Re: 9-decade transimpedance amplifier

> How about this? >
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That could work, sort of. The beauty of the Eckel circuit is the substantial range of its overlapping simultaneous outputs, as you go through range changes.

You won't be trying to digitize an opamp all the way to its saturation, and wait for the next stage to begin working.

If you have a very noisy signal, having two amplifier signal versions, at different gains, lets you process data from two channals and get a clean result.

Note, it has a similar parts count: one opamp per range, and JFET instead of a diode. It'd be a good addition to your bag of tricks.

Also, Eckel's circuit can work through zero, with bipolar input currents.

--
 Thanks, 
    - Win
Reply to
Winfield Hill
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Well, not at the range changes, exactly. As one starts using a lower-gain output, the higher-gain one is still available. Eventually, when you don't care about it anymore, it gets shorted. I'm sorry, if my "no-switching" rule prevented you from discovering an answer. It's certainly not gain switching as we've done it all along.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I'm sorry if the order threw you off, R1 is the highest range and that must be R1. As you decide to add ranges, you get R4 R5, etc. The C2 C3 caps are incidental, omit, most likely. So I think C1 has to be the always-included cap. But we can swap the C2 C3 designators.

Sheesh, I did discover a misleading error, in the "Now for the trick:" paragraph, the active JFETs are p-channel, Q4 and Q2, not n-channel. And the Figure 4x.31 scaling error, that Tom pointed out.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

On a sunny day (26 Jun 2019 11:06:06 -0700) it happened Winfield Hill wrote in :

No, no problem, but I thought somebody else wanted to change resistor values?

We have built a high dynamic range (nine decade) transimpedance ampli The amplifier uses junction-gate field effect transistors (JFETs) to switch ^^^^^^^^ resistors in the feedback of a low input bias current operational amplifier. ^^^^^^^^^^^^^^^^^^^^^^^^^

My idea was a log amplifier...

It is a nice circuit, surprises me that the switching with simple JFETs works so well. Thank you, will keep this, may come in handy one day.

Reply to
Jan Panteltje

Mine can work bipolar, with more diodes. But a photodiode deosn't need bipolar.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

It appears some people were paying attention, and trying to get the word out, past the paywall.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Well, as I said, the Eckel scheme, with outputs running simultaneously, insures fast, accurate digitizing, especially with noisy signals. But John's scheme looks very practical for separating a high Rf first stage from a lower Rf second stage. The first stage would not suffer from any extra capacitances to slow it down. You could safely add switches to the lower-gain stage to get lots of ranges. I might use 10x for 1st to 2nd stage, to insure speed, and do the 3.3x bit later.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I'm not sure you need the 2nd MOSFET. With MOSFET sources to the opamp output, all MOSFETs are off for voltages within your +/-(9V + Vgs-th) window. A MOSFET problem is finding parts with low Coss. E.g., BSS123 and BSS84 are about 7pF. This is across your highest-value feedback resistor.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

But without the 2nd mosfet you'd effectively have anti-parallel diodes across the feedback resistors and be limited to 0.6V?

Good point about capacitance.

piglet

Reply to
piglet

I am currently designing a fiberoptic clock distribution system, and want to light an LED if there is activity on an ECL differential signal pair. The cheap way to do that is with an AC-coupled diode detector and a comparator, with very small pickoff caps driving the diodes. That gets me into understanding the behavior of low-barrier schottky diodes (SMS7621, BAT15) near zero volts. My little multi-range photodiode amp also cares about "sub-threshold" diode behavior.

I did low-voltage measurement on an SMS7621 yesterday, but it ocurrs to me now that I may have been detecting ambient RF, which is after all what those diodes are made for. I should do it again, without the leads to power supplies and DVMs acting like nice antannas.

One day I want to try to visibly light an LED from our ambient RF.

I wonder how good the diode Spice models are at, say, +-0.5 volts, especially close to zero volts. I guess I'll find out. The data sheets are "RF", all about silly things like sine wave dBms.

The more expensive, and less interesting, way to do my signal detector is digitally, namely to clock an EP52 flop off my diff pair and reset it periodically or something.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

On a sunny day (Thu, 27 Jun 2019 06:52:16 -0700) it happened John Larkin wrote in :

I have done that, connect a LED to my CB GPA antenna, it lit up when neighbor across the street used his set. I called him, asked how many watts he was using. eeehh

May also work with your cellphone if close enough.

Reply to
Jan Panteltje

OK, good point, for a bipolar version. But OK for unipolar. But I'd just add an ordinary signal diode.

Carried to the extreme, it rules out JFETs too, and one would have to use John's or Phil's scheme.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Thanks Win, I like the trick. I'm wondering what the application is, where having to switch ranges (with a switch) is a problem. Our diode laser can use a large range of measurement currents (mA to ~10 nA) But not at the same time.

George H.

Reply to
George Herold

A switch, with wiring to the panel, will add 1 to 5pF across the feedback resistor. We often make fast TIAs, with high f_T opamps, that make use of the intrinsic 0.1pF capacitance of many resistors. We go further, and trick the resistor into having even less capacitance, see Figure 8.80-C. These applications could use John's scheme to separate the highest-gain stage from the rest.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

You can sometimes switch things around with low-capacitance diodes or phemts or even multiplexers, so rotary switch capacitance and wiring aren't in the circuit. The switch is cold, just DC.

These Fujitsu relays are dynamite.

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DPDT, fraction of a pF, fraction of an ohm, good to a few GHz. But don't water wash them.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

From

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I see

"The substrate, which functions as Gate 2 of Figure 1, is of relatively low resistivity material to maximize gain. For the same purpose, Gate 1 is of very low resistivity material, allowing the depletion region to spread mostly into the n-type channel. In most cases the gates are internally connected together. A tetrode device can be realized by not making this internal connection."

I have a few surplus JFETs from Linear Integrated Systems which do have an active fourth lead - I believe it's the substrate (gate 2). If I recall correctly one can either tie the substrate to the gate, or to the source, or to a constant voltage which is more negative than the source (in the case of an N-JFET).

Reply to
Dave Platt

Having substrate access sounds like it could be useful - shame the jfets I use (J177, J113, J107, BF256 etc) only have three pins :(

piglet

Reply to
Piglet

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