Biasing That Transistor: The Emitter Follower

We were musing upon the relative paucity of education with respect to the fundamentals of electronic circuitry with discrete semiconductors, so we thought we’d do something about it. So far we’ve taken a look at the basics of transistor biasing through the common emitter amplifier, then introduced a less common configuration, the common base amplifier. There is a third transistor amplifier configuration, as you might expect for a device that has three terminals: the so-called Common Collector amplifier. You might also know this configuration as the Emitter Follower. It’s called a “follower” because it tracks the input voltage, offering increased current capability and significantly lower output impedance.

The emitter follower circuit
The emitter follower circuit

Just as the common emitter amplifier and common base amplifier each tied those respective transistor terminals to a fixed potential and used the other two terminals as amplifier input and output, so does the common collector circuit. The base forms the input and its bias circuit is identical to that of the common emitter amplifier, but the rest of the circuit differs in that the collector is tied to the positive rail, the emitter forms the output, and there is a load resistor to ground in the emitter circuit.

As with both of the other configurations, the bias is set such that the transistor is turned on and passing a constant current that keeps it in its region of an almost linear relationship between small base current changes and larger collector current changes. With variation of the incoming signal and thus the  base current there is a corresponding change in the collector current dictated by the transistor’s gain, and thus an output voltage is generated across the emitter resistor. Unlike the common emitter amplifier this voltage increases or decreases in step with the input voltage, so the emitter follower is not an inverting amplifier.

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Automatic I2C Address Allocation For Daisy-Chained Sensors

Many readers will be familiar with interfacing I2C peripherals. A serial line joins a string of individual I2C devices, and each of the devices has its own address on that line. In most cases when connecting a single device or multiple different ones there is no problem in ensuring that they have different addresses.

What happens though when multiple identical devices share an I2C bus? This was the problem facing [Sam Evans] at Mindtribe, and his solution is both elegant and simple. The temperature sensors he was using across multiple identical boards have three pins upon which can be set a binary address, and his challenge was to differentiate between them without the manufacturing overhead of a set of DIP switches, jumpers, or individual pull-up resistors. Through a clever combination of sense lines between the boards he was able to create a system in which the address would be set depending upon whether the board had a neighbour on one side, the other, or both. A particularly clever hack allows two side-by-side boards that have two neighbours to alternate their least significant bit, allowing four identical boards each with two sensors to be daisy-chained for a total of eight sensors with automatic address allocation.

We aren’t told what the product was in this case, however it’s irrelevant. This is a hardware hack in its purest sense, one of those which readers will take note of and remember when it is their turn to deal with a well-populated I2C bus. Of course, if this method doesn’t appeal, you can always try an LTC4316.

A Parallel Port Synthesiser For Your DOS PC

It is a great shame that back in the days when a typical home computer had easy low-level hardware access that is absent from today’s machines, the cost of taking advantage of it was so high. Professional PCBs were way out of reach of a home constructor, and many of the integrated circuits that might have been used were expensive and difficult to source in small quantities.

Here in the 21st century we have both cheap PCBs and easy access to a wealth of semiconductors, so enthusiasts for older hardware can set to work on projects that would have been impossible back in the day. Such an offering is [Serdef]’s Tiny Parallel Port General MIDI Synthesizer for DOS PCs, a very professionally produced synth that you might have paid a lot of money to own three decades ago.

At its heart is a SAM2695 synthesiser chip, and the board uses the parallel port as an 8-bit I/O port. The software side is handled by a TSR (a Terminate and Stay Resident driver loaded at startup, for those of you who are not DOS aficionados), and there are demonstrations of it running with a few classic games.

If the chip used here interests you, you might like to look at a similar project for an Arduino. The Kickstarter we covered is now long over, but you can also find it on GitHub.

Review: FG-100 DDS Function Generator

I don’t have a signal generator, or more specifically I don’t have a low frequency signal generator or a function generator. Recently this fact collided with my innocent pleasure in buying cheap stuff of sometimes questionable quality. A quick search of your favourite e-commerce site and vendor of voice-controlled internet appliances turned up an FG-100 low frequency 1Hz to 500kHz DDS function generator for only £15 ($21), what was not to like? I was sold, so placed my order and eagerly awaited the instrument’s arrival.

The missing function generator is a gap in the array of electronic test instruments on my bench, and it’s one that maybe isn’t as common a device as it once might have been. My RF needs are served by a venerable Advance signal generator from the 1960s, a lucky find years ago in the back room of Stewart of Reading, but at the bottom end of the spectrum my capabilities are meagre. So why do I need another bench tool?

It’s worth explaining what these devices are, and what their capabilities should be. In simple terms they create a variety of waveforms at a frequency and amplitude defined by their user. In general something described as a signal generator will only produce one waveform such as a sine or a square wave, while a function generator will produce a variety such as sine, square, and sawtooth waves. More accomplished function generators will also allow the production of arbitrary waveforms defined by the user. It is important that these instruments have some level of calibration both in terms of their frequency and the amplitude of their output. It is normal for the output to range from a small fraction of a volt to several volts. How would the FG-100 meet these requirements? Onward to my review of this curiously inexpensive offering.

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Retrotechtacular: The Saturn Propulsion System

“We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard; because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one we intend to win, and the others, too”

When President Kennedy gave his famous speech in September 1962, the art of creating liquid-fueled rocket engines of any significant size was still in its relative infancy. All the rocketry and power plants of the Saturn series of rockets that would power the astronauts to the Moon were breaking entirely new ground, and such an ambitious target required significant plans to be laid. What is easy to forget from a platform of five decades of elapsed time is the scale of the task set for the NASA engineers of the early 1960s.

The video below the break is from 1962, concurrent with Kennedy’s speech, and it sets out the proposed development of the succession of rocket motors that would power the various parts of the Saturn family. We arrive at the famous F-1 engine that would carry the mighty Saturn 5 and start its passengers on their trip to the Moon at a very early stage in its development, after an introduction to liquid rocket engines from the most basic of first principles. We see rockets undergoing testing on the stand at NASA’s Huntsville, Alabama facility, along with rather superlative descriptions of their power and capabilities.

The whole production is very much in the spirit of the times, though unexpectedly it makes no mention whatsoever of the Space Race with the Soviet Union, whose own rocket program had put the first satellite and the first man into space, and which was also secretly aiming for the moon. It’s somewhat jarring to understand that the people in this video had little idea that such an ambitious program would be as successful as it became, or even that in the wake of Kennedy’s assassination the following year there would be such an effort to fulfill the aim set out in his speech to reach the moon within the decade.

The moon landings, and the events and technology that made them possible, are a subject of considerable fascination for our community. We must have covered innumerable stories about artifacts from the Apollo era in these pages, and no doubt more will continue to come our way in the future. Films like this one do not tell us quite the same story as does a real artifact, but their values lies in capturing the optimism of the time. Anything seemed possible in 1962, and those who lived through the decade were lucky enough to see this proven.

Fifty years from now, what burgeoning engineering efforts will we look back on?

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HFT On HF, You Can’t Beat It For Latency

If you are a radio enthusiast of A Certain Age, you may well go misty-eyed from time to time with memories of shortwave listening in decades past. Countries across the world operated their own propaganda radio stations, and you could hear Radio Moscow’s take on world events, the BBC World Service responding, and Radio Tirana proudly announcing that every Albanian village now had a telephone. Many of those shortwave broadcast stations are now long gone, but if you imagine the HF spectrum is dead, think again. An unexpected find in an industrial park near Chicago led to an interesting look at the world of high-frequency trading, or HFT, and how they have moved to using shortwave links when everyone else has abandoned them, because of the unparalleled low latency they offer when communicating across the world.

Our intrepid tower-hunter is [KE9YQ], who was out cycling and noticed a particularly unusual structure adorned with a set of HF beams. These are the large directional antennas of the type you might otherwise expect to see on the roof of an embassy or in the backyard of a well-heeled radio amateur, and were particularly unusual in this otherwise unexciting part of America. There followed an interesting process of tracking down the site’s owners via the FCC permits for its operation, leading to the deduction of its purpose. With other antenna-hunters on the lookout for corresponding sites elsewhere in the world, it seems that this unusual global network hiding in plain sight could soon be revealed.

Unsurprisingly we’ve not covered many shortwave HFT stories. There are however other higher-latency ways to cross the world on HF.

Via SWLing Post, and thanks [W6MOQ] for the tip.

Flashing An LED The Widlar Way

Regular Hackaday readers will be familiar with the work of Boldport’s [Saar Drimer] in creating beauty in printed circuit board design. A recent work of his is the Widlar, a tribute to the legendary integrated circuit designer [Bob Widlar] in the form of a development board for his μA723 voltage regulator chip.

The μA723 is a kit of parts from which almost any regulator configuration can be made, but for [tardate]  it represented a challenge. The μA723 is so versatile that what you can create is only limited by the imagination of the builder. Having done the ordinary before, [tardate] looked toward something unconventional.

The result is modest, a simple LED flasher using the error amplifier as a not-very-good op-amp, building an oscillator at a frequency of about 2 Hz. This is pretty neat and if you are used to the NE555 as the universal integrated circuit, perhaps it’s time to set it aside for the obviously far-more-useful μA723.

Here at Hackaday we harbour at least one fan of the μA723, not to mention also of artful PCBs. If the Widlar looks familiar, we featured the switch mode regulator from the μA723 data sheet on it a few months ago.

Disclosure: [Jenny List] wrote the documentation for Boldport’s Widlar kit.