This is my take on Ikoflexor's low noise measurement preamp.

I used a Vishay photovoltaic MOSFET driver (http://www.vishay.com/docs/81225/ssrvo126.pdf) in an unusual way to make a negative bias on the input JFET. This eliminates the huge capacitor in series with the 1 Ohm gain resistor. In fact there are no electrolytics in the signal path. The back to back PV arrays make a nice bipolar bias reference and you can simply flip the diodes to reverse polarity. On dual supplies this voltage can go outside both rails (when referenced to ground). R3 is selected for ~1/2 the supply at the drain off J1 at the desired drain current. Then when R2 is ~2XR3 the drain voltage will track roughly ½ the supply voltage over a wide range with no further adjustment. The MOSFET driver can be looked at as an isolated current in current out device (gain = .001). This times the 10Meg resistor gives a gain in this loop (FET drain current to gate voltage) of 10,000, or 10,000*gm if looked at as a voltage gain. For these FETs it's around 100 which gives decent enough operating point stability from FET to FET without trim.

The nominal gain is 60dB and BW from .1Hz to >200kHz. the dual op-amp arrangement has DC gain of one and acts at AC like a differential out amplifier. So the input stage finds Vcc/2 at the drain of the FET with optical feedback and the output stage sits at the same voltage.

This op-amp can be an ordinary bi-FET. An additional requirement was that the circuit could be used with 48V phantom power and make for a very simple one box set up. Of course the multi-paralleled FETs at the input would work but phantom power is limited to 10mA.

As built with one 2SK170 it came right out at 1.1nV as simulated and with these values and AD823, .1Hz to 400kHz. No comp-cap is needed at G = 1000, but it will be necessary at lower gains. Even a TL072 will work with Fmax becoming 200kHz or so. If C6 gets too large one can induce motor-boating, I’m sure there is a optimum set of low frequency time constants.

At 6mA current draw this whole circuit could be put under a 48V phantom supply with differential outputs on the two op-amps (gain will be +6dB more). This would require a larger resistance level in the feedback network.

Big advantage, 1Hz BW with no caps in the feedback network and no electrolytics needed anywhere (except bypassing).

(Pictures in Pass Forum)

BTW Ikoflexor my motherboard overheated and failed last week. I lost my entire address book, and it took me a few days to get the energy to assemble this.

I used a Vishay photovoltaic MOSFET driver (http://www.vishay.com/docs/81225/ssrvo126.pdf) in an unusual way to make a negative bias on the input JFET. This eliminates the huge capacitor in series with the 1 Ohm gain resistor. In fact there are no electrolytics in the signal path. The back to back PV arrays make a nice bipolar bias reference and you can simply flip the diodes to reverse polarity. On dual supplies this voltage can go outside both rails (when referenced to ground). R3 is selected for ~1/2 the supply at the drain off J1 at the desired drain current. Then when R2 is ~2XR3 the drain voltage will track roughly ½ the supply voltage over a wide range with no further adjustment. The MOSFET driver can be looked at as an isolated current in current out device (gain = .001). This times the 10Meg resistor gives a gain in this loop (FET drain current to gate voltage) of 10,000, or 10,000*gm if looked at as a voltage gain. For these FETs it's around 100 which gives decent enough operating point stability from FET to FET without trim.

The nominal gain is 60dB and BW from .1Hz to >200kHz. the dual op-amp arrangement has DC gain of one and acts at AC like a differential out amplifier. So the input stage finds Vcc/2 at the drain of the FET with optical feedback and the output stage sits at the same voltage.

This op-amp can be an ordinary bi-FET. An additional requirement was that the circuit could be used with 48V phantom power and make for a very simple one box set up. Of course the multi-paralleled FETs at the input would work but phantom power is limited to 10mA.

As built with one 2SK170 it came right out at 1.1nV as simulated and with these values and AD823, .1Hz to 400kHz. No comp-cap is needed at G = 1000, but it will be necessary at lower gains. Even a TL072 will work with Fmax becoming 200kHz or so. If C6 gets too large one can induce motor-boating, I’m sure there is a optimum set of low frequency time constants.

At 6mA current draw this whole circuit could be put under a 48V phantom supply with differential outputs on the two op-amps (gain will be +6dB more). This would require a larger resistance level in the feedback network.

Big advantage, 1Hz BW with no caps in the feedback network and no electrolytics needed anywhere (except bypassing).

(Pictures in Pass Forum)

BTW Ikoflexor my motherboard overheated and failed last week. I lost my entire address book, and it took me a few days to get the energy to assemble this.