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Between Discrete & Integrated Circuits

Years ago, Tektronix could not justify an expensive, high-performance semiconductor operation solely for producing competition-defeating ICs for oscilloscopes, and formed a joint venture with Maxim called (appropriately) Maxtek. Maxim then participated in the benefits of access to the “super high frequency” processes that Tek had for making BJTs (bipolar junction transistors). Back then, another jointly-shared technology was more prominent than now, that of hybrid circuits .

In previous decades, the desire to increase integration of scopes at Tektronix led at times to hybrid circuits. These circuits are made by screen-printing and laser trimming thick-film resistors or etching small capacitors on a ceramic substrate, and then surface-mounting components on the substrate, sometimes on both sides. Some are shown below.

Pins are placed on a substrate edge, and the resulting subsystem is used as an integrated circuit. Sometimes hybrids were not much larger than through-hole ICs of the time. The high-frequency performance of ceramic is superior to fiberglass etched circuit-board (ECB) material — having less dC/df , called “hook.” All resistors are fabricated in the same process step, and hybrids in this sense are closer to monolithic construction than board-based circuits.

However, surface-mount technology (SMT) has diminished the benefits of hybrids, though it should not be a forgotten option. The BJTs and chip capacitors on the above hybrids are SMT parts. The only significant difference from SMT ECBs is that the resistors and small capacitors are an integral part of the assembly. Except for some specialized applications, hybrid circuits are at a cost disadvantage compared with SMT ECB technology.

As for ECB technology itself, it need not be very discrete. The use of ICs such as op-amps and ADCs on boards as semi-discrete technology continues to be the mainstream level of integration for non-size-critical applications. Transistor arrays for semi-discrete design offer monolithic circuit properties. Intersil has come out with a second generation of CA3000-series BJT arrays, except the PNPs are also fast. The HFA30xx and HFA31xx arrays have an fT spec of multi-GHz. Additionally, Mosfets are available in arrays beginning with the (slow) CMOS logic family part, the 4007UB: three Mosfets of each polarity in configurations suitable for implementing bipolar current mirrors.

Arrays have some of the advantages of monolithic circuitry in a discrete context. For those engineers who like to optimize their designs, finding the right op-amp is not the solution to every circuit design problem. Sometimes, a superior circuit can result from the careful and clever application of a few transistors, especially when matched and thermally connected in arrays.

The old RCA CA3000 series of BJT arrays included the CA3096, a 3-NPN, 2-PNP array with matched NPNs and matched, lateral (low-β) PNPs. The PNPs are slow; fT = 6 MHz, and the NPNs are like 2N3904s, with about 300 MHz fT . These parts have been discontinued by Intersil because over the years it's been selling off a large inventory inherited from RCA. The CA3046 5 NPN array was used extensively in the Tek 2205 20MHz oscilloscope. Although analog scopes are fading, the entire transistor circuitry of the 2205 vertical amplifier looks as if it could be integrated into one present-day IC. Indeed, with a few capacitors hanging on external pins, a many-pin package might contain the entire amplifier and time-base part of this oscilloscope. A bird's-eye view of the Tek circuit diagram of the vertical amplifier output stages is given below.

For a better view of this schematic, click here. (Source: Tektronix)

For a better view of this schematic, click here.
(Source: Tektronix)

U130 (upper middle) is a CA3102, two diff-pairs with BJT emitter current sources, for constructing transconductance multipliers. (The Intersil HFA3101 is similar, with internally cross-connected collectors.) The CA3102 BJTs have fT values of 1.35GHz. This kind of variable-gain amplifier depends on matched p-n junctions, and the array is a semi-discrete design solution that otherwise would require an ASIC or manually matched BJTs — an unattractive alternative.

Transistor arrays are a possible design familiarization path to full integration. In the 1980s, there was a brief flurry of arrays available for discrete prototyping of what were intended to become semi-custom integrated circuits. Such arrays are useful building blocks, and they prepare the engineering mind for the notion of putting more and more on the array, eventually resulting in a monolithic circuit.

What is least integrated about the amplifier are the switches. (The instrument settings or configuration problem is addressed in the instrument-on-a-chip series; see below.) Configuration switching requires additional creative design to integrate, but with high-performance analog switches, both of the slow, high-current relay type and fast, low-voltage type, they reduce or eliminate the need for electromechanical relays.

What were your experiences with hybrid devices — both designing them and using them?

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11 comments on “Between Discrete & Integrated Circuits

  1. eafpres
    July 10, 2013

    @Dennis–are hybrid circuits on the ceramic “boards” still made and used commercially today?  The ones in your figure look very expensive.  Are the pins on the edges custom parts or a catalog item?

  2. Davidled
    July 10, 2013

    I think that this type board may be commonly in military and aerospace industry related to high frequency signal processing.  I am wondering how these PIN are mounted in the other board. It looks like these Pins are easily broken, when packaging it. DIP packages pin style instead of all vertical pins may be more stable in the package viewpoint.

  3. Brad Albing
    July 10, 2013

    @DaeJ – I think that there is metal plating bonded to the ceramic (not sure how exactly, but by some means that is fairly rigid). Then the pins are soldered with a low temperature solder.

  4. Dirceu
    July 11, 2013

        The “4” in a standard FR4 printed circuit board relates to the relative dielectric coefficient. The signals travel at the 0.5 of speed of light on a FR4 based PCB (1/sqrt (4)). I do not know the dielectric coefficient for the ceramic substarte, but I believe the main reason for use in military and space applications are its thermal properties.

  5. D Feucht
    July 11, 2013

    @ eafpres – I have not encountered any of these kinds of hybrid circuits since my Tek days, though my guess is that they are still used in environmentally-demanding applications.

    The edge pins were made by a connector company and, as Brad described, are attached to thin-film metal on the ceramic substrate, to make the electrical connection. We used them in oscilloscopes – usually vertical amplifiers – and had no problem with mechanical reliaility.

  6. RedDerek
    July 16, 2013

    Ceramic hybrids are still used today to keep old circuitry still functioning. I am working with many boards that contain hybrids that date back to the 1970's. Quite a challenge to replace concidering that many parts, not counting resistors and capacitors, are not available. My task then breaks down to how to replace the ceramic hybrid as a black-box such that it is a drop-in; yet still look the same. I will have to blog more on this later as to the techniques used and challenges presented.

  7. Navelpluis
    July 19, 2013

    Dennis, thanks for sharing this with us. Always like to read your stuff, I even have your series of 4 books: For the pure analog guys here loving transistors it is a must-have.

    Back on topic: As a HAM radio guy I see a lot of ceramic substrates. A lot of RF amplifier modules are based on this. In the earlier years this often was beryllium Oxide, be careful, dangerous stuff. We tried to modify amplifier modules to get them from 470MHz back to our HAM radio frequencies we use, say, 435MHz. This often was a tough job to do, and 7 out of 10 impossible due to the way how coils and capacitors are (were) integrated. I learned a lot from this: Often these modules are marbles of engineering design. Why we did this? There is a museum down here dealing about HAM history. It is called 'Museum Jan Corver' and they sometimes receive badges of material from local PTT or distributors. 1500 radios for free is not a bad thing to get. So we hack them to HAM frequencies and we rewrite software, and here we go. Have a look at http://www.jancorver.org/ombouw/nokia/mobira.htm where you can see that we even build PIN switch modules with stripline filters. That was a fun project.

    As a profession I mainly do hardware design. The stuff today is sooo d*m**d small that I always have the feel that I indeed design hybrids all the way 😉 Even with FR4 material.

    One of the reasons why RF and fast telecomms people go away from FR4 is -indeed- the 'hook' spec, but also another: Signal dispersion. The glass fiber is not a constant dielectricum really making our lives miserable above -say- 3GHz. But I have led a group designing a board of 12×15 inch with 3 fat FPGA's on it, talking with each other on 3,15Gbit/Sec with FR4. It can be done, but hey, this is not a 'laid back brass band' project.

    I see less and less hardware design activities around me. People like me seem to fade away to other activities or just stop working due to pension age. I find this a worrying development: Not all electronics problems can be solved with 'available modules'.

    Therefore I often do 'strange' discrete circuitry with pure transistor (pairs), like back in the 70's. Much of it is sensor interface and it cannot be done otherwise. the 'hybrid' often is a small 0,8mm thin FR4 board with these circuits on it. It then really looks like a hybrid indeed…

  8. Brad_Albing
    July 24, 2013

    I never knew that's what that meant. Thanks – good info.

  9. SunitaT
    July 31, 2013

    In addition to circuit dimension and weight reductions over standard PC board packaging, a custom thick-film hybrid circuit simplifies assembly. Enhancements in circuit performances are achieved by shortened circuit paths and closer spacing that yield reduced noise pickup, improved thermal coupling, and improved stability.

  10. D Feucht
    September 30, 2013

    Thanks for the Analog Circuit Design book plug (www.scitechpub.com) and especially for your interesting comments about converting surplus radios to something useful to HAMs.

    I too am wondering how electronics will survive in a world devoid of really good analog circuit engineers. Good sequential and combinatorial logic designers just aren't adequate to do high-performance amplifiers.

    And as for using transistor arrays, that has been one of my obsessions of last year. The Intersil HFA3000-series inspires a new generation of designs where matched on-chip BJTs are in the limelight.

  11. nathandavidson
    September 17, 2018

    I am actually going to ask a very silly question – but this is because as much as I know about analog systems, I'm constantly fumbling with my chips and figuring out which way is up. I mean clearly there is a right way and a wrong way of going things, but can somebody, for the love of God, try to invent something that's usable on both ends without having to worry about which way it to proper orientation?

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