Composite amps

Best result OPA140+BF862 (L=1mH+1mH drain load):

Rf=91k and Rf=1M

Pushmi-Pullyu dual accelerator - input JFET, output pnp BJT DC precision power supply +-3.8V active probe - bootstrapped dual JFET half-bridge CPH6904 (out=in+0.86V) (mediocre, better jfet+ADA4860 or FVF (Flipped Voltage Follower) with BF998 )

Tortoise outrun Achilles ! time: 1.25us ?ircuit speed indifferent to input capacitance !

?heetah enters the arena:

only 1M + 2200pF + OPA656 poor substitute, OPA140+jfet better

The final, a curtain. OPA656 + BF862

GBW=31 GHz ? ((

On Thursday, February 27, 2020 at 6:10:34 PM UTC-5, snipped-for-privacy@gmail.com wr ote:

I'm confused about the two traces are here. The BF862 is unfortunately no more... You can only surf so long on the trailing edge of technology.

Is this for me? Hard to tell the shape of the response for the 91k. But the 1M looks more like a two pole response now anyway.

) 98)

So all the stuff on the left is an active probe? To look at the inverting node without loading down with capacitance? So is this right? The jfet is taking the fast part of the current signal and sending it through to the non-inverting input?

I don't know what the pnp is doing on the output. Unless it's a driver for some coax cable output? It looks the the 'output' (your arrow between collector of pnp and 1k R to -V) should be near -V... are you now biasing PD via the output?

Well at least to C on the inverting node.

Huh the opa656 looks nice.. +/- 5V. Thanks

Have you been lurking here long... I'm just reminded of Jan Pantelje (sp) Which is a good thing. I like Jan.

George H.

With Rf=91k now Cf>0pF must be present

Yes, the circuit is very sensitive to the probe capacitance. Non-inverting node !

JFET is input AC amplifier with feedback through Op Amp TIA))

MMBTH81 is amplifier with gain~10 look in the upper right corner of the book AoE)) no photodiode offset ! slew rate only doubles(( overall gain increases

Capacitance SiPM 60035 = 3400pF Dmitriy P.

Russia is expert at that game.

NT

I didn't mean that you thought it was the best one, but it's sure better than an OPA111. ;)

I'm also partial to the OPA140, which is just about the perfect JFET op amp: 0.5 pA typical Ibias at 25C, 5 nV 1-Hz flatband noise, 30 Hz 1/f corner, 11 MHz BW, 220 uV max offset over temperature, 0.25 uV/K typical drift, $2. Not horrible at all.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

I remembered where the zero offset of the photodiodes is still important ! Excellent mid-infrared photodiodes from Russia -

Ro = 5 ? ... 10 k

Dmitriy P.

OK got it, thanks. I'm so-so at single transistor design.

George H.

Composite voltage follower CFA ADA4860 + BF862

Output signals yellow - circuit A white - circuit B (bootstrapped)

Rise time circuit B (bootstrapped)

Huh? scratch scratch. Dmitriy, I have no idea what you are doing? What is the input? (for one) You'd have to describe what you are doing if you wanted some sort of input/ critic/ ideas. Which would most likely come not from me, I mostly just ask questions.

George H.

very simple - imagine that a capacitor 2200pF is a photodiode with pulse ph

A unity-gain buffer removes the photodiode's parasitic capacitance : circuit A - 2200pF >> 15pF circuit B - 2200pF >> 0.8pF Bootstrap photodiodes known since 1984, may have been used before. The operation of the circuit "A" is limited by the capacitance gate-drain of the JFET transistor.

Versatile composite amplifier configuration

"Shunt bootstrapping technique to improve bandwidth of transimpedance ampli fiers"

fig.3 Output of bootstrap amplifier

my composite voltage follower -

rise time 70ns, noise 150mVp-p Channel 2 = output ADA4860

I wish you all good health.

Composite amplifier EPHEMT + RGC + OPA656 Cdet=2200pF Rf=1M ATF-54143 Igate= 18 nA Bead on gate blm18bb050sn1d

Without e-phemt :

Rise time :

Dmitriy P.

Retrospective "composite amplifier" challenge)) Question for experts and authors of the AoE3. What is the function of capacitors 10nF between base and emitter MMBT6429 ?

Why is there only one capacitor in the service manual between base and emitter ? Dmitriy P.

You're right, my copy of the service manual only shows one capacitor. The symmetrical extra one came in, either when I made the pencil drawing for the draftsman, or they made a drafting error. As for the purpose of the capacitors, we can only guess. It'd be nice to be able to ask the HP engineer(s).

Did we point out in the text that those are cascode transistors, whose only purpose is to isolate the op-amp active nodes from the very high capacitance of the huge IF3602 JFETs? The Darlington is to insure all of the emitter current ends up in the collector, and almost none in the base.

The performance specs for their G=10,000 amplifier are pretty amazing. We have one of those HP 34420A multimeters in my lab, but it's rarely used, it's far too good! And the special nanovolt input connector is awkward.

--
 Thanks, 
    - Win

Some former HP engineers are on:

This replaced the old Yahoo HP group.

Yeah, but so is Gardner lately.

I have a new design, a little thing to keep busy with during the shutdown. It's a GHz o/e converter. The trick is to use both ends of the photodiode, use the current twice. One end goes into a fast AC-coupled amp, and one end into a slow, precise DC gain path, and combine them later.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard