High Performance Laboratory Preamplifier

[Samuel Groner]

Hi

For several applications (noise measurement, THD+N measurement, small signal testing etc.) I want to increase the vertical sensitivity of my scope with an external preamplifier; I set the following design goals:
* 100x gain
* very low input related voltage- and current-noise
* high input impedance, AC-coupled
* very low distortion
* excellent transient response
* 30 Vpp output swing
* input and ouput protected against overvoltage and shorts
And here's what I ended up with: [removed]

An exhaustive discussion of the design would be rather lengthy, so let me just point out a few interesting things:
* the balanced second stage provides very low offset (< 1 uV for matching parts), excellent current balance in the input pair and high open-loop gain (about 130 dB)
* the special biasing of Q2/Q3 provides some sort of common-mode feedback resulting in a simulated CMRR of better than 130 dB below 10 kHz (which is not likely to happen in real life though and rather unimportant for the given application anyway, but it doesn't hurt)
* the bootstrapped cascode Q6/Q7 of the input pair reduces input capacity in order to reduce distortion with high source impedances
* both small- and large-signal transient response is exeptionally good in simulation; the later shows a slew-rate of above 440 V/us

Now a few questions:
* Do you think the drift performance of the amplifier is good enough to skip the DC servo? I'd aim for < 10 mV output offset.
* The drains of the input pair is biased at 5.3 V; is this a well-chosen voltage? I'd have thought that a tad lower wouldn't hurt but SPICE indicated that it won't work.
* Do you think that it is promising to experiment with a three-stage architecture (i.e. as shown in the LT1001 datasheet) in order to improve distortion and precision even further (and probably loose some of the bandwith and transient performance)?
* While simulating I got a lot of "Gmin step failure", "Gmin stepping failure" and "source stepping failed" errors--especially after introducing the bootstrapped cascode, paralleled output devices or DC servo; any hints how to get rid of them?

Thanks for your comments and suggestions!

Samuel

 

[clintrubber]

[quote author="Samuel Groner"]Hi

For several applications (noise measurement, THD+N measurement, small signal testing etc.) I want to increase the vertical sensitivity of my scope with an external preamplifier; I set the following design goals:
* 100x gain
* very low input related voltage- and current-noise
* high input impedance, AC-coupled
* very low distortion
* excellent transient response
* 30 Vpp output swing
* input and ouput protected against overvoltage and shorts
And here's what I ended up with: LabPreAmp_r1.pdf
[/quote]
Can't help it, the quote "to increase the vertical sensitivity of my scope with an external preamplifier" made me immediately think of this circuit:
http://home.hetnet.nl/~chickennerdpig/FILES/Tek122/Tek_Type122_circuit_ampsection_rt.jpg

This 1958 design might even meet most of your requirements
(or could easily be made to meet most of them).

But OK, the design you drew a little bit more practical of course :thumb:

Regards,

Peter

 

[NewYorkDave]

[quote author="clintrubber"]Can't help it, the quote "to increase the vertical sensitivity of my scope with an external preamplifier" made me immediately think of this circuit: [/quote]

It made me think of a decade amplifier. That's what you'd usually use to measure weak signals. I haven't looked over Sam's design carefully but it could probably be adapted to decade switching if he chose to do so.

 

[clintrubber]

[quote author="NewYorkDave"][quote author="clintrubber"]Can't help it, the quote "to increase the vertical sensitivity of my scope with an external preamplifier" made me immediately think of this circuit: [/quote]

It made me think of a decade amplifier. That's what you'd usually use to measure weak signals. I haven't looked over Sam's design carefully but it could probably be adapted to decade switching if he chose to do so.[/quote]
FWIW, the Tek Type 122 is only a modest decade amp: just two settings (times 100 & 1000, and 'approximate' at that).
Could imagine indeed that in general having some more settings would be convenient but I'm sure the 100-requirement fully suits Samuels requirement.

Less fun than DIY would be to put an old analog-style AC-uV meter into service as such a 'decade' amp
(probably 1-2-5-10- or 1-3-10-style).
I have a few old Philips uV-meters with a pre-rectification analog output.
But as said, less fun than DIY.

Regards,

Peter

 

[clintrubber]

[quote author="Samuel Groner"] to increase the vertical sensitivity of my scope with an external preamplifier; [/quote]
As I understand from the noise-requirement & output drive capability you want to send the output-signal also to other things than just a scope, correct ?

Regards,

Peter

 

[Samuel Groner]

Thanks for your answers!

This 1958 design might even meet most of your requirements (or could easily be made to meet most of them).

I don't think it would come close with respect to noise and distortion.

Have you built a prototype yet?

No. may take a few month as I first want to finish other projects (we all have too many, don't we :green: )...

I haven't looked over Sam's design carefully but it could probably be adapted to decade switching if he chose to do so.

It's actually not that easy as one might think--either you loose speed and bandwith at high gains or you have to switch the compensation as well which I would attempt with a little trepidation.

Looks like the output stage is capable of a lot of current.

As I understand from the noise-requirement & output drive capability you want to send the output-signal also to other things than just a scope, correct?

A good part (two third) of the quiescent current is used to drive the feedback network with class A for low distortion, the rest is for a bit of cable capacity, drift and tolerances. I might connect it to a line input as well once in a while (i.e. for a spectral analysis after a notch filter). Short-circuit current is about 150 mA, which is low enough to protect the TO-92s from burning out after a few seconds, but they may not survive prolonged shorts. A TO-126 pair (which eats 250 mA) would have reduced bandwith and current-gain however.

Regarding the need for low noise: my scope (a Tektronix 465) becomes pretty noise a the highest vertical sensitivity setting if you compare it with a mic preamp, so I hope for a visual improvement as well.

Samuel

 

[clintrubber]

[quote author="Samuel Groner"]Thanks for your answers!

This 1958 design might even meet most of your requirements (or could easily be made to meet most of them).

I don't think it would come close with respect to noise and distortion[/quote]
Most likely :wink:
For grins I'll get the specs out tonight. That circuit was intended for an identical/alike application though (increasing scope sensitivity; various measures were taken to do that in a low noise fashion), but that was of course half a century ago.

It's actually not that easy as one might think--either you loose speed and bandwith at high gains or you have to switch the compensation as well which I would attempt with a little trepidation.

Could soon become complicated indeed! The schematic of a decade lab-pre-amp I recall uses quite elaborate switching, but it has to be said that it's also doing a constant 600 Ohm output.

Regards,

Peter

 

[rafafredd]

hey, I could use it as a mic preamp, with 1:1 input transformers...

 

[Samuel Groner]

Hey, I could use it as a mic preamp, with 1:1 input transformers...

Yes, I think it would make quite a good ribbon/dynamic mic preamp! Perhaps a current feedback amp (inverting configuration) afterwards for variable gain? I think I'd use a 1:2 input transformer for best noise figure.

Samuel

 

[Samuel Groner]

May I warily ask if there is a chance that someone here got an answer to one of my questions?

Samuel

 

[clintrubber]

[quote author="Samuel Groner"]May I warily ask if there is a chance that someone here got an answer to one of my questions?

Samuel[/quote]
Questions, there were questions ?! Weren't we discussing decade-amplifiers & tube circuits here ?! :wink:

* While simulating I got a lot of "Gmin step failure", "Gmin stepping failure" and "source stepping failed" errors--especially after introducing the bootstrapped cascode, paralleled output devices or DC servo; any hints how to get rid of them?

I'm not too familiar with SPICE, there's another (unrelated) simulator I'm using, but who knows these things might help. I'm very aware that these will very likely be obvious to you, but let's mention them:

* the circuit you're simulating, is it exactly as drawn in the pdf ? Could SPICE have problems with the in-between nodes @ C2,3 & C4,5 ? (floating for DC)
* what kind of simulation problems do you exactly have ? Convergence-problems for a DC-operating point ? Did 'initializing' help ? (like the voltage across C12, or the 'state' of the various biasing diodes, or the base-voltage of Q6,7 etc

Regards,

Peter

 

[Samuel Groner]

The circuit you're simulating, is it exactly as drawn in the PDF? Could SPICE have problems with the in-between nodes @ C2/C3 and C4/C5 (floating for DC)?

No, I replaced the capacitors with a single 5 pF (or rather I replaced the 5 pF derived by simulation with two 10 pFs). The protection diodes were removed and the servo as well as it never simulated properly with it. I just simulated the servo separately using simple ICs.

What kind of simulation problems do you exactly have? Convergence-problems for a DC-operating point? Did "initializing" help? (like the voltage across C12, or the 'state' of the various biasing diodes, or the base-voltage of Q6/Q7 etc.

DC analysis works well, AC and transient often give up. I tried to initialise the voltages around the cascode (as this seems to start the problems), but this didn't help (obviously as DC is not the problem).

Samuel

 

[bcarso]

You might try using the initial condition tool. Sometimes symmetrical circuits and circuits with bootstrapping give my program problems as well. If you tell it how to get started it may fix things.

Also, as far as the topology goes, I'd put something in the second stage left-hand branch to equalize the collector voltages of the PNPs and hence dissipations.

 

[CJ]

Cabling and shielding will be important, maybe have the 100x amp in a shielded container, with a short set of leads to the circuit.

Lot of work there, why not just buy a B/B 100X instrument amp?

Very educational, though.

 

[Sorr]

Peter,

Thankyou for the Tek 122 cct, I have been looking for it for years! :grin:
Sorr

 

[clintrubber]

[quote author="Sorr"]Peter,

Thankyou for the Tek 122 cct, I have been looking for it for years! :grin:
Sorr[/quote]
Hi Sorr,

My pleasure. I scanned it, only had a paper copy at first.
Later I found it also online, you might want to have the complete stuff:
ftp://bama.sbc.edu/downloads/tek/122_125/
Just curious, how you're using this unit ? (sorry for the OT)

Regards,

Peter

 

[Samuel Groner]

I must have recalled wrongly that operating point analysis worked well--it spites out the same errors. So I set initial conditions at about every node and now it sims like a charm! Thanks for beating me to it, Peter and Brad!

This allowed me to reduce the drain voltage to a more practical 1.7 V. Adding some resistive between Q7/Q9 (bypassed to increase phase margin) helped DC precision as well. The inherent offset is now below 10 nV (!).

Increasing the standing current in the second stage revealed that even higher slew-rates are possible--now we have 535 V/us.

I'll post the updated schematic ASAP.

Samuel

 

[Samuel Groner]

So here's the new revision with a few changes: [removed]

Samuel