Do It With Diodes
February 1961 Radio-Electronics

February 1961 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

"Do it with <fill in the blank>," was a popular form of saying back in the 1960s and 70s. It is a form of double entendre, so people thought it was clever. I never did. This "Do It With Diodes" article from a 1961 issue of Radio Electronics magazine is an example. The term "diode" was not new to the electronics field at the time, as vacuum tube diodes and selenium rectifiers had been around for half a century. However, the newfangled semiconductor form of diodes were just coming on the scene. Germanium and silicon were the compounds available for commercial devices. More exotic materials were still in research laboratories. Author Donald Stoner provides a layman's level introduction to semiconductor diode fabrication and operation. Voltage, current, and power handling capacity was still fairly low. Prices for common diode types had dropped to a point that were making them competitive options for designing into products like radio, televisions, and appliances. Industrial controls and even some military systems were being qualified was well. Six decades later, the article is still useful.

Do It With Diodes

By Donald L. Stoner

What is a diode? Once, this was an easy question to answer. But that was back when you could say that a diode is a device that lets a current flow in only one direction. Today, a diode might be a switch, an amplifier, a capacitor, a voltage regulator, a conductor of currents in more than one direction, or many things not even remotely associated with rectification! The once humble diode may be used to convert light into electricity, ring an alarm when frost threatens or catch burglars who are after the family valuables.

Let's take a look at the operation of a semiconductor junction diode. Such units consist of a sandwich of p- and n-type germanium or silicon. Impurities are added to the p-type material to make its atomic structure short in electrons. However, due to a sharing of electrons, the structure is still quite balanced. The opposite is true for the n-type material - the atomic structure has a surplus of electrons. When the two materials are joined, you might expect the surplus of electrons in one wafer to cross immediately to the other side and make up the deficit. This does not happen. Each material is electrically neutral or balanced and, if an electron from the n-material crossed to the p-material, each of the two pieces would take on a charge. The n-material, having lost an electron, would be positive and, of course, the p-material would be negative. Actually, a field or barrier is set up between the two pieces and only the pressure of voltage will force the reluctant electrons across.

The Barrier

We might consider the barrier as a little gap between two areas. We might even consider the gap an insulator (which it really is under certain conditions) and the two materials the plates of a capacitor. By applying a reverse bias (negative anode, positive cathode) to the two plates, the width of the gap or barrier can be altered. Thus we have a capacitor whose value can be changed by varying a de bias applied to it. Naturally enough, if we reverse the polarity of the bias (positive anode, negative cathode), the barrier will break down and the diode will conduct. When alternating voltage is applied, the barrier is alternately broken down and then built up. Current flows only during the half-cycle the barrier is down.

The barrier capacitance is a very real and usable one. Several manufacturers have developed diodes especially for this purpose and have given them names to suggest their purpose. However, all germanium or silicon diodes exhibit this property and ordinary general-purpose diodes may be used by experimenters. (The diodes made especially for this application have excellent capacitance stability under varying temperature.)

The variable-capacitance diode lends itself to a variety of applications: automatic frequency control, sweep generators, remote tuning control, frequency modulation and others too numerous to mention in the space available. Fig. 1 shows an experimental circuit for demonstrating the variable-capacitance effect. Naturally, to avoid rectification, the rf voltage must not exceed the dc bias. Audio applied across the bias will frequency-modulate the signal.

Diode Switch

When a diode is forward-biased (positive anode, negative cathode), it actually becomes a conductor, just like a piece of wire. As such, it may be used to connect one component to another, like a switch. Audio or rf may be turned off and on so long as the signal through the diode does not exceed the bias voltage. When this happens, rectification will take place. When the diode is reverse-biased, the barrier is re-erected and no rf or audio can flow. The diode may therefore be used as a switch or, more correctly, as a relay.

Fig. 2 gives the general idea of a diode switching circuit. Of course, it is a simple matter to gang up sections to get any number of poles or functions.

I have done this to replace an antenna relay. It has also been used in the place of a signal switching relay in radiotelephone equipment. The advantages of such a device will immediately be obvious. Hard-to-get-at circuits may be conveniently switched, antennas may be changed over up on the tower itself (using only one feed line), and even a receiver bandswitch (and its ganging problems) can be taken over by the diode switch.

The circuitry is simple, there are no moving parts to wear out, and dust cannot cause intermittent operation. The diode must be carefully chosen for each application, however. Silicon power diodes (currently available at low cost) may be used to switch a transmitting antenna. Germanium 1N34's can conveniently switch audio or rf in a high-impedance circuit. In fact, there is nothing that the right semiconductor diode can't switch.

Limit Control

Oscillators must run class-A to be very stable. The amplitude of the oscillation must be limited one way or another so that the peaks don't extend into the nonlinear region of the tube's characteristics. A reverse-bias diode, acting as a clipper, might well be used to limit oscillator amplitude. It is already used in audio work where it is called speech clipping.

The principle may be applied to a variety of circuits. Transistor receivers, for example, use the reverse-biased diode to assist agc action on strong signals. In this case, the reverse bias is overcome by the agc voltage at a predetermined level and the diode becomes a short circuit across an if transformer, thereby causing a reduction in receiver gain. When agc drops again, normal operation resumes.

Zener Diode

Silicon diodes, when subject to a reverse bias, break down when the bias exceeds a certain figure, and conduct. This is the Zener voltage. The voltage at which this effect occurs is very constant and thus the device makes a good voltage regulator. In this application the barrier has been overcome by sheer force. Zener diodes may be used exactly in the same way as voltage-regulator tubes (Fig. 3). The Zener diode normally conducts and, as the supply voltage rises, the Zener current increases. When the supply voltage decreases, the Zener current likewise decreases. This tends to level off voltage changes for improved regulation in much the same manner as a voltage regulator tube.

Zener diodes cover a range from a fraction of a volt to several hundred volts. The currents they handle vary from a fraction of a milliampere to many amperes. Accuracy may well be perfect or, if such perfection is not required, 20%, 10%, 5% or whatever is needed. The 1N430-A, for example, will hold the voltage constant within 0.007 volt over a temperature range of - 55°C to 165°C! This type of diode may be used to regulate an accurate source of voltage for instrument calibration, as a solid-state secondary cell and so on. Naturally one wouldn't use such a diode for simple regulation applications. For this purpose there are less expensive units with wider tolerances.

Zener diodes have a variety of uses. They may control the bias in a tube or transistor class-A, -B or -C stage merely by being inserted in the cathode or emitter lead. Such a system is shown in Fig. 4. The arrangement has been used to bias a 7094 single-sideband linear amplifier. The Zener unit may be connected in parallel with a meter to protect it against overloads. It may take the place of coupling capacitors in audio amplifiers or even electrolytics. In a power supply, the Zener diode will represent around 3,000 μf of capacitance even in an inexpensive unit. So numerous are Zener-diode applications that several books have been written about them. Service technicians may expect to see more and more of them as time goes by.

The Photodiode

If, while forward-biased, a junction diode is exposed to light, the current flowing through it will change. The diode has the properties of (and may actually replace) a photocell. The Philips (Amperex) OAP12, for example, has a built-in lens, is less than 1/8 inch in diameter, generates very low noise and has a host of uses from sound scanning of motion-picture film through computer punch-card scanning to the detection of fire or smoke.

Only the surface has been skimmed when it comes to new applications for modern-day diodes. Tunnel diodes, for example, have not been mentioned, for a full discussion of these devices would more than fill a magazine of this size. The solar cell, too, is really a diode and its applications are numerous. The field of diode application is an exciting one, and one that will pay you to keep abreast of in the months to come. There's something new to be seen almost every day.

Posted May 15, 2024