Front End Amplifier

The Front End Amplifier (FEA) presently in use at Schaller Observatory has evolved over 10 years of development.  The amplifier is a workable compromise between competing design requirements: High Speed (50 KHz) and high sensitivity (0.2 Volts per Nanoampere).

The active device is a quad operational amplifier from Burr-Brown.  It is the OPA404.  The Burr-Brown literature describes this amplifier as a Difet (dielectrically-isolated FET).  "It offers an unusual combination of very-low bias current together with wide bandwidth and fast slew rate." However, I did not select this amplifier from first principles... it was more a matter of trying various amplifiers types and selecting the best performer in the application.

The second key component in the FEA is the high ohm feedback resistor.  I am using a Victoreen, MOX300.  It has a resistance of 200 Megaohm with 1% tolerance.  MOX indicates it is a Metal Oxide Resistive element.  The MOX construction provides high stability. Handle the resistor carefully.  The oily film of your hands will easily affect the resistance.  (Victoreen Inc. 6000 Cochran Rd.  Cleveland, Ohio 44139)

The third key component is a metal die cast box.  Yes, a metal die cast box!  Sometimes referred to as an "RF Tight Box".  Don't waste your time with sheet metal boxes from Radio Shack, or worse yet, plastic boxes and copper tape.  The FEA is absolutely pushing the limits to begin with... feedback, oscillation, AM broadcast stations and 60 Hz noise are formidable foes in this project.  

The fourth key component is a pair of 9 Volt transistor radio batteries, mounted inside the box. An external power supply will pick up noise that you will chase.... forever.

Dead bugs are the fifth key component.  OK, I'm having some fun. What I'm really saying is to use "dead bug" wiring inside the box, most importantly on the input lead of the primary operational amplifier.  The operational amplifier can be socketed, but the actual input lead should be pried up and connected so as to float in mid air.

Here is the: Circuit diagram of the amplifier (Image, 33 Kb).  This circuit is deceptively simple. The most important parameters affecting it's performance are the construction details.

Here is a picture of the completed prototype: FEA Picture (Image, 37 Kb)