## Extra Class question of the day: Toroids

Toroidal inductors are very popular these days. A primary advantage of using a toroidal core instead of a solenoidal core in an inductor is that toroidal cores confine most of the magnetic field within the core material. (E6D10)

Another reason for their popularity is the frequency range over which you can use them. The usable frequency range of inductors that use toroidal cores, assuming a correct selection of core material for the frequency being used is from less than 20 Hz to approximately 300 MHz. (E6D07) Ferrite beads are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of transistorized HF amplifiers. (E6D09)

An important characteristic of a toroid core is its permeability. Permeability is the core material property that determines the inductance of a toroidal inductor. (E6D06)

One important reason for using powdered-iron toroids rather than ferrite toroids in an inductor is that powdered-iron toroids generally maintain their characteristics at higher currents. (E6D08) One reason for using ferrite toroids rather than powdered-iron toroids in an inductor is that ferrite toroids generally require fewer turns to produce a given inductance value. (E6D16)

To calculate the inductance of a ferrite-core toroid, we need the inductance index of the core material. The formula that we use to calculate the inductance of a ferrite-core toroid inductor is:

L = AL×N2/1,000,000

where L = inductance in microhenries, AL = inductance index in µH per 1000 turns, and N = number of turns

We can solve for N to get the following formula:

N = 1000 x sqrt (L/AL)

Using that equation, we see that 43 turns will be required to produce a 1-mH inductor using a ferrite toroidal core that has an inductance index (A L) value of 523 millihenrys/1000 turns. (E6D11)

N = 1000 x sqrt (1/523) = 1000 x .0437 = 43.7 turns

The formula for calculating the inductance of a powdered-iron core toroid inductor is:

L = AL×N2/10,000

where L = inductance in microhenries, AL = inductance index in µH per 1000 turns, and N = number of turns

We can solve for N to get the following formula:

N = 100 x sqrt (L/AL)

Using that equation, we calculate that 35 turns turns will be required to produce a 5-microhenry inductor using a powdered-iron toroidal core that has an inductance index (A L) value of 40 microhenrys/100 turns. (E6D12)

N = 1000 x sqrt (5/40) = 100 x .353 = 35.3 turns

## From the trade magazines – September 14, 2012

Nikola Tesla slideshow: Images and articles from Tesla’s writings. This slideshow contains a sampling of images and excerpts from John Ratzlaff’s collection, which are all obtainable on the Internet. There are also several links to other awesome sites with info on Tesla.

Giga Sample and Direct-RF Sampling ADCs Overview. This video from TI demonstrates TI’s direct RF-sampling ADC IMD3 performance at 700 MHz and 2.7 GHz, allowing the elimination of one or more down-conversion stages. Features: sample rates up to 3.6 GSPS, dynamic performance up to 2.7 GHz inputs, largest high-res Nyquist zone at 1.8 GHz, and pin compatibility with TI’s 10- & 12-bit ADC families.

The basics of FPGA mathematics. For better performance, you can implement some digital signal processing (DSP) algorithms in an FPGA. This article takes a look at the rules and techniques that you can use to develop mathematical functions within an FPGA or other programmable device.

## Extra Class question of the day: Optical devices

Cathode-ray tubes (CRTs) used to the be most common type of display. They were not only used in television sets, but also computer terminals. They have an electron gun which shoots electrons onto a screen which then glows where the electron hits the screen. By sweeping this “beam” both horizontally and vertically, you can display an image on the screen.

To sweep the beam across the CRT, you deflect it by passing it though a set of plates. Varying the voltage will change the angle at which the beam is deflected. Electrostatic deflection is the type of CRT deflection that is better when high-frequency waveforms are to be displayed on the screen. (E6D13)

To accelerate the electron towards the screen, you apply a relatively high anode voltage to it. The higher the voltage, the brighter the CRT will glow. You don’t want to make that voltage too high, however. Exceeding the anode voltage specification can cause a cathode ray tube (CRT) to generate X-rays. (E6D02)

A spot on the CRT screen will glow even after the beam moves onto another spot. Cathode ray tube (CRT) persistence is the length of time the image remains on the screen after the beam is turned off. (E6D01) This characteristic is useful in many different applications.

A more modern type of display is the liquid-crystal display. A liquid-crystal display (LCD) is a display using a crystalline liquid which, in conjunction with polarizing filters, becomes opaque when voltage is applied. (E6D05) The principle advantage of liquid-crystal display (LCD) devices over other types of display devices is that they consume less power. (E6D15)

Unlike the CRT or LCD display, which transform electrical signals into an image, a charge-coupled device is used to transform an image into electrical signals. A charge-coupled device (CCD) samples an analog signal and passes it in stages from the input to the output. (E6D03) One of the things a charge-coupled device (CCD) does in a modern video camera is that it stores photogenerated charges as signals corresponding to pixels. (E6D04)One thing that is NOT true of a charge-coupled device (CCD) is that it is commonly used as an analog-to-digital converter. (E6D14)

## Extra Class question of the day: Digital integrated circuits

Integrated circuits (ICs) are now an integral part (pun intended) of amateur radio electronics. The two main technologies used to manufacture IC are transistor-transistor logic, or TTL, and complementary metal-oxide semiconductor, or CMOS.

CMOS is arguably the most common type of digital IC. An advantage of CMOS logic devices over TTL devices is that the have lower power consumption. (E6C05) CMOS digital integrated circuits also have high immunity to noise on the input signal or power supply because the input switching threshold is about one-half the power supply voltage. (E6C06)

TTL is the other common digital logic IC technology. 5 volts is the recommended power supply voltage for TTL series integrated circuits. (E6C01) The inputs of a TTL device assume a logic-high state if they are left open. (E6C02)

BiCMOS logic is an integrated circuit logic family using both bipolar and CMOS transistors. (E6C12) An advantage of BiCMOS logic is that it has the high input impedance of CMOS and the low output impedance of bipolar transistors. (E6C13)

Tri-state logic devices are logic devices with 0, 1, and high impedance output states. (E6C03) These devices can be made with either TTL or CMOS technology. The primary advantage of tri-state logic is the ability to connect many device outputs to a common bus. (EC604) When a device’s outputs are in the high-impedance state, they act as if they are disconnected.

When working with digital ICs, it is important to recognize the various symbols for the different types of logic gates. In Figure E6-5, 1 is the schematic symbol for an AND gate. (E6C07) In Figure E6-5, 2 is the schematic symbol for a NAND gate. (E6C08) In Figure E6-5, 3 is the schematic symbol for an OR gate. (E6C09) In Figure E6-5, 4 is the schematic symbol for a NOR gate. (E6C10) In Figure E6-5, 5 is the schematic symbol for the NOT operation (inverter). (E6C11)

## Extra Class question of the day: Semiconductor diodes

Diodes are the simplest semiconductor devices. In their simplest form, they have two terminals and conduct current in only one direction, from the cathode to the anode. By manipulating the characteristics of the semiconductor material, manufacturers can make diodes useful in a wide variety of applications.

Take, for example, the Zener diode. The most useful characteristic of a Zener diode is a constant voltage drop under conditions of varying current. (E6B01) This makes it useful in voltage regulator circuits.

Another example is the varactor diode. The varactor diode is a semiconductor device designed for use as a voltage-controlled capacitor. (E6B04) Varactor diodes are often used in tuning circuits.

A PIN diode is a semiconductor device that operates as a variable resistor at RF and microwave frequencies. One common use for PIN diodes is as an RF switch. (E6B12)The characteristic of a PIN diode that makes it useful as an RF switch or attenuator is a large region of intrinsic material. (E6B05) The forward DC bias current is used to control the attenuation of RF signals by a PIN diode. (E6B11)

Two types of diodes used in RF circuits are the tunnel diode and hot-carrier diode. The tunnel diode is a special type of diode is capable of both amplification and oscillation. (E6B03) Tunnel diodes are capable of operating well into the microwave region. A hot-carrier diode is commonly used as a VHF / UHF mixer or detector. (E6B06)

Metal-semiconductor junction is a term that describes a type of semiconductor diode. (E6B08) A Schottky diode is an example of a metal-semiconductor diode. An important characteristic of a Schottky diode as compared to an ordinary silicon diode when used as a power supply rectifier is that it has less forward voltage drop.

In Figure E6-3 (below), 5 is the schematic symbol for a light-emitting diode. (E6B10) Forward bias is required for an LED to emit light. (E6B13)

No matter what kind of diode you are using, it’s very important to not exceed the forward current specification. Doing so, will cause it to fail. Excessive junction temperature is the failure mechanism when a junction diode fails due to excessive current. (E6B07)

## Extra Class question of the day: Optical components and power systems: photoconductive principles and effects, photovoltaic systems, optical couplers, optical sensors, and optoisolators

The photovoltaic effect is the conversion of light to electrical energy. (E6F04) In a device called a photovoltaic cell, electrons absorb the energy from light falling on a photovoltaic cell. (E6F12) The electrons then become free electrons.

The most common type of photovoltaic cell used for electrical power generation is silicon. (E6F10) The approximate open-circuit voltage produced by a fully-illuminated silicon photovoltaic cell is 0.5 V. (E6F11) The efficiency of a photovoltaic cell is the relative fraction of light that is converted to current. (E6F09)

Photoconductivity is a similar phenomenon. Photoconductivity is the increased conductivity of an illuminated semiconductor. (E6F01) The conductivity of a photoconductive material increases when light shines on it. (E6F02) A crystalline semiconductor is the material that is affected the most by photoconductivity. (E6F06)

A device that uses the phenomenon of photoconductivity is the optoisolator. The most common configuration of an optoisolator or optocoupler is an LED and a phototransistor. (E6F03) Optoisolators are often used in conjunction with solid state circuits when switching 120 VAC because  optoisolators provide a very high degree of electrical isolation between a control circuit and the circuit being switched. (E6F08)

A similar device is the solid-state relay. A solid state relay is a device that uses semiconductor devices to implement the functions of an electromechanical relay. (E6F07)

Optical shaft encoders are another device that rely on photoconductivity. An optical shaft encoder is a device which detects rotation of a control by interrupting a light source with a patterned wheel. (E6F05) Optical shaft encoders are used to detect when an operator turns a knob on an amateur radio transceiver.

## Extra Class Question of the Day: Bipolar junction transistor characteristics

Perhaps the most popular type of transistor is the bipolar junction transistor (BJT). Bipolar junction transistors are three-terminal devices, called the emitter, base, and collector.  In an NPN transistor, the emitter and collector are N-type material and the base is P-type material. In a PNP transistor, the emitter and collector are P-type, while the base is N-type. The base is sandwiched between the base and emitter, so there is a diode junction between the base and the collector and the base and emitter.

Refer to Figure E6-1 above. In Figure E6-1, the schematic symbol for a PNP transistor is #1. (E6A07) #2 is the schematic symbol for an NPN transistor. The arrow in both symbols shows the direction of the current flow.

When the base-emitter diode is forward-biased, a current, called the base current will flow. If there is an appropriate voltage between the collector and emitter, this small base current will cause a much larger current to flow between the collector, through the base to the emitter. The amount of base current controls how much collector current flows. This is how transistors amplify signals.

The change in collector current with respect to base current is the beta of a bipolar junction transistor. (E6A06) This is also sometimes called the hfe or current gain of a transistor. The change of collector current with respect to emitter current is the alpha of a bipolar junction transistor. (E6A05)

Another important characteristic of a bipolar transistor is the alpha cutoff frequency. This is a measure of how high in frequency a transistor will operate. Alpha cutoff frequency is the frequency at which the grounded-base current gain of a transistor has decreased to 0.7 of the gain obtainable at 1 kHz. (E6A08)

Resources

What resources have you used for learning how transistors work?

## Updated Audio Guide describes TI and National parts

From my inbox this morning….

The Audio Guide makes it easy to explore TI’s IC solutions for audio applications. In the guide, each audio signal-chain function is highlighted with corresponding device solutions that offer increased application flexibility, higher performance and design longevity. You’ll also find information about new tools as well as selection tables to refine your decisions.

The 2012 Audio Guide features:

• Device solutions for each audio signal-chain function from the combined TI and National portfolio
• Updated selection tables
• More hardware/software tools
• More end-equipment system block diagrams

## From my Twitter feed – 5/1/12

RT @mental_floss: Why is “mayday!” an international distress signal? It comes from the French “venez m’aider,” meaning “come help me!”

Finally got “Unleashing the LM386″ on my blog – amazing old audio amp chip http://t.co/Fi38DV97 #hamr #hamradio

Measuring Battery Capacity w/ an Arduino … very cool! http://t.co/HcBys27g #hamr

## From trade magazines: GE Transistor Manual, analog circuit design, HF op amp filters

This time, I have two items from EE Times and one from MicroWaves&RF…..Dan

Master the first 170 pages of the venerable GE Transistor Manual and you'll be a transistor expert.

The GE Transistor Manual. This editorial by Jack Ganssle reminisces about the old GE Transistor Manual. He notes, “It explains transistor theory in a level of detail that my college classes almost a decade later never approached. Read – and understand – the first 170 pages and you’ll be a transistor expert. But no attempt is made to make the subject easy.” One of the comments contains a link that you can use to download your own copy.

Book excerpt: Analog Circuit Design— A Tutorial Guide to Applications and Solutions, Part 1. Based on the Application Notes of Linear Technology, this book covers the fundamentals of linear/analog circuit and system design to guide engineers with their design challenges. It includes a broad range of topics, including power-management tutorials, switching-regulator design, linear-regulator design, data conversion, signal conditioning, and high-frequency/RF design. VERY good stuff.

Fabricating HF Opamp Filters. Until recently, op amp filters have generally been restricted to circuits operating below 1 MHz. Recent advances, though, are enabling op amps to amplify at frequencies up to 1 GHz.This article explains how to use them for lowpass filters to 150 MHz.