AD639——被遗忘的电路
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Forgotten Circuits (that should be brought back)
Dennis Feucht - November 12, 2012
As electronics technology matures, it shows some signs of aging. As innovation wanes, breakthroughs grow farther apart in time and risk-taking decreases. In earlier decades, some IC companies were willing to put some unusual and conceptually novel parts on the market.
Signetics dared to put out a novel IC, Hans Camenzind’s 555 timer, and it has become a premier legacy IC. What about others that should also be? This article recounts a few of these parts and why it might be good to bring them back.
- AD639 sine converter
- CA3096 BJT array
- MC14500B industrial control unit
- MC14549, MC14559 SARs
- MC4530 dual 5-input majority gate
AD639 sine converter
Long ago, Barrie Gilbert did some investigative work on the 2-BJT diff-amp circuit, which has a hyperbolic tangent (tanh) transfer function;
where I0 is the emitter source current and VT»26mV is the thermal voltage. To reduce nonlinearity, external emitter resistance, RE, is placed in series with the emitter in amplifiers. However, Gilbert applied the engineering adage that “if you can’t fix it, feature it” and put it to good use. Hyperbolic tangents are vaguely related to trigonometric functions, and Alan Grebene at Exar used a single diff-amp in the XR2206 function generator IC to convert a triangle-wave into a sine-wave.
The result was somewhat acceptable for a first-generation effort, though more refinement of the basic idea was carried out by Gilbert. He developed the multi-tanh concept of adding the outputs of diff-amps that had inputs offset from each other by a fixed voltage. This extended the function (and input range) and also led to other novelties, including the one that is used in the AD639 sine converter.
This IC is a trigonometric wonderland in 16 pins, and because its functional capability is so powerful, it was destined to become a legacy IC. Alas, ADI pulled the AD639 from the market without a replacement. Even Barrie did not know why. I do not know why. It seemed like a part destined to become a legend. It can synthesize all the basic trigonometric functions (sin, cos, tan sec, csc, cot) and their inverses.
The sine function is accurate to 0.02 %, better than most function generator sine outputs and better than the THD of many audio amplifiers. The IC has two of them, plus offset circuitry, and a multiplier and divider. It was given niche-market pricing and this IC did not find its way into FG instruments and others requiring accurate or low-THD sine-wave generation. It is specified to 1.5MHz.
Perhaps the only problem was that the AD639 was so appealing, ADI put a premium price tag on it which defeated its spread into the market as a commodity part. Perhaps Rochester Electronics (the leading supplier on the “trailing edge”) could revive it and reap the small fortune from it that it was destined to have. There is no reason to confine the mission of Rochester Electronics to that of a replacement parts supplier for obsolete equipment when they could also be considered the fulfiller of the destiny of great parts for new designs that did not catch on the first time around.
CA3096 BJT array
CA3096 BJT array
Somewhat like the AD639 are the highly versatile building blocks that are transistor arrays. RCA came out with some BJT arrays comprising a CA3000-series line. Some of these parts had fTs of over 1GHz for NPN BJTs that make them good for new designs today.
When RCA was shuffled around, it eventually ended up with Intersil but had lost the fab facility with its old large-geometry process. Tektronix based the design of the vertical amplifier in its 2205 oscilloscope on CA3046 (or the equivalent National LM3046) parts which are attractive for implementing fast two- or four-quadrant multipliers.
Intersil inherited a large but finite supply of these parts that are still available but are dwindling. They ought to be brought back on an existing process. It is not a major development project, and such parts would be extremely useful. Intersil did a replacement in the form of HFA3000-series SOIC parts with multi-GHz fTs but with correspondingly lower breakdown voltages, shown in the table below as excerpted from Intersil parts data.
While the original CA3000-series were good for ±12V supplies, the HFA series are better designed with ±5V supplies, though they can handle up to about 10V. What is much improved in the HFA series are the PNP BJTs which are dielectrically isolated rather than made as lateral transistors in the CA3096, shown below.
The CA3096 is a versatile part, with 3 NPN and 2 PNP BJTs. The one drawback is that the lateral PNPs have an fT of only about 6MHz. (Making the base thin is hard for lateral BJTs.) Yet for numerous circuits, this is not a major impediment.
One example is shown below, a feedback amplifier with a quasistatic gain of 3 and bandwidth of over 50MHz. It has two forward paths, the slow one through the PNP current mirror and the fast path through Q2 of the diff-amp input stage. It uses all five array BJTs. The only other semiconductor part is the avalanche diode, Z1.
Yet you would not be inclined to design this circuit into a new product because of an uncertainty in component supply. And the HFA part does not have the voltage range. A comparable series to the CA3000 but with dielectrically isolated PNPs would be a welcomed addition to the neo-legacy category of ICs.
- AD639 sine converter
- CA3096 BJT array
- MC14500B industrial control unit
- MC14549, MC14559 SARs
- MC4530 dual 5-input majority gate
MC14500B industrial control unit
MC14500B industrial control unit
This 16-pin Motorola IC is a single-bit 1MHz CMOS processor. It has three single-bit registers (flops), an ALU, and executes 16 instructions. More recent microcontrollers blow this part away, but that misses the point. It is a chunk of versatile logic that needs only an external counter for a program counter (PC) and a program memory driven by the PC.
The data memory is also the I/O memory. Four bits of the memory output drive the op-code input on the MC14500B and the rest are I/O addressing of 8-bit bidirectional latches (MC14599B) and 8-input multiplexers or data selectors (MC14512).
The one-bit accumulator is called the result register (RR). Instructions include: Load RR, Load the complement of RR, AND data with RR, Complement data and AND, OR, Complement data and OR, exclusive NOR (equivalence), store and store complement pulse the write line with valid RR output, move input data to input register or to output register, skip next instruction if RR = 0, and pulse flag O out or flag F out. Two other instructions, JMP and RTN also output flag pulses. The JMP flag can be used to load an address into the PC. The RTN instruction outputs a RTN flag and skips the next instruction.
The built-in oscillator generates the clock that drives the PC. The rising edge of the clock increments the PC and while high, the instruction is fetched. During the low phase of the clock, the instruction is decoded and executed.
With bit-serial processing and I/O-intensive, what advantage does this part have nowadays? With the requirement of an additional counter, and program and data memory, this part will remain obsolete because it cannot compete with lower-cost 8-pin to 16-pin flash-programmable ?Cs that are easy to use and are far more powerful. Although the part is interesting, it requires too much bit-twiddling to be welcomed back into production. This one will remain forgotten, despite its inspiration quotient.
- AD639 sine converter
- CA3096 BJT array
- MC14500B industrial control unit
- MC14549, MC14559 SARs
- MC4530 dual 5-input majority gate
MC14549, MC14559 SARs
MC14549, MC14559 SARs
These were originally part of the Motorola 4000-series offering of CMOS (mostly) digital ICs. They are successive-approximation registers. They have 8 bits per IC and can be cascaded for more bits. They are used to build successive-approximation A/D converters. Internally, they have a shift register and a parallel-loading register.
Despite the simplicity of an SAR, it is a useful digital function. The SA algorithm searches a range by making a succession of boolean comparisons beginning at mid-range. If the voltage is greater, the MSB is set and the next bit tested until all bits have been determined. All conversions on n bits take n clock cycles, independent of the digitized value.
With an extra comparator and one or two of SAR ICs driving an extra DAC, a simple ADC can be added to a system with left-over sub-parts. Although this level of integration is semi-discrete nowadays, for many applications with multi-DAC and multi-comparator ICs, and with a need for a simple ADC, this can be a design possibility.
SARs could also be used for auto-ranging, with fewer average steps than sequential ranging. Similarly, the gain of a VGA can be set by a SA search through the large range of the VGA. The weighting of the bits might no longer be binary, but instead be a decade or 1-2-5 sequence. Yet if monotonic, the scheme works.
- AD639 sine converter
- CA3096 BJT array
- MC14500B industrial control unit
- MC14549, MC14559 SARs
- MC4530 dual 5-input majority gate
MC4530 dual 5-input majority gate
MC4530 dual 5-input majority gate
One of the stranger logic functions to make it into integrated form and onto the market was a dual 5-input majority gate, sold by Motorola. If three or more of the five inputs are asserted, the output is asserted. Those of you who like to find novel uses for existing logic parts might have an intriguing time with this part. The output was gated with an XNOR (equivalence) gate from a W input, to set the polarity of the output.
Uses? This part is relevant to unusual applications, but it can coax some creative thinking. It generates a decision when five or fewer subsystems indicate a status in a redundant system. If the vital-sign monitors in a hospital intensive-care unit show three or more of five patients in trouble, a triage state is asserted.
By connecting one input high and one low, then two out of three control computers (as in the Space Shuttle) prevail in a vote on an output assertion. Or if multiple banks of capacitors are asynchronously charged, sufficient charge is available and the firing device enabled if m out of n banks indicate that they are charged. The function is egalitarian; any m out of n will trigger an event. Using inverted input logic, a statically-stable multi-ped robot with fewer than m of n feet on the ground triggers a fault state.
A voting hierarchy is implemented by cascading the output of one of the majority gates into one input of another. Then the first five have one vote in the second five. Although such uses might be practical, it is still an unusual logic function to try to apply. No wonder it is a forgotten circuit.
What would you add to this list?
- AD639 sine converter
- CA3096 BJT array
- MC14500B industrial control unit
- MC14549, MC14559 SARs
- MC4530 dual 5-input majority gate
Dennis Feucht has designed test and measurement instruments, and has been doing power electronics for a long time. He authored the Analog Circuit Design Series.
Related articles:
- What circuits textbooks haven’t told you about feedback amplifier and transistor port impedances
- Benchside chat: BJT arrays
- Quirky chips
- Analog: back to the future
- The 555 timer IC