Ask the Experts Q&A: PCBA & Transistors - KingSheng PCBA

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Q1: When should a design engineer care to account for secondary breakdown?
HS Answer1: Breakdown voltage in a bipolar transistor can be rather complicated. There are several factors that determine the maximum voltage across a given junction. Generally, these include avalanche multiplication, thermal instability, and tunneling. However, as a circuit designer and not a transistor designer, these should not be your direct concern.

Q2: How does a design engineer pick the right value of Hfe for his (paper-n-pencil) design calculation?
HS Answer2: Hfe (or beta) is a characteristic of a given transistor. The only way you can choose Hfe is to select the right transistor -- you cannot change the Hfe. The specifications for a given transistor are found on their individual data sheets. First, you will need to determine what characteristics you need (Hfe, Max Ic, Max Vce, Power, etc.). Consider that, in most cases, Hfe is a function of Ic and decreases with increasing Ic. Once you know the requirements of your transistor, you can start the hunt for the best available device. Again, the "right" Hfe is found in the data sheets, but you must be careful to use the Hfe that corresponds to your operating point.

Q3: Hfe of a transistor used in power applications are typically low. Why?
HS Answer3: The answer is process dependent (what technology is used to make the transistor). Generally, power transistors are required to have higher breakdown voltage than small signal devices. When fabricating devices to operate at higher voltages, it is necessary to make the base-width larger. Base-width significantly affects Hfe (beta).

Q4: I'm from the PCB world, but for the last 8 years I've been at an IC design company. Any suggestions on what I need to know to better interact with the chip designers? Besides going back to school and getting a PhD in chip design -- with a family to support, that's not going to happen.
Answer4: I suggest that going back to school that has a very strong program in the level of chip design, or has a thesis program that allows student to experience chip design.
ST Answer: As a first step, try to use some simulation programs for chip designers.

Q 4-1: OK, I've tried simulation, but it doesn't agree with me! I identify more with the late Bob Peases and Jim Williamses of the world. Design it, build it, test it, learn from it, that's more my style. Sigh, I guess there will always be an impedance mismatch…
ST Answer 4-1: I hear you Jim. It's certainly tough to enter a new realm in engineering, but try to find a mentor in the IC design group that can lead you into their world -- you might want to check out the EDN "IC Design" site. I have posted some really good tutorial articles by IC designers there.
RD Answer4-2: On simulation, linear technology offers LT spice. It's a free schematic capture simulation and waveform viewer. As an ic engineer, there is no breadboards for the ic these days. So we make simulation work. With good models and understanding the limitations of the simulator we get very good agreement between simulators and silicon. If you are trying to learn IC design, look at the past 30 years of Journal of solid-state circuits. That's one place where schematics are published for ICs.
jimfordbroadcomA4-3: Thanks. I've tried LTSPICE, and the price certainly is right, and it's quite easy to use. Probably will have to use it again soon. Also, I've done some work with Agilent ADS. Tough sledding for a lab guy like me. Thanks for the reference to JSSC. I"m sure I can look up whatever I need on IEEEXplore. @steve.taranovich -- thanks, I'll definitely look up the EDN site.

Q5: Do you think devices like the 2n3904 and the 2n2222 will stick around for a long time to come?
HS Answer5: I have no way of knowing about the future of the 2222 and 3904. However, given their popularity I expect they will last for the foreseeable future.

Q6: I'm interested in the thoughts from both experts about the potential for analog front ends with the booming sensors market. More of a market interest; I follow this area as part of my consultancy. At the high level there is all the buzz about Internet of Things, Smart Grid, Home Automation, etc. But practically all of that starts with (mainly analog) sensors that are monitored and eventually communicated. So I see a nearly 1:1 synergy of the big topics and demand for more integrated packages -- sensor, AFE, even the radio part. I see lots of radios like BT or Zigbee or generic 802.15 as SiP or SoC, but not so much fully integrated solutions. In the companies that are traditional analog electronic providers, AFEs have been appearing frequently as new products in the last few years.
RD Answer6: BobDobkin here, the analog front ends for sensors is and will continue to be a big business. Also micromachined parts have the analog front and built in. As I see technology advance is an analog these front ends will get easier to accomplish with better precision.

Q7: Do you see backward/forward integration by companies like yours providing ready-to-go wireless sensors for sensor networks? Or will you stay more in the building blocks?
RD Answer7: Linear will probably stay with building blocks for wireless networking. The cost of developing the wireless circuit means that it needs to be pretty general. So we will end up making several chips that work well together to make a front end.

Q8: What are the biggest design cornerstones that you would give to a design engineer? What are the biggest lessons you've learned, career-wise or technical?
RD Answer8: I think one of the biggest cornerstones is the ability to understand analog circuits on an intuitive basis. After you have been looking it analog circuits long enough, you learn the language and understand how all the pieces work. At that time you can understand what a circuit is doing without equations and by inspection.

Q9: I'm going to change the subject a bit here if I may and bring up the subject of programmable analog arrays. Seems like every few years somebody tries to make a go of it and falls on their face. My own take on this subject is that the value of analog IC's is so tied to the process that making generic building blocks hampers their performance so much that the economics just don't work. Anybody else want to weigh in?
RD Answer9: I have never seen an analog array that worked enough to become popular. There are always too many compromises in the performance. They keep popping up over time and announcing their going to put linear Technology out of business. It hasn't happened yet.
Simple functions can be done. But the processes that are optimized for FPGA are not really good for most linear circuits. Also fine line processes for high density digital is too expensive when you need a large area linear transistor. It's a force fit this does not work well It is very hard to get inductance in an IC. And sometimes it shows up when you don't want it. We have had circuits that oscillated at 3 GHz because of inductance on the die.
One final comment I wanted to put out there about analog arrays -- probably not a good idea to try to make a business out of it. I mean, if a company like Touchstone Semi. with some great products (not analog arrays, mind you) and people couldn't get their stuff going, who will these days?

Q10: Is it possible to implement the functionality of a chip bead ferrite in an IC?
RD Answer10: Ferrite beads are magic! When it comes to stopping parasitic oscillation they are great. There is no equivalent within an IC but you do have other answers. Within ICU have control of the circuit components and the parasitics and the layout. You can find the oscillations in simulation and correct them.

Q10-1: Interesting challenge -- ferrite bead on a chip! As a matter of fact, my colleagues here at Broadcom are quite experts at the Bluetooth radios, and the sales and mktg people have sold billions of them!”
ST Answer10-1: They are magic -- just a piece of bus wire, slide the ferrite bead onto it and solder in place and noise and oscillations magically disappear.
RD Answer10-1: They are at the equivalent of a lossy inductor at high-frequency. But at DC, you only have the resistance of the wire.
Eafpres1 Answer10-1: It could be paired with an SMT wirewound inductor that just happens to act like an antenna at 2.5 GHz. Saw that done once; trouble is there are no guarantees the parts are on-frequency from part to part or lot to lot…
Jimfordbroadcom Answer10-1: Well, the IC designers do use active inductors on the chip once in a while. I understand active vs. passive is a tradeoff between power consumption vs. die area.
Eafpres1 Answer10-1: Even more magical is a common mode choke on a ferrite core. If you design well, the desired currents return on the other line, and cancel fields within the core as if they are not even there. The only attenuation is for common mode noise.

RD Q10-2: I wonder what the origin of of ferrite beads is. Where they have an accident, or were they deliberately constructed.”
Eafpres1 Answer10-2: This abstract says that Dr. Takeshi Takei of Tokyo Institute of Technology invented and patented iron oxide cores. It says TDK was actually founded to capitalize on this invention.