Induction Balance Stuff - Single/Multi Frequency Response, GB, Disc, Measurements, Ideas, Fun, etc.

[Aziz]

Oh man!,

I was really shocked about the simulated "temperature dependency" of the Richard Lee's preamp in LTspice.

With floating power supply rails, where the amplified signal "rides" on these rails, it is difficult to simulate the temperature dependency of the circuit.
After all, I had to switch off any initial operating point definitions (.ic commands) . It worked then.
These are usually required for proper .noise, .ac and .tran time domain analysis.

Now the temperature dependency is really acceptable. The gain varies only appr. 3% with 50 °C temperature difference (0 °C - 50 °C).
Offset voltage? What an offset? We don't need any. We don't want it. We are ignoring it. We have an AC coupled input and output.
Aziz

[Aziz]

Holly cow!

The single NPN/PNP ZTX851 / ZTX951 pair with Ic=10 mA each in Richard Lee's Duraglit Special does get directly
e_n = 0.2 nV/rt Hz
With input impedance of approx. 1.4 Ohm.
Mission accomplished.

Vbe matched two NPN, two PNP does even get 0.17 nV/rt Hz.

[Aziz]

AI Paper: Confidential - Engineering Research Department



pV target signal detection becomes reality soon.
I will revival my Richard Lee's amplifier soon. If I can find the bread board.

And the interesting PDF link to the other implementations and modifications to the Richard Lee's amplifier becomes important (former posting).
Link again: https://www.hifisonix.com/articles/m...it-compendium/

[Aziz]

Hi all,

we are entering the next level now:
e_n = 0.2 - 0.3 nV/rt Hz

And at the very very KISS-Level.


AI Tip: The Richard Lee's common-base amplifier can be used by impedance matching.
And I was wondering, why my former detector wasn't sensitive enough. I have the answer now.

See AI PDF paper.

This is a break-through moment.

Aziz

[Aziz]

I have been looking for a better NE5532 spice model (which is simulating its noise and dynamic current draw too). I have found one, but the .noise analysis isn't working properly.
Even the power consumption part isn't working properly.


Anyway, we can use the Excel table for amplifier noise prediction. Or take a similar other improved op-amp spice model.

[Aziz]

Hi all,

by paralleling two op-amps, the op-amp noise of NE5532 is beeing improved by sqrt(2) factor.
5 nV/rt Hz -> 3.54 nV/rt Hz
At gain 100, it's contribution to the input is 3.54 nV / 100 -> 35.4 pV/rt Hz. So it is completely irrelevant. The dominating noise source is still the pnp input transistor.

And we have effectively doubled the load resistance of 100 Ohm (Rf2) into 200 Ohm compared to a single op-amp. Or doubled the current drive capability of the single op-amp.

What can we do, if the distortion is too much or want to reduce the gain without lowering the load resistance further (Rf2)?
We could slightly increase Rf1 from 1 Ohm to 2.2 Ohm or higher. But this will cause much more noise and we will never get to the 0.3 nV/rt Hz level.

The other option without losing noise performance (keeping Rf1 = 1 Ohm):
We have to upgrade the NE5532 to an other dual op-amp.
- With more drive current capability or
- Rail-to-Rail output.

Rail-to-Rail output option:
As we don't have the big 2.0 - 2.5 V head room to the power supply rails (0 V, 9 V) with the rail-to-rail output option, we could increase the total gain of the amplifier. So we can rise the load resistor Rf2 (for instance doubling it). At the same time, we could voltage divide the output of the amplifier. This voltage divider can be realised in the feedback loop (splitting Rf2 resistor into two Rf2a, Rf2b resistors). There we have it.

This modification is shown here:



I have doubled the total gain (total Rf2 = 100+100 Ohm) and divided the output voltage by two. If I don't change the op-amp, I am limitted to half dynamic range only.
So the load resistance Rf2 is being rised to 200 Ohms and the op-amp won't hopefully clip or distort the signal at the output. You see, we would require rail-to-rail output op-amp here.

Or we could increase the battery voltage. Hell, don't do it. The power consumption... It will turn into a big heater ..

Aziz

[Aziz]

This is it.

Schematics & Noise performance of the pre-amp:


Input impedance:

Full LTspice files included.

[Aziz]

Can we reach that damn fkn level of en = 0.3 nV/√Hz?
Will we have the lowest-noise front-end in the history of metal detection?


The short answer is yes.

I am still optimizing the pre-amp circuit.

[Aziz]

Hi all,

I am really not happy with my discrete transistor amplifier specified for 3.7 V battery voltage. To much distortion. Problems arising if the battery voltage varies between 3 V (almost empty) and 4.5 V. I give it up.

I will look at the old PNP + op-amp designs with 9 V battery voltage.
Transistor collector current Ic around 8 - 12 mA. We can chose either single PNP ZTX951 or two Vbe matched PNP for lower noise.
Aziz

[Aziz]

Hi all,

I will be heading to Berlin tomorrow. 5 days. 650 km with a slow train. A 12 hour trip.
All devs will always delay.

(All the international top spies have a meeting there. We have a top secret meeting near the "Russisches Haus in Berlin", which has been previously closed due to drones in Leipzig Airport recently). (Psssst! Top secret! )

See you.. or later after being arrested..

[Aziz]

BTW,

the IB nulling by the mixer and the line output noise for IB nulling signal is really neglectable. The e_n gets slightly worse (a few pV / rt Hz only) but it won't affect our RX signals.
The dominating noise source is still the preamp. And may be the EMI.

We can limit the residual IB mismatch spec to lower this noise contribution further.

[Aziz]

Hi all,

with e_n = 0.3 nV/rt Hz ULNA (ultra low noise amplifier), we should be able to detect and resolve sub nano Volt RX signals (actually 0.35 nV with Gain=90, EMA-Alpha = 0.05).

[Aziz]

BTW,

I have been inspired by the "Rutherford" circuit solution on page 33.
Link to site: https://www.hifisonix.com/articles/m...it-compendium/
Link to pdf: https://hifisonix.com/wp-content/upl...amps-rev-2.pdf

I have just modified the preamp for the PNP ZTX951 transistor. So changed and adapted.

It is remarkable, that the output stage (NPN) is being part of the feedback loop.
I haven't seen this before.
Aziz

[Aziz]

Hi all,

I couldn't help me once again.

The discrete transistor preamp in post #447 can be modified to work with single 3.7 V Li-Ion cell or 3x 1.2 V Ni-MH cells. Total power consumption will be approx. max. 25 mA.
Instead of the 2xPNP Vbe matching, single PNP ZTX951 could be used. No Vbe matching required at all.

Single PNP solution will still get e_n of 0.3 nV/rt Hz (0.295 nV/rt Hz). THD will be slightly worse due to reduced voltage span.

We would have enough energy in the battery.

I wonder, whether this preamp will do the job too. Regardless of offset voltage variation (irrelevant, AC output) and gain variation (< 0.3 %) over temperature change (delta Temp 20 °C).

This preamp circuit is a nice KISS solution however.
I will think of to realize and test it.
Aziz

[Aziz]

Hi all,

paralelling 3 PNP transistors does not give any significant benefits. And matching 3 transistors does cause much work.

Only 2 parallel matched ZTX951 PNP transistors make sense (trick #1). 4 - 5 mA collector current each.

Now the op-amp considerations:
If we want to go down with the preamp gain (50 - 100 x), the op-amp have to source much current (10 - 15 mA). The THD will suffer.
If we do the current sharing trick (trick #2) with two op-amps parallel, thats ok.
We don't really benefit of the parallel op-amp noise reduction by 3 dB as the op-amp noise is totally irrelevant here (2-3 % of total noise).
This is the reason, why a dual op-amp makes sense. It reduces circuit complexity and allowing much more load of the op-amp output.
Perfect for a cheap NE5532.

If we want to go with gain down to 50 or lower, op-amp output current rises (heavy output load, out of spec). Adding a simple push-pull emitter follower output driver adds complexity to the whole circuit and reduces the phase margin for frequency stability further. We have the choice of using rail-to-rail output op-amp and dividing the gain by 2 for the feedback loop (trick #3). So we can reduce the gain feed back resistor Rf2 down to 50 Ohm.
Resistor Rf1 will always be 1 Ohm. We can't get the 0.3 nV/rt Hz level otherwise.

I hope I won't need trick #3.

Aziz