additional devices are worth it. Compensation tweaks, clipping/overload, stability, etc. haven't been investigated.

Here's the basic circuit, which I'll call the Folded Kaneda-Hawksford.

[kgrlee]

Originally Posted by linuxguru

Here's the basic circuit, which I'll call the Folded Kaneda-Hawksford.

Thanks for this linux. Can you post your transistor models too.

[linuxguru]

LTSpice says that THD20 at 4V into 600R at output stage bias of about 12 mA is -120 dBr. Even if it's lying, it's pretty good - H2 can be reduced further by replacing Q2 with a diode and tying the bases of Q1 and Q3 together. However, I'd prefer higher H2.

[linuxguru]

Originally Posted by kgrlee

... Can you post your transistor models too.

The JFET model is the same as the 2sk117/184/209/2145 posted earlier - you can use the 2sk170 model in a pinch.

Here are the BJT models, downloaded from Infineon and Rohm respectively.

Edit: The output stage models are not critical - I cross-checked with NXP BCX56 and BCX51 models: Icq is within 15 per cent, and THD20 is within a few dB of the Infineon models attached.

[kgrlee]

Originally Posted by linuxguru

The JFET model is the same as the 2sk117/184/209/2145 posted earlier

Can't find this though I've searched back >200 entries looking for your posts.

Here are the BJT models, downloaded from Infineon and Rohm respectively.

Thanks for this Linux

[bcarso]

Originally Posted by bcarso

But even so, if he wanted to persist in viewing the amp as having a voltage gain of -1000, that still "worked": the, say, -10uA of phototube current would develop about -10mV across the 1k, and the gain of -1000 would then cause the output to be about +10V (given adequate opamp open-loop gain). It wasn't the most felicitous way to view things, but that explanation, and the fact that everything worked and had good signal-to-noise ratio, seemed to satisfy him.

The need for loop gain in this particular case had more to do with a fairly low open-loop input impedance for the opamp, not because of the apparent closed loop gain. Scott W., do you recall a module from ancient times that was a sort-of-general-purpose "budget" part? I think the number was the AD118, but of course that number has probably been recycled a few times since --- I couldn't find anything on the web. Maybe it was advertised in Analog Dialogue.