ACTIVE CIRCUIT ELEMENTS

When we have considered electrical circuits, we have come across CIRCUIT ELEMENTS (also known as CIRCUIT COMPONENTS) such as resistors and capacitors . These are PASSIVE components, as they behave according to simple rules when voltages are applied to them. In electronics however, there are a number of components that have a rather more complex behaviour. These components are known as ACTIVE COMPONENTS, and include diodes, LEDs, transistors and operational amplifiers. In this topic we will look at these and see how they can be used in electronic circuitry.

Diodes:

Diodes are devices which operate on the basis of semiconductors p-n junctions. You will remember, from your study of semiconductors in Grade 11, that forward biasing of a diode enables electrons to flow from the n-side of the junction to the p-side, provided the applied potential difference is above the potential difference of the virtual cell (about 0.6 V for Si, and about 0.2 V for germanium).

That it is much easier to explain the behaviour of diodes in terms of electron flow, but in circuits, current is conventionally assumed to flow through the circuit from the positive pole of the source to the negative pole. A typical diode, as well as the symbols that are used to represent diodes in a circuit, is shown on the right.

The conducting properties of a junction diode are depicted in the graphs above. Note that when the diode is reverse-biased, the current (carried by minute quantities of impurities in the silicon, called "minority carriers") is extremely small (about 1 µA or less), and the scales of both the horizontal (potential difference) and vertical (current) axes are not the same.

Since a diode such as the one described above only allows current to pass through if the applied voltage is of a certain polarity, diodes are used as RECTIFIERS, that is, devices that convert an alternating current to a direct current, and as such, find wide applications in electronic power sources.

The diagram above shows how an AC output from a transformer is converted to a pulsed DC current by passing the AC current through a diode. Only the positive values of the applied voltage will forward-bias the diode, allowing the current to flow. The negative regions of the wave will reverse-bias the diode, effectively converting the diode to an insulator, shutting off the current. The rectification is called "half-wave rectification", since only half the cycle produces current.

The light-emitting diode:

Light-emitting diodes, (LED), are semiconductor-based devices that convert electricity into light. They are found in all sorts of places, for example, to indicate whether an appliance is on or off, in digital displays, and in so-called "light boards".

LEDs are based on the principle that when electrons from n-type semiconductors fill "holes" in p-type semiconductors, energy is given out. In a normal diode, this energy appears as heat. If the p-type semiconductor is doped with certain impurities, such as gallium arsenide, the energy is released as photons having a relatively narrow frequency range. Suitable dopants can result in the emission of visible light (red, green, yellow or blue) or infrared light.

Since a diode acts as a switch, the LED will either emit light of be dark. Bar-types LEDs are used to create digital displays, as shown here below. A special circuit lights up individual diodes to create lighted segments forming the characters.

The transistor:

The invention of the transistor in the 1950's by Bardeen, Brattrain and Shockley revolutionised the field of electronics. Previously, electronic circuits depended on glass tubes (called "valves"), which were slow, bulky and relatively short-lived. Transistors are small, cheap and reliable. The computer you are using, and the display you are staring at, make use of thousands of transistors. The basic construction consists of a sandwich of three thin semiconductor layers, forming two p-n junctions. There are various types of transistors some of which are listed below:

Bipolar junction transistors (BJT), coming in two varieties, n-p-n and p-n-p transistors. They are indicated as follows on circuit diagrams:

These transistors have three connections, called the base (b), the emitter (e) and collector (c). The base is connected to a p-type semiconductor in n-p-n transistors, and to an n-type semiconductor in p-n-p transistors.

Field effect transistors (FET). Again, these are of two types, n-channel FETs and p-channel FETs. The symbol for these is

As with the BJTs, FETs have three connections, called the gate (g), the source (s) and the drain (d). In n-channel FETs, the gate is connected to an p-type semiconductor, while in the p-channel FETs, the gate is connected to a n-type semiconductor.

Transistors act as switches and amplifiers. BJTs are controlled by current across the base-emitter junction, whereas FETs are controlled by voltage across the gate-source junction. Low currents (or voltages) prevent current flowing from the collector to the emitter (or from the drain to the source in the case of FETs). Transitors can switch on and off millions of times every second almost indefinitely provided the circuit is well designed.

Operational amplifiers

Before discussing the operational amplifier, it will be useful to discuss the terms "amplifier" and "gain".

What is an amplifier?

In electronics, an amplifier is a device that increases the power of an incoming electrical signal.

What is "gain"?

In electronics, GAIN refers to the ratio of the power (or voltage) entering an amplifier to the power (or voltage) put out by the amplifier. One must be careful to state whether one is dealing with power ratios or voltage ratios. Further, one should be alert as to whether the gain is a simple ratio, or whether it is stated in units of DECIBELS, dB, which is a logarithmic function of that ratio:

Input power = 5mW; output power = 75 mW.

Using a simple power ratio, the gain = 75/5 = 15.

Using decibels: the gain = 10 log1015 = 11.8 dB.

If there is a power loss, the gain is less than 1 (negative decibels), and we are then dealing with a case of ATTENUATION. The two graphs below illustrate what one aims to achieve with an amplifier:

On the left above we have a fluctating signal produced by some instrument (the "raw" signal). On the right, the signal has passed through an amplifier producing a gain of 5.

The OPERATIONAL AMPLIFIER (termed "op-amp" for short) is a device that can provide considerable gains (over 105) over a large range of inputs. Operational amplifiers are complex circuits that include transistors, resistors and capacitors. They are normally provided as an INTEGRATED CIRCUIT (IC) built into a CHIP, with connectors ("pins") whose function is provided by the manufacturer. For example, the 741 operational amplifier is an integrated circuit chip with 20 transistors, 11 resistors and 1 capacitor and 8 pins:

The term "operational amplifier" was applied to the device because it was initially used in computers to perform mathematical operations.

What are opamps used for?

Operational amplifiers can be considered as devices that generate a voltage E = A(V2 - V1), where A is known as the"open-loop voltage gain". It is a known parameter of the device that depends on the frequency of the input voltage(s), having very high values at low frequencies (typically around 1x105). V1 and V2 are the input voltages that are applied to the inverting and non-inverting terminals respectively.

Operational amplifiers are used pricipally to:

Amplifying voltages (with or without phase inversion);

Comparing voltages.

The amplification that may be obtained is not unlimited. The output voltage of the device is limited to about 80% of the supply voltages (+ Vs and -Vs) that are supplied at pins 7 (+9V) and pin 4 (-9V). If the output voltage is greater than that value, the output signal will be truncated and the opamp is said to have reached the SATURATION VOLTAGE. This may be shown by varying the input voltages and connecting the output terminals to a double-beam oscilloscope.

The basic circuit below will deliver an amplified voltage with phase inversion. Part of the output is fed back into the inverting input, via a feedback resistor RF. For display purposes, the graph below is limited to a 5X amplification.

Consider the plot on the right. For the above circuit, the ratio of the resistors RF and RA dictate the amplification that may be obtained (the negative sign takes care of the inverted phase). Note that the amplification is limited by the value of the source voltages. Above a certain value of the input voltage, no amplification takes place. In the linear range of the graph, the gradient of the plot gives the ratio -RF/RA.

Filters:

In electronics, a FILTER is a circuit that removes unwanted frequencies from a signal. Such devices find numerous applications, not least of which in hi-fi audio equipment.

The figure on the left shows a waveform (signal) that actually is the sum of two sine waves of equal amplitude, but different frequencies. These two waves are shown individually in the second figure. The high frequency wave has a frequency that is four times that of the low frequency wave. The purpose of an electronic filter is to remove certain waves with frequencies above a certain value, or alternatively, those waves whose frequencies are below a certain value. A LOW-PASS FILTER will remove waves whose frequencies are above a certain CUTOFF VALUE (third figure from the left), while a HIGH PASS FILTER will remove those waves below a certain cutoff value.

The diagrams above show a simplified ideal situation. In practice, a signal may many different frequencies. Both low-pass and high pass filters do not remove the unwanted frequencies completely, but attenuate them. BAND-PASS filters remove or attenuate frquencies that lie outside a desired range.