Device Recognition

Question File Number 12



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Electronic Devices in Common Use

The Syllabus sets this following requirement

Device recognition: (1 question) Recognition of electrode names of bi-polar transistors, FETs, valves, from diagrams.

The connections to various electronic devices will be explained in sequence. Please refer to your training manuals for pictures or drawings giving examples of the physical construction of these devices.

In all the examples shown below, the electrodes start at the same reference point (usually the source of the electrons) and rotate in a clockwise direction. This is a standard convention throughout electronics.

Semiconductors

diode

A fundamental device in all electronics is the diode. It has two connections, the cathode shown here as 1, and an anode, shown here as 2. It conducts when the anode is positive when compared or referred to the cathode. In other words, the arrow forming the symbol shows the direction of the conventional current flow. Conventional current means a positive-to-negative flow compared to electron flow which is negative to positive.

Pnp2

This device is a transistor - a fundamental device found in semiconductor amplifiers and similar applications. This is a bi-polar transistor - a name used to distinguish it from other types considered below. For our purposes there are are two types of bi-polar transistor. This one is a PNP transistor. Its operation is explained elsewhere in this GUIDE. The significant indicator is the one with the arrow. There are three electrodes, the emitter 1, the base 2, and the collector 3. Start with the emitter as your reference point and remember e-b-c.

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Npn2

This device is also a bi-polar transistor. This one is an NPN transistor. Its operation is explained elsewhere in this GUIDE. The significant indicator is again the one with the arrow. Note that it is drawn in the opposite direction to the earlier example. There are again three electrodes, the emitter 1, the base 2, and the collector 3.

Jfet2

This device is from a different family of transistors. This is a field-effect transistor or FET. Again, to distinguish it from other members of the field-effect family (following below) , this is a junction field effect transistor or JFET. To be more distinctive, this is known as an n-channel JFET. Two electrodes are shown as similar, 1 and 3 on this diagram. This is deliberate and is one of the distinguishing features of the FET. The electrode names are source 1, gate 2 and drain 3. Remember s-g-d.

Jfetp

This device is from the same family of transistors, a junction field-effect transistor or JFET. Note the direction of the arrow on the symbol. This change of direction of the arrow also means a change in the polarity of the supply voltages. Again, to distinguish this JFET from the one considered above, this is a p-channel JFET. Once again, two electrodes are shown as similar, 1 and 3 on this diagram. This is deliberate and is one of the distinguishing features of the FET. Again the electrode names are source 1, gate 2 and drain 3. Remember s-g-d.

Mosfet2

Another member from the field-effect group of transistors. This is a MOSFET - a metal-oxide-semiconductor field-effect transistor. This device is explained elsewhere in this GUIDE.

The device shown is an n-channel device. As with the JFET, there is a p-channel version of the MOSFET too (it has the arrow reversed) - but that is getting beyond the needs for this examination.

There are two gates, this time 'gate 1' and 'gate 2'. Each is insulated from the semiconductor slab. The electrodes, in sequence, are: the source 1; 'gate 1' 2; 'gate 2' 3; and drain 4. It is worth noting that this is a four-terminal device, previous transistors are three-terminal devices.

ic

Integrated circuits (IC's) are as complicated as anyone can make them. Their drawings vary and many different styles of presentation are possible. The drawing may - or may not - give a block diagram of its interior workings. This diagram is for illustration only and shows just one style.

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The odes of Thermionic Valves

diode1

This device is also a diode - a thermionic diode. It has a hot filament 1 which acts as the source of the electron emission, and a cylindrical anode 2 surrounding it. This is the basic valve. These two electrodes are contained in a vacuum-filled (joke!) container, usually made of glass.

diode2

This device is also a diode, this time an indirectly heated valve, The filament now becomes a heater 1 and a very tight sleeve around that, the cathode 2, is made hot to the point when electrons boil from its surface.

triode

A wire-mesh grid 3 is added to the inter-electrode space to control the electron flow from cathode to anode. This is a three-electrode valve - cathode 2, grid 3 and anode 4. This is a triode.

Tetrode2

Add another grid - the screen grid 5 - and the valve becomes a tetrode.

pentode

Add a further grid 5 - the suppressor grid - and the valve is a pentode.

Note that it is placed between the screen grid (shown here as 6) and the anode. The suppressor grid is usually internally-connected to the cathode inside the glass envelope of the valve and separate external connection to it is not always possible.

There are other valve types too, called multi-electrode valves, used for mixing and for other purposes.

Several valves can be mounted in the one envelope. One example is the 1D8G (or 1D8GT) which is a diode-triode-pentode, three valves in the one vacuum space. An early integrated circuit? This is long obsolete, now just a curiosity.



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Compiled Sun Nov 28 2010 at 8:41:44pm


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