BJT (NPN) (Bipolar Junction Transistor) of CE - Configuration

BJT (NPN) (Bipolar Junction Transistor) of CE - Configuration

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.

As the load resistance ( RL ) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib. A transistors current gain is given the Greek symbol of Beta, ( β ).

As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of α. Note: that the value of Alpha will always be less than unity.

Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current ( Ib ), will result in a much larger change in the collector current ( Ic ).

Then, small changes in current flowing in the base will thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors. So if a transistor has a Beta value of say 100, then one electron will flow from the base terminal for every 100 electrons flowing between the emitter-collector terminal.

By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these parameters and therefore the current gain of the transistor 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 a little. This type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common base configuration but its voltage gain is much lower. The common emitter configuration is an inverting amplifier circuit. This means that the resulting output signal has a 180o phase-shift with regards to the input voltage signal.

Bipolar Transistor Configurations

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

Characteristic	Common Base	Common Emitter	Common Collector
Input Impedance	Low	Medium	High
Output Impedance	Very High	High	Low
Phase Shift	0o	180o	0o
Voltage Gain	High	Medium	Low
Current Gain	Low	Medium	High
Power Gain	Low	Very High	Medium

In the next tutorial about Bipolar Transistors, we will look at the NPN Transistor in more detail when used in the common emitter configuration as an amplifier as this is the most widely used configuration due to its flexibility and high gain. We will also plot the output characteristics curves commonly associated with amplifier circuits as a function of the collector current to the base current.

A Bipolar NPN Transistor Configuration

(Note: Arrow defines the emitter and conventional current flow, “out” for a Bipolar NPN Transistor.)

The construction and terminal voltages for a bipolar NPN transistor are shown above. The voltage between the Base and Emitter ( VBE ), is positive at the Base and negative at the Emitter because for an NPN transistor, the Base terminal is always positive with respect to the Emitter. Also the Collector supply voltage is positive with respect to the Emitter ( VCE ). So for a bipolar NPN transistor to conduct the Collector is always more positive with respect to both the Base and the Emitter.

NPN Transistor Connection

Then the voltage sources are connected to an NPN transistor as shown. The Collector is connected to the supply voltage VCC via the load resistor, RL which also acts to limit the maximum current flowing through the device. The Base supply voltage VB is connected to the Base resistor RB, which again is used to limit the maximum Base current.

So in a NPN Transistor it is the movement of negative current carriers (electrons) through the Base region that constitutes transistor action, since these mobile electrons provide the link between the Collector and Emitter circuits. This link between the input and output circuits is the main feature of transistor action because the transistors amplifying properties come from the consequent control which the Base exerts upon the Collector to Emitter current.

Then we can see that the transistor is a current operated device (Beta model) and that a large current ( Ic ) flows freely through the device between the collector and the emitter terminals when the transistor is switched “fully-ON”. However, this only happens when a small biasing current ( Ib ) is flowing into the base terminal of the transistor at the same time thus allowing the Base to act as a sort of current control input.

The current in a bipolar NPN transistor is the ratio of these two currents ( Ic/Ib ), called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, ( β ).

The value of β can be large up to 200 for standard transistors, and it is this large ratio between Ic and Ib that makes the bipolar NPN transistor a useful amplifying device when used in its active region as Ib provides the input and Ic provides the output. Note that Beta has no units as it is a ratio.

Also, the current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is called Alpha, ( α ), and is a function of the transistor itself (electrons diffusing across the junction). As the emitter current Ie is the sum of a very small base current plus a very large collector current, the value of alpha (α), is very close to unity, and for a typical low-power signal transistor this value ranges from about 0.950 to 0.999

α and β Relationship in a NPN Transistor

By combining the two parameters α and β we can produce two mathematical expressions that gives the relationship between the different currents flowing in the transistor.

The values of Beta vary from about 20 for high current power transistors to well over 1000 for high frequency low power type bipolar transistors. The value of Beta for most standard NPN transistors can be found in the manufactures data sheets but generally range between 50 – 200.

The equation above for Beta can also be re-arranged to make Ic as the subject, and with a zero base current ( Ib = 0 ) the resultant collector current Ic will also be zero, ( β*0 ). Also when the base current is high the corresponding collector current will also be high resulting in the base current controlling the collector current. One of the most important properties of the Bipolar Junction Transistor is that a small base current can control a much larger collector current.

Zener Diode MCQ Question And Answer With Viva Question

Zener Diode MCQ Question And Answer

1. Zener diode is designed to specifically work in which region without getting damaged?
a) Active region
b) Breakdown region
c) Forward bias
d) Reverse bias
 Answer - b
The Zener diode is a specifically designed diode to operate in the breakdown region without getting damaged. Because of this characteristic, it can be used as a constant-voltage device.

2. What is the level of doping in Zener Diode?
a) Lightly Doped
b) Heavily Doped
c) Moderately Doped
d) No doping
 Answer - b 
A Zener diode is heavily doped so that the breakdown voltage occurs at a lower voltage. If it were lightly/moderately doped, it would breakdown at a comparatively high voltage and, thus, would not be able to serve its purpose.

3. When the reverse voltage across the Zener diode is increased _____________
a) The value of saturation current increases
b) No effect
c) The value of cut-off potential increases
d) The value of cut-off potential decreases
Answer - c
As the frequency of the incident radiation increases, the kinetic energies of the emitted electron are higher and therefore require more repulsive force to be applied to stop them.
The value of saturation current increases, as the intensity of the incident radiation, increases.
The value of cut-off potential decreases, as the frequency decreases.

4. Zener Diode is mostly used as ____________
a) Half-wave rectifier
b) Full-wave rectifierc
c) Voltage Regulator
d) LED

Answer - c
The Zener diode, once in the breakdown region, keeps the voltage in the circuit to which it is connected as constant. Thus it is widely used as a voltage regulator.

5. Which of the following is the correct symbol for the zener diode?

a)
b)
c)
d)
 Answer - d

6. In normal junctions, the breakdown is same as Zener breakdown.
a) True
b) False
Answer - b

In normal p-n unction diodes, the breakdown takes place by avalanche breakdown which is different than the Zener breakdown. Zener diode is specifically made to operate in that region.

7. The depletion region of the Zener diode is ____________

a) Thick

b) Normal

c) Very Thin

d) Very thick

View Answer

Answer: c

Zener diode is fabricated by heavily doping both p- and n-sides of the junction, which results in an extremely thin depletion region.

8. The electric field required for the field ionization is of what order?

a) 104 V/m

b) 105 V/m

c) 106 V/m

d) 107 V/m

Answer: c

In a Zener diode, a very high electric field is produced for even a very small voltage. The electric field required for field ionization is of the order 106 V/m.

9. In a circuit the load current is 5 mA and the unregulated output is 10 V. If the voltage drop across the Zener diode is 3 V, what should be the value of resistance?

a) 50 Ωb

b)100 Ωc

c)125 Ωd

d)150 ΩA

Answer: d

The value of R should be such that the current through the Zener diode is much larger than the load current.

Imagine Iz = 20 mA. The total current is therefore 24 mA.

Thev voltagedrop = 3 V

Resistance = 3 V/24 X 10-3 AR=125 Ω.

Viva Questions

1)  What is a zener diode? 

A  heavily  doped  P-N  junction  which  has  a  sharp  breakdown  voltage  is  called  as  a  zener diode. 

2) What is diode?

The  diode is a  semiconductor device  having  p-n junction. 

3)  On what factor does the  breakdown voltage  of a  zener diode  depends?

The  breakdown  voltage  of  a  zener  diode  depends  upon  the  amount  of  doping. If  the diode  is  heavily  doped,  the  depletion  layer  will  be  thin  and  consequently  the  breakdown of  the junction will  occur  at a lower  reverse  voltage. 

4)  What is breakdown voltage?

When the reverse  bias of  a  zener  diode  is increased,  a  critical voltage  is reached  at which the  reverse  current  increases  sharply  to  a  high  value.  This  critical  voltage  is  called   breakdown voltage.

5)  What is the difference  between  an ordinary  diode  and a  zener diode?

i)  A  zener  diode  is  like  an  ordinary  diode  except  that  it  is  properly  doped  so  as  to  have  a sharp  break down voltage.

ii)  It has  a  sharp breakdown voltage  called zener  voltage.

iii) When forward biased, zener diode  characteristics are  just  that of  ordinary  diode.

iv) A  zener diode  is always  reverse  biased.

6)  Why zener diode  is always  reverse  biased?

Because it utilize  reverse  characteristics for acting  like  a  voltage  regulator.

7)  Mention the  uses of  zener diode.

A  zener  diode  is  used  as  a  voltage  regulator  to  provide  a  constant  voltage  from  a source  whose  voltage  vary  over sufficient range.

8)  What do  you mean by  Characteristics of a  Zener Diode?

A  Graph  showing  the  variation  of  voltage  applied  across  the  terminals  of  a  diode  to  the corresponding  current  is  called  characteristics  of  a  diode.  In  case  of  junction  diodes,  there are  two types of characteristics, forward  and reverse  characteristics.

9)  Explain the flow of  current in zener diode  under reverse  biasing  condition?

In  case  of  reverse  biasing  condition,  the  external  field  is  established  in  a  direction  such  as to  help  internal  field.  Under  this  biasing  the  holes  in  P-  region  and  electrons  in  N-  region are  pushed  away  from  the  junction  with  the  result  that  the  depletion  or  barrier  layer  is thickened  and  hence  increases  the  potential  barrier,  therefore,  the  flow  of  current  stops.  Only  a  few thermally  generated minority  carriers produce  a  very  small  current.

10) Explain the flow of  current in zener diode  under forward biasing  condition?

In  the  case  of  forward  biasing  condition,  the  zener  diode  behaves  like  an  ordinary  p-n junction  diode.  In  this  case  the  external  field  is  established  in  a  direction  such  as  to oppose  the  internal  field.  The  external  field  is  much  stronger  than  internal  field.    In  this arrangement  holes  move  along  the  external  field  from  p-region  to  n-region  and  electrons move  opposite  to  the  field  from  n-region  to  p-region,  that  is,  the  majority  carriers  from each  side  move  across  the  junction.  The  potential  barrier  or  depletion  layer  at  the  junction wiped  out  and  a  substantial  current  flows  depending  upon  the  density  of  n-  and  p- carriers.

11) What do  you mean by  doping?

The  process of introducing  the impurity  in a  semiconductor is called doping.

12) What is Zener breakdown?

If  the  reverse  potential  across  the  zener  diode  is  increased  beyond  a  certain  value,  the current  increases  very  rapidly  due  to  zener  breakdown.  Zener  breakdown  occurs  when  the applied  electric  field  or  potential  is  so  high  that  the  valence  band  electrons  are  pulled  out to  the  conduction  band  in  large  numbers  resulting  in  breakdown.  Thus,  a  zener breakdown,  direct  rupture  of  covalent  bonds  take  place  by  thermally  generated  carriers having  acquired high  energy  due  to strong  electric  field.

13)  What do  you mean by  avalanche  breakdown?

When  the  reverse  voltage  is  made  sufficiently  high  the  thermally  generated  electrons  and holes  acquire  high  energy  from  the  applied  potential  and  make  ionizing  collisions  with the  atoms  of  the  crystal.  These  collisions  produce  further  electrons,  which  in  turn  collide with  further  atoms.  The  commutative  effect  of  such  collisions  results  in  the  breakdown  of the  junction.  Due  to  this  avalanche  multiplication  the  reverse  current  increases  abruptly  to high value. This is called  avalanche  breakdown and may  damage  the  junction. 

14) What are  the differences between avalanche  breakdown and zener breakdown?

In  general  zener  breakdown  occurs  below  8V  and  avalanche  breakdown  occurs  at  higher voltages  (~20  V).  The  Zener  breakdown  is  characterized  by  the  soft  knee,  whereas avalanche  breakdown  is  hard  knee  type.  Zener  breakdown  voltage  has  a  negative temperature  coefficient,  while  the  avalanche  breakdown  voltage  exhibits  positive temperature  coefficient.

15) How the  width of the  depletion region in the reverse  biased diode varies with  the impurity      concentration.

The  width  of  the  depletion  region  of  a  reverse  biased  diode  varies  as  the  square  root  of the impurity  concentration.

16).  How  the  value  of  the  potential  barrier  depends  on  the  amount  of  doping  of  the semiconductor?

The  value  of  potential  barrier  decreases  with  heavy  amount  of  doping  of  the semiconductor. 

17). Why  the silicon diode  is preferred  compare  to  germanium  diode? 

Silicon  diodes  are  preferred  compare  to  germanium  diodes  because  of  its  higher temperature  to current capability. 

18). Under what condition a  zener diode  behaves like an ordinary  p-n junction diode? 

A  zener  diode  behaves  as  an  ordinary  p-n  junction  diode  when  it  is  used  in  forward  bias conditions. 

19). What is the main application of  a  zener diode? 

An  important  application  of  zener  diode  is  its  use  as  voltage  regulator.  The  regulating action  takes  place  due  to  the  fact  that  in  reverse  breakdown  region;  a  very  small  change  in voltage  produces  a  very  large  change  in current. 

FET , Field Effect Transistor

FET 

Field Effect Transistor

First Image Of Transistor

Julius Edgar Lilienfeld (April 18, 1882 – August 28, 1963) was an Austro-Hungarian American physicist and electical engineer, credited with the first patents on the field-effect transistor (FET) (1925) and electrolytic capacitor (1931). Because of his failure to publish articles in learned journals and because high-purity semiconductor materials were not available yet, his FET patent never achieved fame, causing confusion for later inventors.

Lilienfeld moved to the United States in 1921 to pursue his patent claims, resigning his professorship at Leipzig to stay permanently in 1926. In 1928, he began working at Amrad in Malden, Massachusetts, later called Ergon Research Laboratories owned by Magnavox, which closed in 1935. In the United States Lilienfeld did research on anodic aluminum oxide films, patenting the electrolytic capacitor in 1931, the method continuing to be used throughout the century. He also invented an "FET-like" transistor, filing several patents describing the construction and operation of transistors, as well as many features of modern transistors.

The optical radiation emitted when electrons strike a metal surface is named "Lilienfeld radiation" after he first discovered it close to X-ray tube anodes. Its origin is attributed to the excitation of plasmons in the metal surface.

The American Physical Society has named one of its major prizes after Lilienfeld.

The field-effect transistor (FET) is an electronic device which uses an electric field to control the flow of current. FETs are devices with three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source.

Types  of FET Transistor

FETs are of two types- JFETs or MOSFETs.  JunctionFET

A Junction FET

The Junction FET transistor is a type of field effect transistor that can be used as an electrically controlled switch. The electric energy flows through an active channel between sources to drain terminals. By applying a reverse bias voltage to gate terminal, the channel is strained so the electric current is switched off completely.

The junction FET transistor is available in two polarities which are;

N- Channel JFET

N channel JF

N channel JFET consists of an n type bar at the sides of which two p type layers are doped. The channel of electrons constitutes the N channel for the device. Two ohmic contacts are made at both ends of the N-channel device, which are connected together to form the gate terminal. The source and drain terminals are taken from the other two sides of the bar. The potential difference between source and drain terminals is termed as Vdd and potential difference between source and gate terminal is termed as Vgs. The charge flow is due to flow of electrons from source to drain. Whenever a positive voltage is applied across drain and source terminals, electrons flows from the source ‘S’ to drain ‘D’ terminal , where as conventional drain current Id flows through the drain to source. As current flows through the device, it is in on state. When a negative polarity voltage is applied to the gate terminal, a depletion region is created in the channel. The channel width is reduced, hence increasing the channel resistance between the source and drain. Since the gate source junction is reverse biased and no current flows in the device, it is in off condition. So basically if voltage applied at the gate terminal is increased, less amount of current will flow from the source to drain. The N channel JFET has greater conductivity than the P channel JFET. So the N channel JFET is more efficient conductor compared to P channel JFET.

 P-Channel JFET

P channel JFET consists of a P type bar, at two sides of which n type layers are doped. The gate terminal is formed by joining the ohmic contacts at both the sides. Like in an N channel JFET, the source and drain terminals are taken from the other two sides of the bar. A P type channel, consisting of holes as charge carriers, is formed between source and drain terminal.

P channel JFET bar

A negative voltage applied to the drain and source terminals ensure the flow of current from source to drain terminal and the device operates in ohmic region. A positive voltage applied to the gate terminal ensures the reduction of channel width, thus increasing the channel resistance.  More positive is the gate voltage; less is the current flowing through the device.

Circuit Diagram

Characteristics of p channel Junction FET Transistor

Given below is characteristic curve of p channel Junction Field Effect transistor and different modes of operation of the transistor.

Characteristics of p channel junction FET transistor

Cutoff region: When the voltage applied to the gate terminal is enough positive for the channel width to be minimum, no current flows. This causes the device to be in cut off region.

Ohmic region: The current flowing through the device is linearly proportional to the applied voltage until a breakdown voltage is reached. In this region, the transistor shows some resistance to the flow of current.

Saturation region: When the drain source voltage reaches a value such that the current flowing through the device is constant with the drain source voltage and varies only with the gate source voltage, the device is said to be in saturation region.

Break down region: When the drain source voltage reaches a value which causes the depletion region to break down, causing an abrupt increase in the drain current, the device is said to be in breakdown region. This breakdown region is reached earlier for lower value of drain source voltage when gate source voltage is more positive.

 

MOSFET Transistor

MOSFET transistor

MOSFET transistor as its name suggests is a p type (n type) semiconductor bar (with two heavily doped n type regions diffused into it) with a metal oxide layer deposited on its surface and holes taken out of the layer to form source and drain terminals. A metal layer is deposited on the oxide layer to form the gate terminal. One of the basic applications of field effect transistor is using a MOSFET as a switch.

This type of FET transistor has three terminals, which are source, drain, and gate.  The voltage applied to the gate terminal controls the flow of current from source to drain.  The presence of an insulating layer of metal oxide results in the device having high input impedance.