Syllabus Sections:-

Solid State Devices

3n.1 Understand that doping of semiconductor material (silicon and germanium) produces p-type (electron deficient) and n-type (electron rich) semiconductors.

In modern electronic equipment today there are many components that are in the "family" called "semiconductor".

The name "semiconductor" correctly implies that a material is neither a conductor nor an insulator. An insulator has high resistance to the passage of electrons and a conductor has low resistance to the passage of electrons. The resistance of the "semiconductor" lies between those two extremes.

The two materials in general use that form semiconductors are SILICON and GERMANIUM.

Molecular structure

Just for a moment we have to split the atom of silicon and germanium. The atoms of these two elements have a nucleus and respectively 3 and 4 rings of electrons. It is the outer rings that we are concerned with as these both have 4 electrons. Both of these outer rings have the capability of joining with an adjacent atom to form a crystal lattice.

What happen in the other rings is of no consequence with regards to this course. It is important that you understand that each atom has 4 electrons in the outer ring and that those in the silicon outer ring being nearer to the nucleus which has a positive charge has a greater hold than those in the outer ring of the germanium atom simply because they are that little bit further away from the nucleus and thus the attraction between them is less.

This distance from the nucleus is important as in silicon the outer electrons are NOT FREE to move from the lattice when they have combined with other atoms of silicon (and are thus an insulator) where as those in the germanium being further away can become detached (and so they have a tendency to being a conductor).

Making semiconductor material

This complex process takes both silicon and germanium (separately) and refines them to high standards of purity and then after all that hard work makes them impure again but in very controlled conditions called "doping".

This "doping" process introduces atoms which have either 5 or 3 electrons in this outer ring. The material where there are 5 electron in the outer ring is given the name N - Type Material as it is and it appears to be NEGATIVELY charged due to the "extra" electron (electron rich) where as if there is 3 electrons in the outer ring the material is given the name P - Type Material and appears to be positively charged due to an apparent absence of electrons (electron deficient).

Both Silicon and germanium can be doped to form N or P type material.

P - Type Material appears to be positively charged

This absence of electrons in P - Type material leaves a space in the outer ring that can be filled by another electron and is as such called a "hole".


Understand current flow in terms of electron and hole movement. Understand the formation and effect of the depletion layer.

We will take these two parts of the syllabus together.

Let us look at the situation where we have a two pieces of semiconductor material, one piece N-Type and the other P-Type that are fused together. Now we have the situation, just like a party where free spirited boys and girls arrive in different cars from different directions and enter the hall via different doors but they all have one aim look for partners.

"depletion layer"

Thus we have the holes of the P-type free to move towards the electrons of the N-Type and the electrons are free to move towards the holes. So, just like the boys and girls sorting themselves out, a scrum occurs until pairing has taken place, the Holes and the Electrons move about in an action called "diffusion" across the material junction, which forms an area, just like the party hall, called the "depletion layer" where the free electrons have jumped into the migrated holes and eventually the holes in this area are filled which stops any further migration (diffusion).

You have learned in the ILC that a flow of electrons is a current thus as there is a movement of electrons and holes there is in essence a current flow but it is not sustained.

Summary:

  • Silicon and Germanium are the most common materials that make up semiconductors and each can be N-Type or P-Type semiconductors.

  • The N-Type material has a surplus of electrons and thus appears negatively charged.

  • The P-Type material has one too few electrons - a HOLE - and thus appears positively charged.

  • when P and N type material are fused together diffusion of holes and electrons occurs forming a "depletion layer" which stabilizes the junction between the two materials as holes are filled by electrons preventing further diffusion.

  • the movement of holes and electrons is in essence a current flow until holes are filled by electrons.

Understand how the p-n junction forms a semiconductor diode.

P-N JUNCTION

If we take our fused N and P type material mentioned above and now attach wires to each of the P and N type material we have a JUNCTION DIODE or semiconductor diode. The material will have already formed its depletion layer and is waiting to do something useful. It is waiting for energy to be applied from an external source such as a cell or battery.

Understand that an applied potential difference can cause electrons to flow across the PN junction (forward bias) or prevent electron flow (reverse bias) depending on polarity.

ELECTRON FLOW the flow of electrons from Negative to Positive

forward bias

If we connect up a circuit such as shown at the left the electrons from the N will be attracted by the batteries' positive terminal and this attraction will be great enough to over come the depletion layer which previously stoped any further activity between the electrons and holes and eventually the electrons will find their way into the connecting wire and to the positive terminal.

At the same time this will leave holes in the N material which will be filled by more electrons entering the circuit from the negative terminal. A flow of electrons will then continue with the rate of the flow restricted by the resistor in the circuit. This circuit is said to forward bias the diode. It was said earlier that there needed to be energy applied to the circuit for current to flow and it is found that no current flows until a pressure (voltage) of about 0.6V for silicon and 0.3V for germanium diodes had been applied. This voltage is called the "barrier voltage" and is the energy that needs to be applied to help the electrons through the depletion layer.

CONVENTIONAL CURRENT FLOW ------- Current Flow from Positive to Negative

You may recall the diagrams above from your ILC course. Circuit A represents a reverse biased diode whilst Circuit B represents a forward biased diode.

Reverse bias

If the diode is reversed biased, (as in circuit "A" above) no (or negligible) current flow will occur and electrons will build up at the battery end of the N material and Holes at the Battery end of the P material. This condition is called reverse bias and as a generalization other than "leakage current" no current flows.

Peak Inverse Voltage PIV

Diodes when used in home construction must be rated properly for the use to which they are to be put.

You must consider :-

  1. Peak Reverse Voltage and

  2. Maximum Average Current.

Peak Inverse Voltage ( or Peak Reverse Voltage ) is the maximum voltage that a diode can withstand in the reverse direction without failing and starting to conduct. If you exceed the PIV the diode may be destroyed. Thus the diodes must have a PIV rating that is higher than the maximum voltage that will be applied to them when reverse biased.

In a DC only circuits, diodes should have a Peak Inverse Voltage rating greater than the highest voltage to which diode will be exposed.

In an AC circuits, such as power supplies, diodes should have a Peak Inverse Voltage rating up to 2.8 times the maximum RMS voltage (RMS is 0.707 of the peak voltage) of the transformer's secondary winding (depending upon the rectifier design).

Maximum Average Forward Current is the average forward current that a diode can conduct without being damaged.

In DC only circuits the Maximum Average Current is considered to be the current that the diode will continuously conduct.

In AC circuits such as power supplies the Maximum Average Current Rating of a diode should be twice the DC current that the supply will deliver at full load. For example; If a power supply can deliver 1 amp the rectifier diodes should have at least a 2 amp current rating.


3n.2 Recall that a Zener diode will conduct when the reverse bias potential is above its designed value and identify its V/I characteristic curve.

ZENER DIODE

Schematic symbol

In the standard diode we have established that only a negligible current flows when the diode is reverse biased the "leakage current", but if the voltage is increased then it can reach a value when the diode just cannot prevent a flow of current and the diode can fail dramatically.

With the ZENER diode, as the reverse bias voltage is increased from zero it acts the same as any other diode and resists the passage of all but leakage current. Then when the voltage rises to its designed value, the depletion layer allows current to flow and the voltage remains at a stable level. So long as the current passing through the device does not exceed its rated handling capability the ZENER continues to function. This is achieved "somewhere" in the circuit with a current limiting resistor. However if the current passing is too great then the zener will suffer from failure.


3n.3 Understand that the depletion layer in a reverse biased diode forms the dielectric of a capacitor and that the magnitude of the reverse bias affects the width of the layer and the capacitance.

"varicap diode"

This part of the syllabus is referring to a type of diode called the Variable Capacitance diode or "varicap diode".

All diodes to a greater or lesser extent exhibit the phenomenon of an increasing width of the depletion layer when a reverse bias is applied. However Diodes that are made to be especially susceptible to the widening of the depletion layer, which in turn varies the capacitance associated with the diode.
Diode D2 in the diagram B is acting as a varactor diode.

More voltage less capacitance

As the reverse bias (voltage) is increased so the depletion layer widens and the capacitance decreases

The depletion layer can be thought of as the plates of a capacitor, and just like a capacitor by widening the gap between the plates the capacitance drops. With the Varactor diode as the reverse bias is increased so the depletion layer widens and the capacitance decreases and by reducing this applied reverse bias the capacitance then increases.

The depletion layer in the diode is acting as not only the plates of a capacitor but also the dielectric of a capacitor.


3n.4 Understand the 3 layer model of the transistor (npn and pnp) and the channel model of the FET.

The 3 layer model of the transistor

Bipolar transistors each have 2 junctions and 3 separate connections. The NPN transistor has a thin layer of P type material sandwiched between 2 thicker N type layers. In fact in the manufacture they are not separate layers, as such, just area of the N and P doping making depletion layers at each of the two N / P boundaries.

The PNP transistor has a thin layer of N type material sandwiched between 2 thicker P type Layers. As with the NPN the N and P doping makes the depletion layers at each of the two N / P boundaries.

As the drawings show the layer which forms the middle of the sandwich is called the BASE, the others are called EMITTER and COLLECTOR respectively.

The arrowhead on the emitter symbol distinguishes the transistor as being either NPN or PNP.

The arrowhead also points in the conventional direction of current flow.

Bipolar transistors are constructed such that the junctions are so close together that electrons flowing across the base/emitter junction will control the flow of electrons in the collector/emitter junction. The result being that a small amount of current flowing in the base/emitter junction will control a much larger current flowing in the collector/emitter junction.

When we were discussing the diode a junction is said to be forward biased when the P type material is connected to the positive supply and the N type material is connected to the negative supply.

Look at the drawing of an NPN transistor connected across a 9V battery with no BIAS voltage or nothing connected to its base connection, no current can flow between the collector / emitter junction because it is reverse biased (or turned off) (Positive battery connection to the N-Type material).

No current can flow between the collector/base junction because it too is reversed biased (or turned off) the transistor behaves as if it where 2 diodes connected back to back.

When the transistor is forward biased or turned on (0.6V for silicon, 0.3V for germanium), current flows across the base/emitter junction, but because the collector/emitter junction is physically so close, current flows across this junction also. With both junctions conducting most of the current flows across the collector/emitter junction since this is the path of least resistance, hence the base current is less than the collector/emitter current.

The transistor now no longer behaves like 2 diodes because the base current makes the collector current flow despite being reverse biased. The current flowing between the collector/emitter is much greater than the current flowing through the base/emitter. (Typically 25 to 800 times greater) and standards are improving all the time.

Understand the channel model of the FET.

The N type FET (Field Effect Transistor)

The FET is another family of transistors and is available in a number of different forms.
As with the bipolar transistors N and P channel type exist but we will consider the N type as shown to the left.
The P doping creates a GATE by encircling the N type material and causing an increase in the depletion layer thereby shutting off any flow of electrons Drain to Source.

NOTE: The "looped" connection shown in the diagram is there only to indicate a coupling between the two cross-sectional areas of P doped material.

In the real world the connections are as shown in the bottom diagram on the left. The main current flow is drain to source with a voltage being applied to the gate to control the drain to source flow of current.

The FET is a voltage controlled device where as the NPN and PNP transistors are current control devices. Bias voltages are applied to the GATE and thus to the SUBSTRATE P material it controls the flow of current between SOURCE and DRAIN.

With no voltage applied to the Gate current would flow through the FET Drain to Source.

To stop the flow of current a NEGATIVE voltage have to be applied to the gate that is lower than the source voltage which is usually at 0V to repel electrons from the channel between Drain and Source, so reducing it's conductivity. As the voltage becomes more negative the conductivity of the FET stops at what is called the pinch off voltage as it stops the flow of electrons.

Thus a POSITIVE voltage applied to the gate has the effect of attracting more electrons into the channel, and so increasing its conductivity.

So generally the Gate voltage applied will be lower than the Source voltage.

Both N and P type devices can be made. When the voltage applied at the gate has the effect of cutting down the current flow in the channel, the operation is said to be IN THE DEPLETION MODE

The gate is therefore normally forward biased with respect to the source. The increased gate voltage is used to increase current flow, the operation of the FET is said to be IN ENHANCEMENT MODE.

In most cases, ENHANCEMENT MODE devices are made without the conducting channel.

  1. Always keep new mosfet's in their conductive foam around their leads until soldered in place.
  2. Always short the leads of a mosfet together before un-soldering it.
  3. Never touch the mosfet leads with your fingers
  4. Never plug a mosfet into a holder when the circuit is switched on.

FET'S can be used in circuits similar to bipolar transistors but they give LOW VOLTAGE GAIN, and are only used when their peculiar advantages are required.

FET Advantages

  1. FET'S have a very high input resistance
  2. FET'S perform very well as switches, with channel resistances switching between a few hundred ohms to several Megohms as the gate voltage is varied.

  3. If a graph is drawn of channel current Ids(current drain source) against Vgs (Voltage gate source). The graph is noticeably curved in a shape called a square law. This type of characteristic is particularly useful for signal mixers in superhet receivers.

Dual gate mosfets are used as mixers and RF amplifiers in FM receivers. The shape of their characteristic also gives less distortion in power amplifiers, and HIGH POWER FET'S are now commonly used in HI FI and also find application in the power output stages of transmitters.

FET SUMMARY

FET's are devices that depend on a junction action different to that of bipolar transistors.

JUNCTION FET'S are usually operated with their single junction Reverse biased.

MOSFET'S have almost infinite gate resistance, and the leads should not be touched unless first shorted.

FET'S are used in applications where their high input resistance, good switching characteristics and low noise factor outweigh their poor voltage gain.


3n.5 Understand the basics of biasing bipolar and FET transistors (including dual gate devices).

BIASING OF TRANSISTORS.

There are commonly 3 types of bias systems for transistors they are: -

1. SIMPLE

So called as you can see that the Emitter is common to both input and output

Firstly, the SIMPLEST uses a single resistor connected between the supply rail (+ for NPN - for PNP) and the base of the bipolar transistor. This type of bias is seldom used for linear amplification these days, because it is difficult to find a suitable value of bias resistor. In this simple system the value of resistor depends on the value of current gain or Hfe of the transistor so that a bias resistor suitable for one transistor will not work properly with another, even if its the same type and number, as the values of Hfe are wide and varied. Also the value of resistor maybe critical so that one preferred value of resistor maybe too high, the next one down, too low.

The simple system is unsuitable if the transistor is to work in varying degrees of temperature, because the voltage needed between the base and emitter for a given collector current, decreases as the transistor warms up. As the simple system cannot compensate for this, so the transistor turns harder on, so increasing the collector current further still, unless the current is limited by a collector load resistor, thermal runaway occurs and the transistor is destroyed.

2. CURRENT FEEDBACK TYPE

THE CURRENT FEEDBACK bias system represents a considerable improvement over the simple system, because the bias resistor is returned to the collector of the transistor rather than the supply rail. This small change makes the bias to some extent self-adjusting, so stabilising the bias.

The connection of the bias resistor causes DC feedback, which means that the level of DC voltage at the COLLECTOR affects the amount of DC BIAS CURRENT at the BASE of the transistor.

Let's see what happens. A change in either the transistor itself or the load, which causes the collector current to increase will, because of the presence of the collector resistor, cause the collector voltage to drop, because the voltage is less where the base resistor is connected, the voltage and current at the base is less, this drop off in base current will return the collector current to somewhere near to its original value.

Alternatively a change causing the collector current to drop will cause the base current to rise, so re-instating the collector current. All NEGATIVE FEEDBACK SYSTEMS work in a similar way, keeping conditions unchanged despite other variations (AC feedback which has the effect of reducing stage gain will be considered later).

The disadvantage of this arrangement is that the AC feedback that results reduces the stage gain.

3. FIXED VOLTAGE TYPE

The FIXED VOLTAGE bias system is the most commonly used of all. A pair of resistors forms a potential divider across the supply to set the voltage at the base terminal, and a resistor placed in series with the emitter controls emitter current flow by DC NEGATIVE FEEDBACK.

If the transistor has a higher gain, then it tends to pass more collector current, which in turn results in more emitter current. This increases the voltage drop across the emitter resistor, and raises the emitter voltage.

As the base voltage is fixed and the emitter voltage rises, there is less voltage across the base-emitter junction, which then tends to turn the transistor off. It meets equilibrium at the (designed) operating point.

AC feedback does not occur because of the emitter capacitor which bypasses AC components to earth, not allowing any signal voltages at the emitter to oppose the signal at the base. Note that  if the capacitor is removed all the gain disappears !!

In this type of circuit the replacement by one transistor with another, has little effect on the level of steady bias voltage at the collector or base, This biasing arrangement is therefore ideal for mass produced circuits which must behave correctly even when fitted with substitute transistors.

FET BIASING.
(Drawing of a FET in depletion mode with biasing)

Gate must be negatively biased with respect to the source

For correct bias of an FET, the voltage at the gate must be negatively biased with respect to the source voltage - or to put it another way, the source voltage must be positive with respect to the gate voltage. In the circuit drawn the positive voltage is derived from the voltage drop across the resistor in series with source. The gate voltage is kept at zero volts or ground level by the resistor connected from the gate to the negative rail.

Biasing of an FET is slightly less complex than for BIPOLAR transistors

SUMMARY.

The purpose of biasing a transistor is to set its output current to a value which permits the best use of its transfer characteristic.

For a linear amplifier having a resistive load, the most useful bias setting is when the collector voltage is close to half the supply, (Class A).

The biasing method chosen must be stable, and thermal runaway must not occur.

Bias failure can be caused by either a short circuit or open circuit bias components. Either will greatly affect the working of the transistor as an amplifier.

DUAL GATE FET

The FET can also be made as a dual gate device. The original diagram of the FET is shown above (left) and the Dual Gate FET shown above (right). Note in the dual gate that the signal is on one gate and the main bias is on the other gate.


3n.6 Identify different types of small signal amplifiers (e.g. common emitter (source), emitter follower and common base) and explain their operation in terms of input and output impedances, current gain, voltage gain and phase change.

The diagrams below you must be able to identify in the exam - so look for the differences.

General information

Impedances / Gain

As mentioned above the Common Emitter is so called as the emitter is common to both the input and the output in that with regard to signals, it is coupled to ground via a capacitor and both input and output have a grounded connection. The emitter resistor does have some involvement in connecting the emitter to ground, but its main function is to set the correct DC operating conditions. The output signal is an inverted, larger copy of the original.

Phase change There is a phase change between input and output as shown in the drawing it being 1800 out of phase.

Voltages gain High (about 100)

Current gain High (50 - 800)

Input resistance Medium (about 5k)

Output resistance High (about 40k)

The Emitter follower is so called as the signal that comes from the emitter is a copy of the input signal. Although there is no voltage gain, (a very small LOSS may be experienced), there is a considerable current gain. Circuits such as this one have an application where a very small load should be applied to the preceding stage, whilst allowing a good drive signal to the following stage. A typical use is for a buffer amplifier for an oscillator, where the oscillator must be loaded very little by the stage it is driving to avoid frequency changes.

Phase change There is NO phase change between input and out put as shown in the drawing.

Voltages gain Unity (1)

Current gain High (50 - 800)

Input resistance High (several tens of k)

Output resistance Low

The Common Base is so called as the base is common to both the input and the output in that with regard to signals, it is taken to ground via a capacitor and both input and output have a grounded connections. The output signal is a larger copy of the original.

Phase change There is NO phase change between input and out put as shown in the drawing.

Voltages gain Medium (10 - 50)

Current gain Unity (1)

Input resistance Low (about 50)

Output resistance High (about 1M)

CURRENT GAIN hfe ( previously it was )

The amount of current flowing between the collector and the emitter of a BIPOLAR transistor is much greater than the current flowing between the base and the emitter, but the amount of current flowing in the collector is controlled by the base current. The ratio COLLECTOR CURRENT TO BASE CURRENT varies according to the collector current flowing. (An undesirable characteristic!) All comments regarding hfe in the remainder of this section must be understood in relation to this unwanted effect!

The constant is commonly known as current gain and the symbol used to indicate current gain is hfe ( previously it was ). A low gain transistor might have a gain of around 20 - 50, Power transistors sometimes have a gain of only 10, a high gain transistor might have a gain of 300 - 800 or even more.

The equation for the calculation of gain is

The current flowing in the collector = the hfe (gain) times the current flowing in the base.

Tolerance values of hfe are very large, so that transistors of the same type or even the same batch may have widely different hfe's. Published figures of transistor gains are only typical values. If an exact gain is wanted then the transistor will have to be tested. The secret is not to design a circuit where the maximum gain is required from a transistor, but to design such that many different devices can be used for the same circuit.

MEASURING GAINS OF TRANSISTORS. APPLICATIONS OF BIPOLAR TRANSISTORS.

TRANSISTOR FAILURE.

SUMMARY.

Bipolar transistors

  • Bipolar transistors consist of three regions - EMITTER, BASE, COLLECTOR, - with 2 junctions.

  • Current flows between collector and emitter only when current flows between base and emitter.

  • A transistor has GAIN when collector current divided by the base current is greater than 1.

  • Connecting transistors in different ways changes gain and input/output impedances.

FIELD EFFECT TRANSISTORS.

To be strictly correct, the so-called FIELD EFFECT TRANSISTOR is not a transistor at all, as the word TRANSISTOR is derived from TRANSFER RESISTOR and the FET doesn't work like that at all. The FET relies upon the presence and the effects of an electric field.

There are 2 types of FET - The JUNCTION FET and the METAL OXIDE SILICON FET or MOSFET.

Both work by controlling the flow of current carriers in a narrow channel of silicon. The main difference between them lies in the way the flow is controlled.

Firstly the JUNCTION FET. A tiny bar of N or P type silicon has a junction formed near to one end. Connections are formed at either end of the silicon bar (see drawing) and also to the junction material (p type for N type FET).

The P type connection is called the GATE, the end of the bar nearest the gate is called the SOURCE, and the connection at the other end is called the DRAIN.

A junction FET is normally used with the junction reverse biased (it has a negative voltage on it for an N channel as opposed to what you might expect a positive one) so that a few moving carriers are around the junction (keeping it turned off) making the bar of silicon itself a poor conductor.

With less reverse bias (or less negative volts) on the junction the silicon bar will conduct better, and so on as the amount of reverse bias on the junction decreases the FET conducts better.

When a VOLTAGE is connected across the SOURCE and DRAIN the amount of current flowing between them depends on the amount of reverse bias (or negative volts) on the GATE and the ratio SOURCE - DRAIN CURRENT/GATE VOLTAGE is called the MUTUAL CONDUCTANCE the symbol for which is Gm. This quantity is a measure of the effectiveness of the FET as an amplifier of current flow.

Because the GATE is REVERSE BIASED, practically NO GATE CURRENT FLOWS, so that the RESISTANCE between GATE and SOURCE is VERY HIGH, much HIGHER than a BASE EMITTER junction of a BIPOLAR transistor, This uncommonly high resistance is put to good use, for instance in voltage measuring circuits NO LOAD is put on the circuit being measured.


3n.7 Recall the characteristics and typical circuit diagrams of different classes of amplifiers (i.e. A, B, A/B and C).

Classes of amplification

Several methods exist for biasing transistors; typically class A, B, C.

Class A


Class A. is biased so that the collector voltage never bottoms, nor is the current flow cut off. Output current flows during the whole AC cycle; it is this bias that is used for linear voltage amplification in low power stages. IT IS ALSO USED IN SOME HI FI AUDIO AMPLIFIERS AS IT HAS THE LOWEST DISTORTION COMPARED TO B, AB, AND C CLASSES.

Class A suffers from 2 disadvantages.

1. Current flows in the transistor at all times, so the transistor needs to dissipate heat. (Power stages will get VERY hot!).

2. This loss of power in the transistor inevitably means less power is available for dissipation into the load. Even under ideal conditions with everything matched, efficiency is only 33%

CLASS B


The bias of a class B amp is so set that only half of the AC cycle is amplified. The bias is such that with no input signal, the transistor is biased, but at a level insufficient to turn it on. Application of the input signal (on positive half cycles) drives the transistor base into conduction, causing an amplified flow of collector current.

For RF applications a tuned circuit as the collector load would be used, which stores energy and releases it to replace the other half of the cycle.

The efficiency of this type of circuit is approximately 50%.

CLASS AB


SO CALLED BECAUSE THE AMPLIFIER STAGE DOES 2 THINGS. 1 IT AMPLIFIES THE WHOLE WAVEFORM. (LIKE CLASS A) 2 IT EMPLOYS 2 TRANSISTORS TO AMPLIFY EACH HALF OF THE WAVEFORM (LIKE CLASS B) SO HENCE AB. POORLY DESIGNED AB AMPLIFIERS ARE KNOWN TO HAVE "CROSSOVER DISTORTION" WHEN THE AMPLIFIER IS NEITHER AMPLIFYING 1 HALF OR THE OTHER OF THE WAVEFORM

For audio applications the other half of the cycle is amplified by a second transistor, and transformers are used to split the incoming signal to each transistor, and recombines the amplified results. To avoid audio distortion, some forward bias is applied to the transistors, such that a small current flows in the absence of drive signal. As this results in operation that is neither true class A or B, it is known as class "AB".

CLASS C


Class C amplifiers amplify less than half the AC cycle. With no bias resistor from the positive rail only one to the negative rail. This resistor keeps the transistor turned off until the positive going signal voltage overcomes the negative bias and amplification takes place. Commonly, in medium to high power RF stages, this resistor will be replaced by an RF choke.

Efficiency of a Class C amplifier is quite good and typically, a stage efficiency of 66% may be obtained.

Once again as with Class B, for RF service the collector load resistor is replaced with a tuned circuit, the stored energy in it replaces the missing half cycle. The Class C amplifier is rich in harmonics; the tuned circuit in the collector selects the desired frequency. Note that such is the level of harmonics that further filtering would be required before this signal were applied to an antenna.



3n.8 Understand the concept of the efficiency of an amplifier stage and be able to estimate expected RF output power for a given DC input power, given the stage's efficiency.

When dealing with a power amplifier stage it is possible to estimate the expected output RF power from the DC input power given the stage's efficiency.

If the DC input power is 100 watts with a known efficiency of 60%, then 60% of 100 will be the RF output power (60 watts of output).

The remainder of the input power is converted into heat, which must be safely removed from the output device by a heat sink (for transistors) or air flow (for thermionic valves).


THYRISTORS

Thyristors or to put it another way, SILICON CONTROLLED RECTIFIERS are like a diode in that they have an anode and cathode or positive and negative end. They only conduct when a pulse is received at the gate, and do not stop conducting until the current flowing through them is zero. They have many uses in electronics, typically power supply protection and motor speed control circuits.


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