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Syllabus Sections:- Solid State Devices
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 :-
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. 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.5 Understand the basics of biasing NPN and PNP bipolar transistors and FET transistors (including dual gate devices). BIASING OF TRANSISTORS. There are commonly 3 types of bias systems for transistors they are: - 1. SIMPLE
2. CURRENT FEEDBACK TYPE
3. FIXED VOLTAGE TYPE
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 FET BIASING.
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. ----------------------------------------------------------------------------------------- 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.
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.
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. MEASURING GAINS OF TRANSISTORS. APPLICATIONS OF BIPOLAR TRANSISTORS. SUMMARY. Bipolar transistors
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.
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.
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