Logging Equipment Op Amps
Or How I Learned to Love the CA3140

Operational amplifiers or "op amps" as they are commonly called are used in many well logging tool and surface equipment designs.  The op amp is an incredibly versatile analog electronics building block used in just about every modern downhole logging tool in one capacity or another.  Our favorite op amp is the CA3140 and we will tell you why; but first a very brief history.

The term operational amplifier is generally believed to have been coined in 1947.  The name derives from the use of these high gain amplifiers to perform mathematical operations in analog computer applications.  Much research went into this area during WWII because of the need for artillery gun directors and bomb sites.  However, much of the foundation that made the early vacuum tube (valve) op amps possible was laid at Bell labs in their quest for better telephone repeater amplification technologies.  The early operational amplifiers were vacuum tube affairs, but gave way to solid state devices by the 1960's.

There were hybrid op amps in the early 1960's, a few of which showed up in surface logging electronics.  The first true integrated circuit op amp appeared in 1963, Bob Widlar's Fairchild ľA702.  The ľA702 was not well received due to a number of quirks, but in 1965, another Widlar design, the ľA709 was a great success, a design milestone and the first widely used monolithic IC op amp.  Many designs followed with Widlar moving to National Semiconductor and creating the LM101, followed less than a year later in 1968 with the Fairchild ľA741, one of the most popular op amps ever created.  There are many op amps that have been used in logging electronics, and a detailed list is beyond the scope of this effort, but the precision op amps are often seen in logging electronics with the OP7 and its descendents like the OP27 and OP37 being quite common.

In the early days of IC op amps, all manufacturers were using the same NPN bipolar process, and speed was limited due to the slow PNP transistors that could be made.  The LM118/218/318 introduced in 1971 was an early attempt to solve the problem, but was not a huge improvement (note that in the National Semiconductor scheme a 1xx chip indicates the military temperature rating, 2xx industrial temperature rating, and 3xx the commercial temperature rating).  In the early 1970s, the only truly fast IC process belonged to Harris, and the HA2500 and HA2600 series op amps became popular in downhole logging tools, especially for nuclear detector preamp circuits.  Many technicians hate the expensive Harris chips, and they were in fact overkill for many applications.

FET input op amps have some decided advantages especially in downhole tool applications, but the development of decent FET input monolithic IC op amps did not occur as early as chip manufacturers would have wished due to significant engineering problems.  There were some hybrid designs around; for example the LH0062 was popular with a number of logging tool manufacturers.  In 1974, National Semiconductor introduced the ion implantation process, finally enabling the fabrication of better monolithic FET input op amps.  National introduced the LF155/156/157 series, but it was not wildly successful due to some quirks related to asymmetrical topology.  PMI (later acquired by Analog Devices) introduced the OP15, OP16, and OP17 to compete with the LF155/156/157 parts.  The OP17 was particularly popular with logging tool repair technicians, and was regarded as almost a universal replacement part for nuclear logging tools.  In 1978, Texas Instruments (TI) entered the FET op amp market with their TL06x, TL07x, and TL08x devices, which became standards.  Computer Sonic Systems used the TI devices along with a few other logging equipment manufacturers.

In 1974, RCA introduced the CA3130 which uses a P-Channel MOS input and a CMOS output stage.  The CA3130 found use in a few logging equipment designs, but it was the later introduction of the CA3140 that caught the attention of logging tool designers.  The CA3140 utilizes a MOSFET input and a bipolar output, making it a far more "bullet proof" device than the CA3130.  The CA3140 has sufficient bandwidth to work nicely in scintillation tool preamp circuits, and Comprobe uses it for that purpose in its tools.  Buckeye in Ohio even used a pair of CA3140 op amps in their neutron preamp circuits (the very high input impedance of FET input op amps makes this possible).  Every CA3140 ever made is rated for the military temperature range, a feature one must pay dearly for in other op amps (this is also true for the CA3130 and a few other devices in the CA series, but sadly the dual version of the CA3140, the CA3240, is available only in the industrial temperature range).  RCA sold the CA series to Harris who eventually sold it to Intersil, and Intersil continues to produce the CA3140.  While far from perfect, with an MSRP as low as 29 cents each for a versatile mil spec temperature range op amp, you gotta to love the CA3140!

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