Home » , , » The Many Op Amp Categories

The Many Op Amp Categories

Many of the earliest ADI modular op amps used bipolar transistors for the input stages, and they all used bipolar transistors in later stages. Matched duals of either bipolar or FET types were scarce in the early 1960's, but these were incorporated into designs soon after announcement. An early listing of ADI op amps has five categories; general purpose, low bias current, low drift, wideband, and high voltage/current (Reference 20: Capsule Listing of Analog Devices Op Amps, ADI bulletin of April 1968). This list was expanded considerably in only a year (Reference 21: Selection Handbook and Catalog Guide to Operational Amplifiers, ADI catalog of January 1969).
In the general purposes types, the models 111 and later the 118 were popular units, due to a combination of good basic specs and attractive prices. The varactor types already mentioned led performance for low bias current types, but there were also FET input types such as the early model 142 with bias currents in the tens of pA range.
In the low drift category, various chopper amplifiers such as the 210, 211, 220, and later the 232 and 233, and the 260 led in performance, There were also low drift chopper-less amplifiers such as the 180 and 183, using precision bipolar transistor front ends. There was considerable support for choppers over the next few years (References 22-24: Ray Stata, "Applications Manual 201, 202, 203 & 210 Chopper Stabilized Operational Amplifiers," ADI application note, 1967 (Use of 200 series modular chopper amplifiers), ADI Staff, "Circuit Advantages of Chopper Stabilized Operational Amplifiers," ADI application note, September 1970 (Operation and use of inverting-type modular chopper stabilized op amps), Peter Zicko, "Designing with Chopper Stabilized Operational Amplifiers," ADI application note, September 1970 (Operation and use of non-inverting-type modular chopper stabilized op amps)).
Model 121 Op Amp
A design done by Dick Burwen for ADI was the model 121, a fast, fully differential op amp, in 1966. This design demonstrates some useful circuit techniques in Figure 11.
The ADI model 121 wideband DC op amp
Figure 11: The ADI model 121 wideband DC op amp
One of the techniques is how to make a high speed, low noise input stage, which is by means of the L1-L2 chokes. At low frequencies the chokes shunt the otherwise noisy degeneration resistors, R8-R9. It also shows the use of relatively heavy bypassing and decoupling internal to the op amp (a necessary practical step, but possible within the confines of a module).
The model 121 NPN input stage runs at a high tail current, for fast slewing. But, note that the R2 and R5 resistors compensate the input bias current, which would otherwise be high. These (selected) resistors provide a temperature tracking bias from the floating diode source, CR1-CR2. This bias scheme was patented by Burwen, and was also used in other ADI op amps of the period (Reference 25: Richard S. Burwen, "Input Current Compensation with Temperature for Differential Transistor Amplifier," US Patent 3,467,908, filed Feb. 7, 1968, issued Sept. 16, 1969 (A method of compensating for bias current in a differential pair amplifier)).
As can be noted, the model 121 used a current source for the 2N2975 matched NPN pair input stage, Q2, to optimize CMR. In critical locations, 1% metal film resistors are also used. The gain-of-five stable model 121 had a gain of 25,000 (or 88dB), achieved a slew rate of 250V/μs, and sold for $98 in 1968 (see Reference 20, again).
Share this article :


Post a Comment

Please wait for approval of your comment .......