LEDs are very familiar as the glowing indicator lights encountered on the front panels of almost every piece of audio technology. As with all diodes, it is important to differentiate between the anode and the cathode when connecting an LED into a circuit. In the case of LEDs there are two common signifiers to look out for, indicating the cathode or negative pin of the device, a shortened leg and a flat region on the side of the LEDs body. LEDs are just diodes that emit light, and can occasionally found used in a circuit where a normal diode might be expected, rather than exclusively as a front panel indicator light. So long as the lower than usual V_R max is taken into account when using an LED in a circuit, there is no reason not to try one and see how it sounds.

Of the very many special purpose diode types which exist, the most common is probably the Zener diode. Zeners are wired into a circuit in the reverse orientation to normal, and are used to provide very well defined reference voltages. While many diodes can be easily damaged or destroyed by reverse biasing them all the way to their breakdown voltage, Zener diodes are designed to operate at this point on their characteristic curve, and this is how they provide their stable voltage reference. Figure 16.5 shows the i-v characteristic curve for a three point one volt Zener diode. Its forward characteristic is more or less identical to a standard silicon diode, but in reverse bias it conducts at a specified voltage much lower than a normal diode’s V_R max.

Zener diodes can be seen in two of the circuits included in the circuit catalogue in Appendix D. In both cases they are performing their usual job of providing a fixed reference voltage. In the phantom powered electret microphone circuit the Zener utilised is a 12V device, while a 5V1 Zener can be seen deployed in the noise gate circuit. Notice in both cases that the diode is inserted into the circuit counter to the normal diode orientation. The anode connects to the low voltage side, with the cathode wired at a point of higher potential, in order to reverse bias them and bring them into conduction at their designed breakdown voltage.

At first glance the device in the electret mic circuit may appear to be inserted the wrong way round, but notice that its top (the anode) is connected to ground, while its bottom connects through to the +48V coming into the circuit from the phantom power supply. Sometimes the layout conventions outlined in Chapter 7 on circuit diagrams are not strictly adhered to. In this instance it simply leads to a much neater schematic diagram to arrange things with the ground point in the middle of the diagram.

Another commonly encountered type of diode is the Schottky diode. These have a low switching voltage and operate very quickly, making them ideally suited to high frequency circuits. These high switching speeds are generally of no great benefit in audio circuits which operate in a very limited bandwidth when compared to the likes of radio communications circuitry. Their low switching voltage is however sometimes used to advantage in audio circuits. The main disadvantages of Schottky diodes are their relatively large reverse leakage current and junction capacitance. They also tend to have a lower value for V_R max, and so can not withstand as large reverse voltages as most standard diodes can. The specifications for the 1N5817 Schottky diode included in Table 16.1 illustrate these points well.

Schottky diodes do not actually consist of a standard p-n junction as standard diodes do, but rather are formed by the junction of a semiconductor with a metal. This is what gives them their low forward voltage drop and fast switching action. A silicon diode has a typical forward voltage of 600–700mV, while the Schottky’s forward voltage is usually in the range 150–450mV. This lower forward voltage requirement allows higher switching speeds and better system efficiency. A Schottky diode can be seen employed in the low power audio amplifier circuit in Appendix D. Here advantage is being taken of its low switching voltage. It is being utilised as a reverse voltage protection measure (the 386 IC in this circuit is easily damaged by reverse polarity power connection). The low forward voltage drop means that as little voltage as possible is lost to the rest of the circuit in normal operation. There are certainly better reverse polarity protection measures which could be employed, but this has the advantage of being trivially simple to implement.

These days diodes are almost exclusively made from silicon (Si) semiconductor material, but germanium (Ge) based diodes used to be much more common. While germanium semiconductors have fallen out of general usage in most electronics applications due to their poor performance in most respects when compared to their silicon equivalents, some audio circuits call specifically for germanium diodes for their particular sonic characteristics. In particular, many distortion circuits specify (or at least suggest) Ge diodes. The gentler slope of their i-v characteristic curve means that they can produce a softer distortion effect than their silicon counterparts. Silicon diodes switch from fully off to fully on over a very small voltage range and as such the clipping they introduce can be very harsh. Germanium diodes provide a slightly gentler alternative.

Their very low initial conduction voltage relative to silicon devices also makes them better suited when dealing with very low level signals. A small signal might be all but used up by the time it has managed to switch on a silicon diode, but the same signal will be able to drive a reasonable amount of current through a germanium device. Sometimes a Schottky diode would also work well in such applications, however the much larger junction capacitance of a Schottky can often rule them out, making germanium devices the best option.

When wiring up an LED as a standard indicator light as shown in Figure 16.7, a current limiting resistor is always needed. The value of the resistor depends on three things, the voltage drop and working current of the LED, and the DC voltage level which will power the circuit. Once these are known, the optimum resistor value can be calculated. This value is then usually rounded down to the nearest E12 standard resistor value.

Typical values for a standard red LED might be something like 2.1V and 20mA for normal operation. Details will be found in the manufacturer’s data sheet for the specific device to be used. If a 9V power supply is to be used then the calculation proceeds as follows.

Step 1: Voltage across resistor: V_R1 = Vtotal − Vdiode = 9 − 2.1 = 6.9V

Step 2: Value of resistor: ${\text{R}}_1=\frac{{\text{V}}_{\text{R}1}}{{\text{I}}_{\text{R}1}}=\frac{6.9}{0.02}=345$

Step 3: Next largest E12 value: R₁ = 390Ω

Some components can be damaged if power is inadvertently connected to a circuit the wrong way round. Various approaches can be taken to mitigate or prevent any potential problems. One of the simplest solutions is to connect a diode in series with the power supply. If the connections are reversed, the diode will be reverse biased and no current will flow, thus saving components from potential damage. The down side to this simple solution is that the voltage dropped across the forward biased diode in normal operation is lost to the circuit it is protecting.

In order to minimise the impact, the circuit shown in Figure 16.8 indicates a Schottky diode as opposed to a standard silicon diode. As has been explained, the Schottky diode has a lower forward voltage drop than the more common silicon diode, round about 0.3V, versus about 0.7V. This may not seem like much, but when a circuit is powered from a small power supply of maybe 5V or even 3.3V, as some are, it can make all the difference. For even lower insertion losses the MOSFET solution presented in Chapter 17 can be used.

One of the most widely recognised diode circuits (by name if nothing else) is the classic diode ring modulator circuit (Figure 16.9). The circuit takes its name from the ring of diodes at the heart of the circuit. The ring modulator combines two input signals, effectively multiplying them together. This can produce a rich mixture of harmonic and anharmonic frequency components in the output signal, which can result in interesting metallic and bell like sounds. The right combination of inputs are needed in order to obtain really good results, and the signals levels need to be well balanced for best effect.

This circuit has some visual similarities to the ring modulator of the previous section, but it is completely different. The most important thing to notice is that, although there are again four diodes in a loop, here they do not form a ring, but rather a bridge. Notice that in the ring modulator ring the four diodes chase each other’s tails around the circle. Here, on the other hand, the four diodes all flow left to right. This means that the loop here operates in a very different way. The configuration is called a bridge rectifier, and as Figure 16.10 illustrates, it takes the bottom half of an AC signal and folds it back up on itself. This process is called full wave rectification, and is described in Chapter 5: Signal Characteristics.

The circuit shown in Figure 16.10 is the business end of a typical linear power supply circuit. The transformer takes in mains voltage and converts it down to a more user friendly level. The bridge rectifier then full wave rectifies it. This is usually followed by a large smoothing capacitor and possibly a regulator circuit, to turn it into a nice steady DC voltage ideal for powering DC circuits.

An variation on this circuit, using a transformer with a centre tap output, allows the four diode bridge in Figure 16.10 to be replaced with just two diodes to achieve the same full wave rectified result. Penfold (1994) closes with a simple mains power supply project which uses this approach. It adds a large 1000µF smoothing capacitor, followed by an LM317 based regulator circuit similar to Figure 18.9, to produce a well regulated DC power supply suitable for powering most of the circuits in this book.