Can anyone take a guess at the topology of an SR560 (1mhz 100Meg pre-amplifier)

[AlessandroAU]

Hi all,

I'm interested in replicating the basic front-end of the SR650 http://www.thinksrs.com/products/SR560.htm

Can anyone take a guess on what type of topology they would use? Jfet > Bipolar?. Would it be all hand picked and matched discrete components? Anyone have any reference material for similar spec'd preamp circuits?

[T3sl4co1l]

Hmm, with only 1MHz bandwidth it can probably be done with a stock pair of op-amps.  Current feedback or high GBW types will be required to get that gain, but still only $10 of amps, really.

You'd go to the trouble of using discretes (and fancy matching and stuff) if you needed similar performance (maybe not so much gain; it would be useless on top of the noise level), and maybe DC accuracy too, over a 100MHz+ bandwidth.

Tim

[Alex Eisenhut]

Why not read the user manual? It's right there.

"U106 is an NPD5564 low-noise matched
FET pair, which, along with U102 and U103
form the first differential amplifier stage.
U102 compares the currents in the drain
loads of U106, and U103 maintains the sum
of those currents at a fixed level by varying
the total current in both FETs. "

[Null]

SR560 front end is discussed in AoE 3rd edition, in one of the "Designs of the Masters" examples I think, don't have the book with me. I believe the obsolete dual JFET is replaced with LSK389 in current production... from memory, don't have the book at hand.

[Marco]

Why not read the user manual? It's right there.

"U106 is an NPD5564 low-noise matched
FET pair, which, along with U102 and U103
form the first differential amplifier stage.
U102 compares the currents in the drain
loads of U106, and U103 maintains the sum
of those currents at a fixed level by varying
the total current in both FETs. "

That sounds like a variation of this kind of topology (for which I do not know the name).

[acbern]

That sounds like a variation of this kind of topology (for which I do not know the name).

Not really. The link relates to a BJT-based differential amp, while the 560 uses a FET diff amp in its input stage. You should use a BJT amp only with very low impedance sources (some ten ohms). FET based amps are for high impedance sources (as is more often the case, hence its use in the SR560)

[holko]

I scored a SR560 cheaply from Ebay a few months ago, haven't got around to fix it up yet but I know for sure that the input is a dual matched J-FET connected to a TI/BB opamp. When I get around to fix the instrument I'll sure do a sketch of the input circuitry.

[Marco]

Not really. The link relates to a BJT-based differential amp, while the 560 uses a FET diff amp in its input stage. You should use a BJT amp only with very low impedance sources (some ten ohms). FET based amps are for high impedance sources (as is more often the case, hence its use in the SR560)

I'm talking more about the concept of turning the differential voltage between two inputs into a current, which you can convert to an output voltage with a resistor, which has some advantages over the traditional inamp. No reliance on resistor matching for CMRR for instance.

As is the amplifier I linked is only useful for AC coupling any way.

[dom0]

Have a picture

Av = 100
en ~ 2 nV²/Hz
ft ~20 MHz

[Marco]

Oh well, just a plain inamp with composite discrete FET opamps then. Disappointing :/

[T3sl4co1l]

Like I said, it doesn't need anything very special.

Tim

[AlessandroAU]

Why not read the user manual? It's right there.

"U106 is an NPD5564 low-noise matched
FET pair, which, along with U102 and U103
form the first differential amplifier stage.
U102 compares the currents in the drain
loads of U106, and U103 maintains the sum
of those currents at a fixed level by varying
the total current in both FETs. "

I missed that completely, quite embarrassing, thank you.


Also thanks dem0 for that schematic and thanks for the AoE 3rd edition reference, I hadn't come across that book.