Fundamentals of electronics - Electronic components

Resistance is a component which opposes the passage of electric current, dissipating energy in the form of heat. It is labelled with the letter R and is measured in Ohm. As we'll later see, when we'll be describing Ohm's law, resistance binds tension V and current I in one single formula. In particular, if we apply a tension V to a resistance R the passing of a current I is generated and the three quantities in question are bound in the following formula:

Equation 4.1. Ohm's Law 

This component is made up of two parallel metal plaques placed at a very short distance from one another. If we apply a tension to the two plaques, the latter are capable of maintaining the accumulated charge, thus generating an electric field within the slit that separates the two, which can be likened to the passage of a current as shown in the figure:

Charge of a capacitor

The quantity of charge which a capacitor is capable of absorbing is called Capacity (C) and is measured in Farad. The figure shows a capacitor with a capacity C, to which a tension V is applied. The formula that ties capacity, tension and accumulated charge is the following:

Equation 4.2. Charge of a capacitor 

When we apply a tension to a capacitor which was discharged, the latter will begin to charge up until it reaches its maximum charge limit it can accumulate. Beyond this limit the capacitor is no longer capable of storing charge and, if the tension is removed, the capacitor shall remain charged. A charged capacitor has a constant tension at its extremities and if it gets connected to a resistance it discharges onto the latter and creates a current. The two processes of charge and discharge of a capacitor are not instantaneous but take place during a certain time-lapse which depends on the capacitor's characteristics and the circuit it is inserted in. Whilst the capacitor is charging, we have a movement of charges of opposite polarities which accumulate on the two plaques, and this movement of charges generates a current. This behaviour is at the heart of the functions of high-pass filters [Filters ] . Let's imagine that we apply a tension with a sinusoidal rate to the capacitor. If the sinusoid's rate is such that positive semi-wave is faster than the capacitor's charge-time, the latter won't reach its maximum charge limit in time, and the negative semi-wave shall come on and discharge it. In this way the passage of current within the capacitor never interrupts itself. Vice versa if we have a low frequency, the capacitor reaches its highest charge before the positive semi-wave runs out and in that precise instant blocks the current-flow. So, a capacitor blocks the flow of low frequencies (which cause its complete charge which in turn interrupts the charge-flow) and can be used as a high-pass filter.

Simple High-pass filter

When a conductor is immersed in a magnetic field, the latter attracts the electrons within the conductor, making them move and therefore generating a current. Vice versa, in the proximity of a conductor which has a current flowing through it , a magnetic field whose force lines are distributed as follows is generated:

Magnetic field inducted by a current in a conductor

In circuits the inductor is labelled with the letter L and its inductance-value is measured in Henry. An inductor is basically a conductor bound in a spiral shape. When a current passes through it, a magnetic field is generated whose line forces become distributed as follows:

Magnetic field inducted by a current in an inductor

An inductor can be used effectively as low-pass filter filter, by making the most of a characteristic of inertia of the magnetic field. By applying a current with a sinusoidal rate a magnetic rate is generated which is also sinusoidal. However, if the frequency is too high, the negative semi-wave generates a magnetic field with line-forces that are opposite to those generated by the positive semi-wave which haven't yet had time to run out: the current-flow is in this way blocked. The following figure shows us an example of a low-pass filter filter circuit:

Simple low-pass filter filter

Combining inductors and capacitors it's possible to build up band-pass filter circuits:

Simple band-pass filter

When we apply to a capacitor a signal which contains a variety of different frequencies, like an audio signal for example, the capacitor will react differently to every separate frequency. Moreover, each component having been built with materials which have a certain resistance, to identify the component's behaviour we use a quantity-value which takes into account these characteristics. The quantity is named Impedance and is labelled with the letter Z. In a capacitor it has the following value:

Equation 4.3. Impedance of a capacitor 

This formula indicates that the impedance of a capacitor depends on the frequency. Moreover, it has two components: the first is called resistivity and gives the value of the component's effective resistance. The second is called reactance and introduces dependency on frequency. Finally the symbol j indicates that reactance is an imaginary number. Don't worry! We won't be going any deeper than this. What we have said up until now is sufficient to understand the significance of these factors in relation to audio signals and the circuits that manipulate them. Notice that in f=0 (as in continuous current), the capacitor's current becomes infinite, simulating an open circuit, whilst in f=infinity the impedance coincides with resistance.

Similarly, in inductors we have an impedance-value of:

Equation 4.4. Impedance of an inductor 

Notice how in f=0 impedance coincides with resistance, whilst in f=infinity the inductor behaves like an open circuit. From this point of view, the capacitor and the inductor have opposite behaviours.

Questo componente permette il passaggio di corrente in un solo verso. Applicando una tensione con un certa polarità ai suoi capi si ha uno scorrimento di corrente. Applicando la polarità opposta non si ha passaggio di corrente. Il simbolo utilizzato nei circuiti per rappresentarlo è il seguente:

Diode

A particular kind of diode is the LED (Light Emitting Diode). This component has the faculty of freeing a band of photons (in English: it lights up!) when a current flows through it.

A transistor is obtained by setting two diodes up in a certain way. It has three connectors: base, collector and emitter.

Its symbol is the following:

Transistor

It is used in a variety of modalities and set-ups. What is interesting from a sound-engineering perspective is its amplification functions.

A transistor is capable of supplying power-amplification as well as a tension or current-amplification. Let's take a look at an example of it in action.

If we apply a little variation in tension between the emitter and the base, the current experiences a relatively high excursion on the emitter. One fraction of this variation in the current is gathered by the collector, thus increasing the difference in potential between the base and the collector. So, one little variation in the potential applied between the base and the emitter, produces a rather high change in the tension between the base and the collector, resulting therefore in tension-amplification.

This kind of amplifier is capable of amplifying a difference in the signals. The symbol used to identify it is the following:

Operational Amplifier

It is commonly used as an input stage for balanced connections, described in detail in the chapter pertaining to connections [Balanced Electric Connections ] and in VCA faders [VCA controls ] .

This component makes the most of electromagnetic induction ^{[6 ]} of conductors that are in their bound form. If in the proximity of a coil through which current is flowing, we place another coil, the magnetic field of the first will take over the second, thus generating a current flowing through it. The number of spirals of each coil determines the difference between the two currents and consequently determines the relationship between the tensions at the two extremities of the two coils. So, a transformer, as its name suggests, transforms one tension into another. The following figure shows a transformer in which its primary coil has 20 spirals and the secondary has 10 spirals. If we apply a tension of 10 V to the primary coil we obtain a tension of 5V on the secondary one:

Transformer

Another important characteristic of transformers is their capability to act as impedance adapters. As we shall see when we'll be speaking about the amplification chain in its relative section, it is necessary, when connecting two components, that the output impedance of the first and the input of the second have values that respect a precise relationship. When it is necessary to change the impedance-value (in other words, to make an adaptation in impedance), without changing the other electrical quantities, we can recur to a transformer in which the relationship between the numbers of primary and secondary spirals is acted upon.