September 21, 2009



The junction gate field-effect transistor (JFET or JUGFET) is the simplest type of field effect transister. It can be used as an electrically-controlled switch or as a voltage-controlled resistance . electric charge flowflows through a semiconducting channel between "source" and "drain" terminals. By applying a bias voltage to a "gate" terminal, the channel is "pinched", so that the electric current is impeded or switched off completely.


The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers (p-type), or of negative carriers (n-type). Contacts at each end form the source and drain. The gate (control) terminal has doping opposite to that of the channel, which it surrounds, so that there is a P-N junction at the interface. Terminals to connect with the outside are usually made ohmic.


JFET operation is like that of a garden hose. The flow of water through a hose can be controlled by squeezing it to reduce the cross section; the flow of electric charge through a JFET is controlled by constricting the current-carrying channel. The current depends also on the electric field between source and drain.


The JFET gate is sometimes drawn in the middle of the channel (instead of at the drain or source electrode as in these examples). This symmetry suggests that "drain" and "source" are interchangeable, so the symbol should be used only for those JFETs where they are indeed interchangeable (which is not true of all JFETs).

Officially, the style of the symbol should show the component inside a circle (representing the envelope of a discrete device). This is true in both the US and Europe. The symbol is usually drawn without the circle when drawing schematics of integrated circuits. More recently, the symbol is often drawn without its circle even for discrete devices.



The COMMON-COLLECTOR CONFIGURATION (CC) is used as a current driver for
impedance matching and is particularly useful in switching circuits. The CC is also referred to as an
emitter-follower and is equivalent to the electron-tube cathode follower. Both have high input impedance
and low output impedance.In the CC, the input is applied to the base, the output is taken from the emitter, and the collector is
the element common to both input and output.

Circuit diagram:-


GAIN is a term used to describe the amplification capabilities of an amplifier. It is basically a ratio
of output to input. The current gain for the three transistor configurations (CB, CE, and CC) are ALPHA
(a), BETA (b), and GAMMA (g), respectively


different properties of the three configurations.



The COMMON-EMITTER CONFIGURATION (CE) is the most frequently used configuration
in practical amplifier circuits, since it provides good voltage, current, and power gain. The input to the CE
is applied to the base-emitter circuit and the output is taken from the collector-emitter circuit, making the
emitter the element "common" to both input and output. The CE is set apart from the other configurations,
because it is the only configuration that provides a phase reversal between input and output signals.

Circuit diagram:-


The COMMON-BASE CONFIGURATION (CB) is mainly used for impedance matching, since it
has a low input resistance and a high output resistance. It also has a current gain of less than
In the CB, the input is applied to the emitter, the output is taken from the collector, and the base is
the element common to both input and output.

Silicon-controlled rectifier


A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state device that controls current. The name "silicon controlled rectifier" or SCR is Generel electronics's trade name for a type of thyrister. The SCR was developed by a team of power engineers led by Gordon Hall and commercialised by Frank W. "Bill" Gutzwiller in 1957.




An SCR is a type of rectifier, controlled by a logic gate signal. It is a four-layer, three-terminal device. A p-type layer acts as an anode and an n-type layer as a cathode; the p-type layer closer to the n-type (cathode) acts as a gate. It is unidirectional in nature.


It consists of a four layers pellet of P and N type semiconductor materials. Silicon is used as the intrinsic semiconductor to which the proper impurities are added. The junctions are either diffused or alloyed. The Planar construction is used for low power SCR's, here all the junctions are diffused. The Mesa type construction is used for high power SCR's. In this case junction J2 is obtained by diffusion method and then the outer two layers are alloyed to it because the PNPN pellet is required to handle large currents. It is properly braced with tungsten or molybdenum plates to provide greater mechanical strength. One of these plates is hard soldered to a copper stud, which is threaded for attachment of heat sink. The doping of PNPN will depend on the application of SCR.


In the normal "off" state, the device restricts current to the leakage current. When the gate-to-cathode voltage exceeds a certain threshold, the device turns "on" and conducts current. The device will remain in the "on" state even after gate current is removed so long as current through the device remains above the holding coupling. Once current falls below the holding current for an appropriate period of time, the device will switch "off". If the gate is pulsed and the current through the device is below the holding current, the device will remain in the "off" state.

If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge into the gate to trigger the device into the "on" state; this is referred to as "dv/dt triggering." This is usually prevented by limiting the rate of voltage rise across the device, perhaps by using asnubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly and the partially-triggered SCR may dissipate more power than is usual, possibly harming the device.

SCRs can also be triggered by increasing the forward voltage beyond their rated break down voltage (also called as break ver voltage), but again, this does not rapidly switch the entire device into conduction and so may be harmful so this mode of operation is also usually avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device to device.

SCRs are made with voltage ratings of up to 7,500 V, and with current ratings up to 3,000 RMS amperes per device. Some of the larger ones can take over 50 kA in single-pulse operation. SCRs are used in power switching, phase control, chopper, battery charger, and inverter circuits. Industrially they are applied to produce variable DC voltages for moters (from a few to several thousand HP) from AC line voltage. They control the bulk of the dimmers used in stage lighted, and can also be used in some electric vehicles to modulate the working voltage in a jacabson circuit. Another common application is phase control circuits used with inductive loads. SCRs can also be found in welding power suplies where they are used to maintain a constant output current or voltage. Large silicon-controlled rectifier assemblies with many individual devices connected in series are used in high voltage DC converter stations.

Two SCRs in "inverse parallel" are often used in place of a TRIAC for switching inductive loads on AC circuits. Because each SCR only conducts for half of the power cycle and is reverse-biased for the other half-cycle, turn-off of the SCRs is assured. By comparison, the TRIAC is capable of conducting current in both directions and assuring that it switches "off" during the brief zero-crossing of current can be difficult.

Typical electrostatic discharge (ESD) protection structures in integrated circuits produce a parasitic SCR. This SCR is undesired; if it is triggered by accident, the IC can go into latch up and potentially be destroyed.

September 20, 2009

Transistor codes..

NPN transistors
(typical use)
BC107NPNTO18100mA45V110300mWAudio, low powerBC182 BC547
BC108NPNTO18100mA20V110300mWGeneral purpose, low powerBC108C BC183 BC548
BC108CNPNTO18100mA20V420600mWGeneral purpose, low power
BC109NPNTO18200mA20V200300mWAudio (low noise), low powerBC184 BC549
BC182NPNTO92C100mA50V100350mWGeneral purpose, low powerBC107 BC182L
BC182LNPNTO92A100mA50V100350mWGeneral purpose, low powerBC107 BC182
BC547BNPNTO92C100mA45V200500mWAudio, low powerBC107B
BC548BNPNTO92C100mA30V220500mWGeneral purpose, low powerBC108B
BC549BNPNTO92C100mA30V240625mWAudio (low noise), low powerBC109
2N3053NPNTO39700mA40V50500mWGeneral purpose, low powerBFY51
BFY51NPNTO391A30V40800mWGeneral purpose, medium powerBC639
BC639NPNTO92A1A80V40800mWGeneral purpose, medium powerBFY51
TIP29ANPNTO2201A60V4030WGeneral purpose, high power
TIP31ANPNTO2203A60V1040WGeneral purpose, high powerTIP31C TIP41A
TIP31CNPNTO2203A100V1040WGeneral purpose, high powerTIP31A TIP41A
TIP41ANPNTO2206A60V1565WGeneral purpose, high power
2N3055NPNTO315A60V20117WGeneral purpose, high power
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.
PNP transistors
(typical use)
BC177PNPTO18100mA45V125300mWAudio, low powerBC477
BC178PNPTO18200mA25V120600mWGeneral purpose, low powerBC478
BC179PNPTO18200mA20V180600mWAudio (low noise), low power
BC477PNPTO18150mA80V125360mWAudio, low powerBC177
BC478PNPTO18150mA40V125360mWGeneral purpose, low powerBC178
TIP32APNPTO2203A60V2540WGeneral purpose, high powerTIP32C
TIP32CPNPTO2203A100V1040WGeneral purpose, high powerTIP32A
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.

Field Effect transister (FET)

A field-effect transistor (FET) is a type of transister commonly used for weak-signal amplification (for example, for amplifying wireless signals).The device can amplify analog or digital signals.It can also switch DC or function as an oscillator.Field-effect transistors are fabricated onto silicon integrated circuit (IC) chips.A single IC can contain many thousands of FETs, along with other components such as resistors, capacitors, and diodes.

circuit symbol:-

fet-field-effect-transistor.jpg (450×340)
in real....

In the FET, current flows along a semiconductor path called the channel. At one end of the channel, there is an electrode called the source. At the other end of the channel, there is an electrode called the drain. The physical diameter of the channel is fixed, but its effective electrical diameter can be varied by the application of a voltage to a control electrode called the gate.The conductivity of the FET depends, at any given instant in time, on the electrical diameter of the channel. A small change in gate voltage can cause a large variation in the current from the source to the drain. This is how the FET amplifies signals.

The junction FET has a channel consisting of N-type semiconductor (N-channel) or P-type semiconductor (P-channel) material; the gate is made of the opposite semiconductor type. In P-type material, electric charges are carried mainly in the form of electron deficiencies called holes. In N-type material, the charge carriers are primarily electrons.In a JFET, the junction is the boundary between the channel and the gate.Normally, this P-N junction is reverse-biased (a DC voltage is applied to it) so that no current flows between the channel and the gate.However, under some conditions there is a small current through the junction during part of the input signal cycle.


Field-effect transistors exist in two major classifications.These are known as the junction FET (JFET) and the metal-oxide- semiconductor FET (MOSFET).

Advantages & disadvantantages:-

The FET has some advantages and some disadvantages relative to the bipolar transister.Field-effect transistors are preferred for weak-signal work, for example in wireless communications and broadcast receivers.They are also preferred in circuits and systems requiring high impedance.The FET is not, in general, used for high-power amplification, such as is required in large wireless communications and broadcast transmitters.

Bipolar Junction Transistor


simple diodes are made up from two pieces of semiconductor material, either Silicon or Geranium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual diodes end to end giving two PN-junctions connected together in series, we now have a three layer, two junction, three terminal device forming the basis of a Bipolar Junction Transistor, or BJT for short. This type of transistor is generally known as a Bipolar Transistor, because its basic construction consists of two PN-junctions with each terminal or connection being given a name to identify it and these are known as the Emitter, Base and Collector respectively.

The word Transistor is an acronym, and is a combination of the words Transfer Varistor used to describe their mode of operation way back in their early days of development. There are two basic types of bipolar transistor construction, NPN and PNP, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made. Bipolar Transistors are "CURRENT" Amplifying or current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing current applied to their base terminal. The principle of operation of the two transistor types NPN and PNP, is exactly the same the only difference being in the biasing (base current) and the polarity of the power supply for each type.

Bipolar Transistor Construction:-

The construction and circuit symbols for both the NPN and PNP bipolar transistor are shown above with the arrow in the circuit symbol always showing the direction of conventional current flow between the base terminal and its emitter terminal, with the direction of the arrow pointing from the positive P-type region to the negative N-type region, exactly the same as for the standard diode symbol.

There are basically three possible ways to connect a Bipolar Transistor within an electronic circuit with each method of connection responding differently to its input signal as the static characteristics of the transistor vary with each circuit arrangement.

  • 1. Common Base Configuration - has Voltage Gain but no Current Gain.
  • 2. Common Emitter Configuration - has both Current and Voltage Gain.
  • 3. Common Collector Configuration - has Current Gain but no Voltage Gain.

The Common Base Configuration.

As its name suggests, in the Common Base or Grounded Base configuration, the BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a Current Gain for this type of circuit of less than "1", or in other words it "Attenuates" the signal.

The Common Base Amplifier Circuit

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are In-Phase. This type of arrangement is not very common due to its unusually high voltage gain characteristics. Its Output characteristics represent that of a forward biased diode while the Input characteristics represent that of an illuminated photo-diode. Also this type of configuration has a high ratio of Output to Input resistance or more importantly "Load" resistance (RL) to "Input" resistance (Rin) giving it a value of "Resistance Gain". Then the Voltage Gain for a common base can therefore be given as:

Common Base Voltage Gain:-
The Common Base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or RF radio amplifiers due to its very good high frequency response.

The Common Emitter Configuration:-

In the Common Emitter or Grounded Emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of connection. The common emitter amplifier configuration produces the highest voltage, current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased junction, while the output impedance is HIGH as it is taken from a reverse-biased junction.

The Common Emitter Amplifier Circuit:-

In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance (RL) is connected in series with the collector, the Current gain of the Common Emitter Transistor Amplifier is quite large as it is the ratio of Ic/Ib and is given the symbol of Beta, (β). Since the relationship between these three currents is determined by the transistor itself, any small change in the base current will result in a large change in the collector current. Then, small changes in base current will thus control the current in the Emitter/Collector circuit.

By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these parameters and therefore the current gain of the amplifier can be given as:

Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal.

Then to summarise, this type of bipolar transistor configuration has a greater input impedance, Current gain and Power gain than that of the common base configuration but its Voltage gain is much lower. The common emitter is an inverting amplifier circuit resulting in the output signal being 180o out of phase with the input voltage signal.

The Common Collector Configuration:-

In the Common Collector or Grounded Collector configuration, the collector is now common and the input signal is connected to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms, and it has relatively low output impedance.

The Common Collector Amplifier Circuit:-
The common emitter configuration has a current gain equal to the β value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector and base currents combined, the load resistance in this type of amplifier configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as:

This type of bipolar transistor configuration is a non-inverting amplifier circuit in that the signal voltages of Vin and Vout are "In-Phase". It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector amplifier configuration receives both the base and collector currents giving a large current gain (as with the Common Emitter configuration) therefore, providing good current amplification with very little voltage gain.

Bipolar Transistor Summary:-

The behaviour of the bipolar transistor in each one of the above circuit configurations is very different and produces different circuit characteristics with regards to Input impedance, Output impedance and Gain and this is summarised in the table below.

Transistor Characteristics:-

The static characteristics for Bipolar Transistor amplifiers can be divided into the following main groups.

Input Characteristics:- Common Base - IE ÷ VEB
Common Emitter - IB ÷ VBE
Output Characteristics:- Common Base - IC ÷ VC
Common Emitter - IC ÷ VC
Transfer Characteristics:- Common Base - IE ÷ IC
Common Emitter - IB ÷ IC

with the characteristics of the different transistor configurations given in the following table:

Input impedance
Low Medium High
Output impedance
Very High High Low
Phase Angle
0o 180o 0o
Voltage Gain
High Medium Low
Current Gain
Low Medium High
Power Gain
Low Very High Medium


Electronics is the study and use of electrical that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves. and semiconductors. The pure study of such devices is considered as a branch of physics, while the design and construction electronic circuits to solve practical problems is called electronic engineering.

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