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Solid-State Modular and Hybrid Op Amp Designs

There were new as well as old companies involved in early solid-state op amps. GAP/R was already well established as a vacuum tube op amp supplier, so solid-state op amps for them were a new form of the same basic product. With GAP/R and others in the 1960's, the Boston area was to become the first center of the solid-state op amp world.
Elsewhere, other companies were formed to meet market demand for the more compact transistor op amps. Burr-Brown Research Corporation in Arizona fell into this category. Formed by Robert Page Burr and Thomas Brown in 1956, Burr-Brown was an early modular op amp supplier, and supported their products with an applications book (Reference 9: Burr-Brown Applications Staff, Handbook of Operational Amplifier Applications, 1st Ed., Burr-Brown Research Corporation, 1963 (Burr-Brown’s early semiconductor oriented op amp application manual)). The Burr-Brown product line grew steadily over the years, emerging into a major supplier of precision amplifiers and other instrumentation ICs. Texas Instruments bought Burr-Brown in 2000, merging the product lines of the two companies.
The GAP/R model P65 solid-state op amp
Figure 7: The GAP/R model P65 solid-state op amp
On the other hand, in 1960 GAP/R was a transitioning company, and while they maintained the vacuum tube op amp line for some time, they stayed away from solidstate op amps until quality silicon transistors could be found. GAP/R began to introduce solid-state op amps in the early 1960's. George Philbrick was simply unwilling to produce germanium transistor solid-state op amps, and he also had specific ideas about the optimum amplifier topology that could be used— more on this below.
The new solid-state op amps were to transition power supply and signal range standards from ±300V/±100V down to ±15V/±10V, a standard that still exists today. And of course, new packaging for the op amps was to emerge, in several forms.
The GAP/R P65, shown above in Figure 7, was a general purpose device. It was designed by Alan Pearlman, with later revisions by Bob Malter, and was produced from 1961 through 1971. The first stage Q1-Q2 used a pair of matched 2N930s, with a tail current of 66μA, and had hand-selected bias compensation (the SEL resistors).
The second P65 stage of Q3-Q4 ran at substantially more current, and featured a gainboosting positive feedback loop via the SEL and 47kΩ resistors. The common-emitter output stage was PNP Q5, loaded by NPN current source Q6. The two-stage NPN differential pair cascade used in the P65 design was to become a basic part of other GAP/R op amps, such as the P45 (described below).
Small value feedforward capacitors sped up the AC response, and an output RC snubber provided stability, along with phase compensation across the Q1-Q2 collectors. The transistor types shown represented the original P65, but later on the P65A used better transistors (such as the 2N2907), and thus could deliver more output drive.
Another GAP/R solid-state op amp was the P45, shown below in Figure 8 as a photo of the card-mounted op amp, and the schematic. The P45 was designed by Bob Pease, and was introduced in 1963 (Reference 10: Robert A. Pease, "Design of a Modern High-Performance Operational Amplifier," GAP/R Lightning Empiricist, Vol. 11, No. 2, April 1963. (The designer's description of the P45 op amp. Note— Q8 polarity correction via RAP to WJ email of 5/23/2001)). The edge connector card package shown was used with the P45 and P65, as well as many other GAP/R solid-state amplifiers.
The GAP/R model P45 solid-state op amp
Figure 8: The GAP/R model P45 solid-state op amp
The P45 design was aimed at fast, inverting mode applications. With a class AB output stage, the P45A could deliver ±10V at ±20mA to the load. Gain was rated a minimum of 50,000 at 25°C into a load of 500Ω. One of the more outstanding specifications of the P45 was its gain-bandwidth product of 100MHz. In 1966, a P45A cost $118 in quantities of 1-4 (Reference 11: Philbrick Solid-State Operational Amplifiers, GAP/R bulletin of 3/1/1966). Both the P65 and P45 ran on ±15V, the new power standard, and were intended for input/output signal ranges of ±10V.
As mentioned, the cascaded NPN differential pair topology used in the P45 and P65 designs was to become a basic part of other GAP/R op amps. A feature of the design was the controlled positive feedback path, from the Q3 collector back to the Q4 base. Offset was controlled by a potentiometer connected between the BAL pin and +15V in the P45, with a similar arrangement used in the P65 (Fig. 7, again).
In the P45 the first two gain stages are followed by PNP common-emitter stage Q5, which provides a great deal of the voltage gain. Emitter followers Q7 and Q8 buffer the high impedance node at Q5's collector, providing a low impedance source to the load.
In 1962 Alan Pearlman and partner Roger R. (Tim) Noble formed their own Boston area company, Nexus Research Laboratory, Inc. Nexus competed with both GAP/R and Burr-Brown in the growing solid-state op amp field (and ultimately with a third local company). The Nexus mission was to deliver solid-state op amps to customers for printed circuit board mounting, thus the Nexus designs used a rectangular, potted module package. They were so popular that they influenced GAP/R to follow suit with modular designs of their own.
In 1962, George Philbrick himself did the layout of another P65 derivative, the PP65, which was one of the first GAP/R modules. Shown in Figure 9 below, this square outline, 0.2" centered, 7-pin footprint was to become more or less a modular op amp standard. It used five pins for output/power/offset on one side, with the two input pins on the opposite side.
The GAP/R model PP65 potted module solid-state op amp
Figure 9: The GAP/R model PP65 potted module solid-state op amp
It might be easy for some to dismiss the importance of a package design within a chart of op amp progress. Nevertheless, the modular package format opened up new opportunities, and allowed the op amp to be treated, for the first time, as a component. This opening of application opportunities enhanced op amp growth, significantly(This pattern was to be repeated again and again with op amps, and continues even today, with miniature SOIC packages displacing DIP and other through-hole packages).
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