VALVES & THERMIONIC EMISSION

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VALVES & THERMIONIC EMISSION

OK, people who speak American English tend to call these tubes. However Ill use the word Valve as I am British and (almost!) speak English rather than American.

Its simpler than calling them something like Vacuum-state Thermionic Devices all the time!

Thermionic Emission

Metals at room temperature have a lot of electrons inside them which can move around in response to an applied electromagnetic field. However under normal conditions the negative charges on all these electrons are cancelled out by the positive charges on the atoms of the metal.

If we heat up the metal, however, we give the electrons more kinetic energy. This may mean that some have so much energy that they can leap out of the piece of metal into its surroundings. However when they do this the metal, having lost an electron, now has a positive charge. The result is an electrostatic attraction between the (negatively charged) electrons that have leapt out of the metal and the (positively charged) metal they have left. This tends to pull them back.

The result of the above is to produce what is called a space charge effect. The hot metal becomes surrounded by a cloud of electrons, that have jumped out of the metal, but are then drawn back by the attraction between the electron and the metal. Taken overall, the system is still electrically neutral since we have the negative electrons and the positive metal. Add their charges and we still can get zero. It is just that some of the negative charge is displaced from the metal to its surrounding. In this case, if we heat one of the metal pieces of our diode valve it becomes surrounded by this space charge.

The usual practice with valves is to use the heated metal as a cathode i.e. as the part of the system that will supply electrons when we apply suitable voltages. Hence as youll see with the circuit symbols shown later, the cathode often has two leads. This allows us to pass a current though the cathode and heat it up.

The above explanation leaves out lots of details. For example, it is possible for some material surfaces to create a space charge around them even if we use them at room temperature. However here Ill ignore such details as the valves that are used most widely do require the cathode to be heated for the valve to work.

The result is illustrated in the above diagram. When we heat up the metal we get a cloud of electrons that are boiling off the metal surface and then (usually!) falling back again.

This property of boiling off electrons is called Thermionic Emission as the emission of electrons is produced by the heat.

Diode Valves

The easiest way to understand how valves work is to start with the simplest types and work upwards to the more complicated ones. The most basic was the earliest type that was invented. This is the Thermionic Diode. It basically consists of two parts or Electrodes, surrounded by an envelope which allows them to operate in a vacuum.

The standard circuit symbol for a diode valve is as shown above. (Please note, though, that the symbols may differ a little from one diagram to another, depending on the preferences of the person who drew the diagram!) The anode and cathode are made of good conductors (e.g. metals), but are separated by an insulating envelope and the vacuum inside.

To start, lets consider connecting the diode up as shown below.

Here what we have done is heat the cathode by applying a voltage between the two cathode heater leads, H1 and H2. We have also connected the cathode and anode together via a resistor. If we do this we get a surprising result. We find that the anode develops a negative potential with respect to the cathode, and some current will flow through the resistor. Note that in this case the positive and negative signs shown on the above diagram dont represent voltages we have applied from an external source. They indicate what the valve generates!

Unless we know what is happening inside the diode this result is puzzling as we seem to have created electrical power from nowhere and perhaps violated the law of energy conservation! Is this the solution to our global energy crisis and we can give up fossil fuels?... Afraid not. What is happening is that when we heat the cathode we create a cloud of electrons in the vacuum near the surface of the cathode. Most of these electrons will stay near the cathode. But a few will have energy to leap far enough from the cathode to be able to cross the vacuum and strike the anode.

Since the anode isnt heated, it will grab any electrons that hit it, and they wont have enough kinetic energy to escape it again. As a result a number of electrons end up sitting on the anode that have crossed the vacuum to reach it.

If we dont connect the anode to anything, then as some electrons gather on the anode they give it a negative charge. This would tend to produce a negative potential, which then tends to repel any other electrons that approach the anode. Hence unless we give the arriving electrons some way to escape, they build up until they repel any further borders! However if we connect the anode back to the cathode via an external resistor, the arrivals can flow back home to the cathode via the path through the resistor. As they do this, the anode potential relative to the cathode becomes less negative and some more electrons will have enough energy to cross the vacuum gap, thus continuing the flow. The broken blue lines with arrowheads in the above diagram show the direction of the electron flow. However note that for daft historical reasons we define conventional current to be in the opposite direction to the actual electron flow. Thus in conventional terms wed say a current flows through the resistor from C to A (positive to negative potential).

The energy which we see dissipating as the electrons pass through the resistor is part of the kinetic energy which they removed from the cathode when they were flung out of it by the thermal motions inside the cathode. Hence the energy is drawn from the heat we supplied to the cathode. Alas, no perpetual motion or energy crisis solution, since we have to supply energy to the cathode to drive this process.

The amount of current wed see will depend on various factors. These include the temperature of the cathode, its surface area, the distance to the anode, etc. One of the most important of these factors is the details of the surface of the cathode. Most practical valves have cathodes which are treated or coated to enhance their ability to release electrons when heated. Despite this, with most practical valves, the actual current level we get if we carry out the above experiment tends to be very small. This is quite deliberate as in most applications we want the diode to have very low current leakage when the potential difference between anode and cathode is around zero volts.

In practice it is also usual for a valve to have an Indirectly Heated cathode. This means that the heater element is a wire inside the cathode, insulated from the external outer surface that acts as the electron emitter/cathode. This gives more flexibility in choice of materials, and also helps avoid any of the heater voltages or currents from affecting signals on the actual cathode. When we wish to represent the presence of an indirectly heated cathode the circuit symbol is altered as indicated below.

In practice it is fairly common to simply omit showing the details of the heater altogether. This simplifies circuit diagrams since the heater, and the wiring that powers it, can be assumed to be present with almost any normal valve used in electronic circuits. As a result many diagrams show symbols which look like the simple directly heated valve type, but are actually only showing the cathode, and omitting showing the heater. The giveaway for what is being represented is indicated above. In particular, if there is only one wire shown connected to the cathode we can assume that the valve is indirectly heated, but the heater wiring is being left off the diagram to make the diagram easier to read. Note that although the above shows diode valves, the same conventions tend to apply for other forms of valve such as triode, pentodes, etc.

DIODES & TRIODES

Diode Characteristics

Now consider what happens when we deliberately apply a potential difference between the cathode and the anode and look to discover what effect that will have on the flow of electrons.

The diagram above illustrates what happens when we apply a potential difference between the anode and the cathode. (Note that the heater is working, and the cathode is hot, but the heater isnt shown.)

When we apply a voltage to make the anode positive with respect to the cathode we attract the electrons in the vacuum space towards the anode, and push them away from the cathode. This has the effect of making it easier for electrons to reach the anode. It also tends to reduce the density of electrons near the cathode, making it easier for more to boil off the cathode. The result is that when we apply a potential difference this way around we tend to increase the rate at which electrons flow from cathode to anode. Thus, making the anode positive relative to the cathode increases the current flow. This sign of applied potential is called Forward Bias.

When we apply a voltage the other way around, and make the anode negative with respect to the cathode we repel the electrons in the vacuum space away from the anode, and make it harder for them to escape the cathode. Since the anode isnt hot, it does not tend to release any electrons itself. The result is that when we apply a potential difference this way around we tend to make it even harder for any electrons to cross the vacuum space. The current flow is therefore almost zero. This sign of applied potential is called Reverse Bias.

If you look in a basic textbook on Physics or Electronics you may find an analysis of the behaviour of a pair of metal plates, one of which is heated. This leads to an algebraic expression that describes how the current tends to vary with the applied voltage. This has the form

where is the anode-cathode current, is the potential difference between anode and cathode, and is a factor whose value depends on the size and shape of the anode, cathode, etc. This relationship is called Childs Law.

However we should treat this result with caution as it makes some simplifying assumptions that often do not describe a practical valve very well. It also ignores the small leakage of current we described earlier that occurs even if we dont apply any external voltage. Despite these words of caution, the basic behaviour is that when forward biassed, the current tends to rise rapidly as we increase the voltage, and when reverse biassed, the current is almost nil. This is the basic rectifying behaviour which we associated with diodes in electronics. The variation of current with voltage is also nonlinear, and the device does not obey Ohms Law. Hence we can also use valve diodes as nonlinear devices.

It is quite common for the makers to put two actual diode arrangements into one vacuum container, and hence make a valve that functions as a pair of diodes. Typically, the circuit symbols for these show two anodes and one cathode. since that is the usual arrangement.

The Triode

Diodes are useful for tasks like rectifying, but to obtain gain and be able to amplify signals different forms of valve were developed. The simplest of these is the Triode. As the name implies, this has three electrodes whereas a diode has just two. The extra electrode is called a Grid as this describes its usual physical form.

The above diagram represents a triode arrangement. The grid is a fine mesh or winding of wire, placed in between the anode and cathode. Usually it is located much nearer to the cathode than the anode.

We can now consider applying potential differences in two ways: , a potential difference between anode and cathode.

, a potential difference between grid and cathode.

These are applied as indicated in the above diagram.

Electrons in the space charge region near the cathode now experience an electric field which has two components - that due to the anode, and that due to the grid.

The electric field produced by a potential difference depends on how close together the objects are located. Since the grid is close to the cathode, a given voltage on the grid exerts much more force on electrons near the cathode than would the same voltage on the more distant anode. This means the current is much more sensitive to changes in the grid voltage than to changes in the anode voltage.

To understand what this implies we can use an example. Lets assume that we have a triode where the distance between the anode and cathode is 10 mm, but the grid is just 1 mm from the cathode. We start off by applying a potential of +200 Volts to the anode with respect to the cathode. This has the effect of producing an electric field near the cathode of +200/10 = 20 Volts/mm which tends to try and accelerate electrons in the space towards the anode. The result of this field will be to tend to set up a steady flow of electrons, and hence a large anode-cathode current.

However, if we now apply a potential of, say, -10 Volts to the grid with respect to the cathode this produces an electric field near the cathode of -10/1 = -10 Volts/mm.

One of the basic rules of electromagnetic fields in space is Field Superposition. This means that we can just add together the fields produced by different elements or objects provided we get the signs and directions correct. In this case, the values above mean that wed get a resulting field near the cathode of 20 - 10 = 10 Volts/mm. The result of the -10 Volts on the grid is almost as if we had turned down the anode voltage attracting the electrons by 100 Volts! Indeed, if wed increased the grid voltage to -20 Volts, the electrons near the cathode would see no net electric field due to the presence of the anode and grid, and the flow from cathode to anode would drop to almost zero.

As a result of the field superposition, and having the grid near the cathode, we can therefore use relatively small voltage variations on the grid to control, vary, or even cut off, the anode-cathode current level. This behaviour is the basis of how we can then use valves like a Triode to amplify signals. A small change in grid voltage can be used to produce much larger variations in voltage and current at the anode.

We can use a modified version of the Childs Law equation to estimate what we can expect the anode-cathode current we get to approximately be

when forward biassed, and zero as with the diode when reverse biassed. However in this case forward bias means that , so depends on both the applied potentials.

The value is often called the valves Amplification Factor. The value for a given design of valve will depend on various details the obvious one being that we get a higher value if we can hold the grid very near to the cathode. However there are all kinds of detailed effects and problems I have ignored here, which also limit what can be made. In practice, real triodes tend to have amplification factor values somewhere in the range 10 100.

Since we try to put the grid near the cathode, we can expect it to be in or near the region where the space charge density is significant. Hence there is a tendency for the grid to pick up a flow of electrons from the space charge cloud of electrons, unless we apply a large enough negative voltage to the grid to push the electrons away, close to the cathode, and cut off the flow through the valve. This is leads to one of the possible difficulties of trying to make a triode with a high amplification factor. It tends to mean putting the grid closer to the cathode, where the charge density is high, and more electrons then tend to hit the grid.

In normal use, most of the electrons tend to fly through the gaps between the wires of the grid, but some will hit the grid and be lost from the anode-cathode current. Cutting down on the size of the wires in the grid, or placing them further apart may reduce the grid current. Alas, it also tends to weaken the ability of the grid to apply an electric field to its surroundings, so reducing the amplification factor for that reason.AMPLIFIER & MORE COMPLICATED VALVES

If you have an interest in a topic like audio/hi-fi you may already know that valve amplifiers are still popular with some enthusiasts. These dont all use triodes. Instead, some use Pentodes. As the name implies, a pentode has five electrodes a cathode, an anode, and three grids. These extra grids are included to try and deal with some of the practical drawbacks of the triode.

The usual symbol for a pentode valve is shown above. The Control Grid is the electrode that acts in the same way as the single grid in a triode. i.e. it is where we input the signal that controls the anode-cathode current. The Suppressor Grid and Screen Grid are the additional electrodes. To understand their purpose lets consider some of the practical problems that arise with the triode valve. We can do this by using the circuits shown below.

The above illustration shows two simple arrangements that we might use as experimental amplifiers. In each case the intention is that we use the control (signal) grid for our input and the anode for our output. Usually, we want the amplifier to provide a reasonably large voltage gain. This means that when we change the input voltage, , by a small amount we wish to arrange that the output, , changes by a much larger amount. The arrangement on the left uses a triode. That on the right uses a pentode. Note that the above circuits arent actually very useful as practical amplifiers, so if you look in textbooks you will see various, more complex, arrangements which differ in detail from the above. However the simple circuits illustrated above will serve to show the functions of the new grids.

Since we have arranged for both circuits to show a high gain, we can expect that when we change by a small amount, , will change by a much larger amount. Now in practice the electrodes are all conducting objects placed close together. This means there will be some capacitance between them. The result is that to change the potential differences between the electrodes we have to charge/discharge these capacitances. This means that we may have to provide quite high input currents to the control grid if we wish to change the grid voltage (and hence also the anode voltage very quickly. This tends to restrict the ability of the triode to amplify at high frequencies.

The pentodes Screen Grid is used as an electrostatic screen in between the control grid and the anode. This grid is held at a steady potential midway between that on the anode and the cathode or control grid, so tends to shield the control grid from seeing the changes in potential of the anode. Hence it tends to reduce the effective capacitance between the control grid and the anode. The result is that less current is needed to drive the input signal variations of the control grid, making it easier for the valve to be used at higher frequencies than a triode.

Some valves only have two grids the control grid and the screen grid and are called tetrodes. However simple tetrodes have their own problems. Some of the electrons which travel from the cathode and hit the anode may arrive with enough kinetic energy to knock free some electrons from the anode. These may then interfere with the valve operation in various ways. Any that remain around the anode may produce an unwanted negative space charge which will deter fresh electrons from arriving from the cathode. Others may strike the grids, or even the cathode, again producing unwanted flows of charge that degrade the operation of the valve. This problem can become particularly noticable if we allow the variations of the anode voltage to swing the anodes potential to being negative compared with the screen grid. When this happens we could get quite a large unwanted flow of electrons from the anode to the screen, and the result is a distortion or kink in the way the anode voltage varies as we change the input voltage on the control grid.

The Suppressor Grid is employed to catch electrons that have been released from the anode. This grid uses just a few wires, placed near the anode, and usually connected to the cathode potential. Most of the electrons coming from the cathode fly through the gaps in all the grids as they have been given a lot of kinetic energy by the cathode-anode potential difference. So most of them whizz past the wires in the grids. However most of the electrons knocked out of the anode will have a relatively small amount of kinetic energy, As a result, they tend either to fall back to the anode, or drift into the nearest grid which is now the suppressor grid. Having done this, they dont have any effect on the input current required to drive the control grid, and have little effect on the valves gain, etc.

The above explanations are quite simplified, but should indicate why pentodes may be preferred over triodes of we wish to obtain high gain and high bandwidth. The details of the bias arrangements in practical circuits will vary according to the requirements (and the preferences of the designer!) but the basic idea is that: The Control Grid is normally used to input the signal to be amplified

The Suppressor Grid is normally connected to the same potential as the cathode so as to sweep away any unwanted electrons liberated from the anode.

The Screen Grid is normally connected to a steady potential somewhere in between the anode and cathode potentials, and shields the input signal from seeing a large input capacitance.

In practice it is quite common for a single valve envelope to contain two actual valve devices. This helps keep down the weight and size of the devices, and may allow them to share a heating arrangement for their cathodes.

The above shows an example of a simple, perhaps old-fashioned, hi-fi power amplifier design from the 1950s/1960s. The details of this circuit are too complex to deal with here, but if you examine the circuit diagram you can see that although it shows four gain devices, they are actually paired. The valves shown as V3A and V3B are in the same glass envelope and sold as an ECL86. Similarly, V4A and V4B are another ECL86.

You will also see that the above diagram does show the output devices (V3B and V4B) as only having two grids, which implies they are tetrodes. In fact audio output valves often employed another form of valve called the Beam Tetrode. The most well-known examples in the UK being the KT66 and KT88. These tetrodes only have connections for four electrodes, but they include a pair of beam forming plates that direct the electron flows. These plates are internally connected to the cathode, and serve the same function as the suppressor grid. The KT in the type numbers for the KT66 and KT88 stands for Kinkless Tetrode. It lets us know that although these valves only apparently have four electrodes, they have been modified internally to remove the kink in the transfer curve which I mentioned earlier.

The above is only a very brief introduction to the family of thermionic valves. Although they have largely been replaced by solid-state devices in most applications, valves are still used for special purposes, and some audio enthusiasts continue to prefer valve amplifiers for listening to music.

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