Transconductance, Anyone?
     

My, You Look FETching
Question posed in the Aug. 19 issue
(Exam level: CBRE)


About FET transistors, which of the following is most accurate?

a. The drain is the normal input element akin to a grid in a tube

b. The gate is the normal input element akin to a grid in a tube

c. The source is the normal input element akin to a grid in a tube

d. The screen is the normal input element akin to a grid in a tube

e. None of the above because an FET is a transistor and so a current amplifier unique to itself.


SBE certification is the badge of professionalism in broadcast engineering. To help you get in the exam frame of mind, Radio World Engineering Extra poses typical questions in this series of articles. Although similar in style and content to exam questions, these are not from past tests nor will they be on future exams in this exact form.

Development of anything in new and uncharted areas usually takes an unusual path. Most often we do not know where we are going, where our work, our experiments will take us — the unexplored country.

Transistors were no exception. The FE of our question (the Field Effect principle) was recognized by Austrian-Hungarian physicist Julius Edgar Lilienfeld, and a generalized patent was filed in Canada on Oct. 22, 1925. Oddly, Lilienfeld didn't follow up with any research articles about his work. In 1934, a derivative device was patented by German physicist Dr. Oskar Heil.

AT&T's Bell Labs history indicates that John Bardeen and Walter Brattain actually set out at first to determine if they could use the very high-impedance input of these FET devices to create an amplifier with a voltage to current transfer function exhibiting transconductance similar to tube amplifiers. While working to this end, they experimentally observed that when electrical current was introduced into a junction contact attached to a crystal of germanium, the output power was larger than the input. This event was the birth of the bipolar junction transistor, BJT. This current amplifier arrangement satisfied their need and the BJT generation was born, placing the FET on the back burner.

Two fellows named Kahng and Atalla built the first MOS (metallic oxide semiconductor) transistor at Bell Labs in 1960.

The earliest workable FET application that appeared was a MOSFET (metal oxide semiconductor field effect transistor) but their cause célèbre, the best application of this technology, was an integrated circuit where gas deposition and other fabrication concepts allowed CMOS (complementary metallic oxide semiconductor) techniques to be easily implemented.

Fig. 1: Electronic symbols for the junction field effect transistor. G stands for gate, the normal input terminal, D stands for drain and S stands for source.
The high impedance of CMOS means little power is needed from the source, minimizing loading of the previous stage. High gain and relative high output power are achievable and make FETs ideal for certain uses. This small power demand of the input and the small quiescent power consumption of the device are reasons FETs usually are more power efficient than other transistor types. Power efficiency is an important concern in IC development where an LSIC or VLSIC can have over 1 million individual active transistors (as in a Pentium microprocessor) and inefficiency is translated directly into destructive heat.

At the other end of the complexity spectrum, one simple application for a FET is the gain and impedance conversion needed in condenser microphones. A small change in reactance is created when acoustic pressure waves deflect a capacitive element surface (e.g., when your air talent speaks into the condenser microphone) which is differentiated into a tiny voltage change with negligible energy. A FET stage amplifies this tiny signal and produces enough voltage gain to feed a microphone cable with a standard level. When you place "phantom power" on your console microphone channel this little circuit is what you are powering.

Fig. 1 shows two common FET configurations in schematic representation and similarly, Fig. 2 is a triode tube that in function is similar to the FET. The gate then is functionally similar to the grid of the tube, so answer (b) in the question on page 6 is correct.

Fig. 2: A triode, three-element tube showing the input grid connection to the left. The grid in a tube is analogous to the gate on an FET.
A screen is an additional control element in a tube, such as a 4CX1000A or 5CX1500B, both of which are used extensively in FM transmitters (Fig. 3). This "red herring" choice, (d), has nothing to do with the construction of an FET, and so is incorrect.

As noted above the BJT and FET are both transistors but only the BJT is normally considered a current to current amplifier.

HOW DOES IT WORK?

In ultra simple terms, a voltage applied to the gate (relative to the source), alters the current flow from source to drain. In the most ordinary "N" channel type FET, a negative voltage pinches off the current flow between source and drain, the depletion mode. Conversely a positive voltage increases the current flow between source and drain, the enhancement mode.

Fig. 3: A tetrode, four-element tube showing the additional 'screen' control input between the grid and plate.
Since the first FET came to market, at least a dozen different special-use FETs are in current manufacture for applications as varied as microwave receive pre-amps and high-speed, high-current switching.

In 2006, more than 1.7 billion MOSFETs were vended (mainly in LSI and VLSI ASICs), nearly 60 million for every person on the planet.

Where would we be without them?

By happy coincidence, Mario Orazio, the Masked Engineer in our sister publication TV Technology, explores the universe of MOSFETs more fully in the Oct. 21 issue of that publication, for those who would like some more background. Read the article online.

Note: The deadline for signing up for the next cycle of SBE certification exams is Dec. 31, 2009 for exams given in local chapters between Feb. 5 and Feb. 15, 2010.

The question for next time is shown in the box below.

Charles "Buc" Fitch, P.E., CPBE, AMD, is a frequent contributor to Radio World. Missed some SBE Certification Corners or want to review them for your next exam? See the "Certification" tab under Columns at radioworld.com.


Protect Me, CB!
Question for next time
(Exam level: CBT)


When is a fuse more desirable than a circuit breaker as an overcurrent protection device?

a. A fuse is never more desirable in a circuit

b.A fuse is always more desirable in a circuit

c. A fuse is more desirable when the current is over 100 amps for economic reasons

d. A fuse is always more desirable because it means you get a service call

e. A fuse is more desirable when instantaneous interruption is needed

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