Syllabus Sections:-

Solid State Devices

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 stopped 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 IL 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.

2I1  16 Recall that a Zener diode will conduct when the applied reverse bias potential is above its designed value and identify its V/I characteristic curve.


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


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


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.


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.


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.

Here is an example of the biasing of a PNP transistor  Very similar to the NPN but the decoupling capacitor has been omitted in the drawing.

There are other factors affecting biasing and you can read about these in the page 33 under the section called "Bias stability"


Lets take a look at the circuit symbols for the Mosfet single and dual gate.


Above left is the symbol for a single gate mosfet and above right is for the dual gate mosfet.
The G indicate GATE and the D indicates Drain and the S indicates Source in the single gate mosfet.
In the dual gate the only difference is that there vare two gates which can be used independently and indicated by G1 and G2


(Drawing of a FET in depletion mode with biasing)

It is surprising to note that the 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.

Note: in an FET the Drain takes the place of the Collector, where as the Source takes the place of the Emitter and Gate takes the place the Base of an NPN transistor.

There is much more description on the Mosfet in the page 36 and page 37 together with more detailed drawings to have a look at.



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.


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.

NOTE: Circuits shown will use an NPN transistor connected in common emitter / common source mode.

2I4  17 
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 de-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 through the collector is controlled by the amount of base current.

The diagram above shows the basic NPN transistors with connections B base C collector and E emitter. It must be understood that the Base terminal is always positive with respect to the Emitter and that the Collector supply voltage is positive with respect to the Emitter, it is hoped you can see the little + and - in the image. For a bipolar NPN transistor to conduct the Collector is always a greater positive value than both the Base and the Emitter values. With this in mind we can continue to discuss the NPN transistor.

Electrons flowing through the Base is called the base current Ib similarly flowing though the Collector is the Ic and from the Emitter a current of Ie. As you should expect there is a relationship between these three currents !! The amount of current flowing through the Emitter equals  the current through the collector plus that through the base so Ie = Ic + Ib.

Thus you should be able to see that there is a relation ship between the Collector current and the Base current and both have an effect on the Emitter current.

We now introduce a new term "current gain"  and the symbol used to indicate current gain is hfe ( which is also known as and in an exam could be either !!)

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 current flowing through the collector = the hfe (or ) times the current flowing in the base.

The equation for the calculation of gain is Ic = hfe x Ib  or  Ic = x Ib   

So in a NPN Transistor it is the movement of the  "negative electrons" through the Base (the input circuit) that causes the transistor to turn on and provide the energy to cause a link between the Collector and Emitter circuit (the output circuit which could be on the emitter or collector). This link between the input and output circuits is the feature of transistor action and which causes the amplification  of the input signal on the Base to the output circuit all due to the control which the Base exerts upon the Collector to Emitter current.

Tolerances of hfe/ values are very large, so that even transistors of the same type or of the same batch may have widely different gains. Published figures of transistor gains are only typical values. If an exact gain is wanted then the transistor will have to be tested and selected to do the job required. 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.




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.


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.

2I4   17  continued

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%


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%.



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 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.

2I5  18  Understand the feedback requirement to sustain oscillations in an oscillator.

Oscillator purpose is to provide accurate and controllable waves of know frequency. To sustain oscillations feed back is required from the output to the input in a controlled way to limit the output to just a single frequency.

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