.. - ' demaw's workbench Doug DeMaw

Diodes in one form or another have existed since the beginning of radio. Without them, we would not have radio as we know it today. The first receivers in the early days of radio were based on the diode action of galena or carborundum crystals, the surfaces of which were used in combination with a catswhisker contactor.

The junction of these two objects formed a rectifier diode that demodulated an amplitude-modulated broadcast signal (AM) and converted it to a pulsating de current that caused an audio response in a pair of earphones. That crude receiver was known as a crystal set or crystal detector.

A

D

G

POINT-CONTACT GERMANIUM

CATH.1 ~ODE SILICON JUNCTION

CATH.

CATH.

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ANODE

ANODE

+12V <ON)

ZENER REGULATOR

220

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do with Diodes It is ironical that we had solid-state devices

before we had vacuum tubes! Vacuum-tube diodes appeared, and they were used as rectifiers and detectors for decades to follow. The solid-state selenium and copper-oxide rectifiers appeared in midstream, but vanished in favor of modern solid-state diodes.

We now have all manner of solid-state diodes that are used as de switches, power­supply rectifiers, voltage regulators (Zener diodes), signal detectors, balanced modulators and voltage-reference devices. Germanium and silicon diodes are the prevalent types today.

ANT DIODE DETECTOR

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AUDIO OUT

SHUNTt- !. ~ITCH I

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RFC

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AC

AC

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+12V (ON)

Diode Inner Structure

Modern diodes are the "point contact" or "junction" types. The former species is structured in a similar manner to the old galena/catswhisker type, because a tiny wire contactor touches a piece of germanium crystal within the glass body of the diode. The familiar 1N34A and 1N60 diodes fit this description.

A germanium diode conducts at a lower "barrier" voltage than does a silicon junction diode. This is typically 0.3 to 0.4 volt. A silicon diode conducts at approximately 0.7 volt. Also, the germanium diode exhibits

HALF-WAVE RECTIFIER

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BIAS SUPPLY

+0.7V +12V

22

H 10 MHz FREQ. MULTIPLIER

+9V REG. 10

IN

20-MHz

--------. E--Q OUT

10 pf

Figure 1 -- Practical examples of circuits that contain solid-state diodes. Each of these circuits is discussed in the text.

92 March 1990 MONITORING TIMES

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less internal capacitance than does its silicon brother.

These germanium traits make the diode ideally suited for weak-signal detection and use from VLF into the microwave frequen­cies. Radar receivers in WW II, for example, used germanium diodes as detectors immedi­ately after the antenna.

Silicon diodes contain a junction or sand­wich of P and N type crystal. The larger the diode the greater the junction capacitance. The small silicon diodes (such as the 1N914) have a capacitance of roughly 3.5 pF. Large rectifier diodes have much greater capa­citance, owing to the increased junction size. This makes them unsuitable for signal detection except at very low frequencies.

All solid-state diodes have an internal resistance. Of major concern is the "forward resistance." This is measured from the anode to the cathode with a standard ohmmeter. Silicon diodes have resistances from, say, 5 to 15 ohms. It depends on the diode type and how it was manufactured. The back resistance (cathode to anode) is usually one to several megohms. Germanium diodes have a much lower back resistance (100,000 ohms or somewhat greater).

The combination of diode capacitance and forward resistance establish a time constant that determines how well or poorly the diode will perform at radio frequencies. The larger the time constant the lower the useful oper­ating frequency. This time constant deter­mines also the effectiveness of small diodes that are used as "high-speed switches." In other words, the greater the diode time constant, the slower the available switching time.

Diode Ratings

Of major concern to us is the PIV (peak inverse voltage) or PRY (peak reverse voltage). You will see both abbreviations used.

PIY or PRY ratings indicate the maximum instantaneous value of reverse voltage (cathode to anode) that can occur across the diode junction without damaging the device. One example of PRY is when rectifier diodes have a large capacitor (filter) connected to the output side of the diodes in a positive power supply, and the primary voltage to the trans­former is turned off. The charge contained in the capacitor is presented to the diode cathodes while there is zero voltage on the anodes.

It is for this reason that rectifier diodes should have a much higher PRY rating than the maximum de voltage in the power supply. I like to use, for example, 1000 PRV diodes in a 300-V power supply.

Diodes also have a maximum safe current rating. We need to pay close attention to

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this factor when replacing defective diodes or designing a circuit. If you build a 12-Y de power supply that is used to supply a circuit that draws two amperes, use diodes that are rated at five amperes or greater. This allows plenty of safety factor. The larger diodes don't cost much more than do the smaller ones. I use 50- or 100-PRY diodes in my 12-y power supplies.

Diode temperature is an important consid­eration also. No diode, during operation, should more than warm to the touch. If the diodes are hot, they should be affixed to a heat sink or replaced with huskier units, assuming there is no circuit fault that is causing excessive current flow.

Diode Applications

Figure 1 shows a number of practical applications for solid-state diodes. These are simplified illustrations to aid your under­standing of how the diodes may be employed.

Circuit B demonstrates the simplicity of a diode detector that may be used to convert an AM radio signal to audio. Circuit C shows how to use a half-wave rectifier to obtain a positive or negative output voltage. The diode and the filter capacitor must be reversed (dashed lines) in order to obtain a negative output.

The circuit at D shows how two diodes may be used to serve as a series de switch. The RF chokes (RFC) prevent the signal from being lost to ground.

Figure lE shows how diodes can be used to select two or more crystals in an oscillator circuit. This type of circuit may be used also for switching other RF components, such as tuned circuits. Figure lE is a simple 0.7-V positive bias supply, such as those used for linear Class-A RF amplifiers that use power

MONITORING TIMES

transistors. Two identical diodes used in series will yield 1.4 V, and so on. D4 in this circuit should be a 50 PRY, 1-A rectifier diode.

A Zener diode is shown at G of Figure 1. The series resistor should be chosen to cause approximately 18 mA of de current to be drawn by D5 when using 400-mW or 1-W Zener diodes. Zener diodes are available for a large number of voltage values. Detailed information about designing Zener­diode regulators is found in The ARRL Handbook.

The final example in Figure 1 (H) shows how a small-signal diode may be used as a frequency multiplier. In this example we see a frequency doubler from 10 to 20 MHz. By selecting the proper tuned circuit after the diode we can make it work as a tripler or quadrupler. The diode output power decreases as the order of multiplication is increased.

Since the diode is a low-impedance device it is necessary to use it with the low­impedance interface shown in Figure lH. In our example, we may add a transistorized amplifier at the doubler output in order to increase the power of the 20-MHz signal. Some excellent examples of diode frequency multipliers are provided in Solid State Design for the Radio Amateur (an ARRL, Inc. publication).

In Summary

I have merely skimmed the surface in this discussion. Diodes have many other applica­tions, but page space does not permit us to explore further. I suggest that you photo­copy these pages and file them in your note­book for future reference.

March 1990 93