bipolar transistor

Bipolar Transistor

Bipolar Transistor Basics

In the Diode tutorials we saw that simple diodes are made up from two pieces of semiconductor material, either silicon or germanium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in series that share a common P or N terminal. The fusion of these two diodes produces a three layer, two junction, three terminal device forming the basis of a Bipolar Junction Transistor, or BJT for short.

Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistor’s ability to change between these two states enables it to have two basic functions: “switching” (digital electronics) or “amplification” (analogue electronics). Then bipolar transistors have the ability to operate within three different regions:

  • Active Region   –   the transistor operates as an amplifier and Ic = β.Ib
  • Saturation   –   the transistor is “Fully-ON” operating as a switch and Ic = I(saturation)
  • Cut-off   –   the transistor is “Fully-OFF” operating as a switch and Ic = 0
bipolar transistor

A Typical
Bipolar Transistor

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, PNP and NPN, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made.

The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively.

Bipolar Transistors are current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch. The principle of operation of the two transistor types PNP and NPN, is exactly the same the only difference being in their biasing and the polarity of the power supply for each type.

Bipolar Transistor Construction

bipolar transistor construction


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

Bipolar Transistor Configurations

As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement.

  • Common Base Configuration   –   has Voltage Gain but no Current Gain.
  • Common Emitter Configuration   –   has both Current and Voltage Gain.
  • Common Collector Configuration   –   has Current Gain but no Voltage Gain.

The Common Base (CB) 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 “1” (unity) or less, in other words the common base configuration “attenuates” the input signal.

The Common Base Transistor Circuit

common base configuration


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 transistor arrangement is not very common due to its unusually high voltage gain characteristics. Its input characteristics represent that of a forward biased diode while the output characteristics represent that of an illuminated photo-diode.

Also this type of bipolar transistor 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 ( Av ) for a common base configuration is therefore given as:

Common Base Voltage Gain

common base transistor gain

Where: Ic/Ie is the current gain, alpha ( α ) and RL/Rin is the resistance gain.

The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or radio frequency ( Rf ) amplifiers due to its very good high frequency response.

The Common Emitter (CE) Configuration

In the Common Emitter or grounded emitter configuration, the input signal is applied between the base and the emitter, 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 bipolar transistor connection.

The common emitter amplifier configuration produces the highest 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 PN-junction, while the output impedance is HIGH as it is taken from a reverse biased PN-junction.

The Common Emitter Amplifier Circuit

common emitter configuration


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:

bipolar transistor alpha beta relationship

common emitter current gain

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 is 180o “out-of-phase” with the input voltage signal.

The Common Collector (CC) Configuration

In the Common Collector or grounded collector configuration, the collector is now common through the supply. The input signal is connected directly 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 common collector, or 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 while having a relatively low output impedance.

The Common Collector Transistor Circuit

common collector configuration


The common emitter configuration has a current gain approximately 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 the base current combined, the load resistance in this type of transistor 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:

The Common Collector Current Gain

common collector gain

Common Collector Current Gain

This type of bipolar transistor configuration is a non-inverting 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 transistor 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.

We can now summarise the various relationships between the transistors individual DC currents flowing through each leg and its DC current gains given above in the following table.

Relationship between DC Currents and Gains

transistor currents transistor alpha and beta equations
transistor base currents
transistor collector currents transistor emitter currents

Bipolar Transistor Summary

Then to summarise, 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 whether this is voltage gain, current gain or power gain and this is summarised in the table below.

Bipolar Transistor Configurations

bipolar transistor configurations

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

Characteristic Common
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

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.


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  • B
    Bob Sonier

    I had downloaded this tutorial as an aid for an electronics class presentation for a high school class and believe that the schematic for the Common Collector circuit is in error. The schematic as shown is for a Common Emitter circuit. The figure with the four Bipolar Transistor Configurations shows the correct hookup.

    • Wayne Storr

      Hello Bob, The diagram to which you refer shows the transistor connected in a common collector configuration were the transistors collector is connected directly to the supply voltage making it common through the supply. In the common collector, also known as an emitter follower, the input signal is effectively connected between the base and collector, while the output being generated between the emitter and collector is taken from across the emitter load as shown in the diagram.

  • L

    Quote: Then, small changes in current flowing in the base will thus control the current in the emitter-collector circuit.

    Physically, this is impossible. A small current can NEVER control a larger current. Instead, it is the base-emitter voltage which controls Ic.
    I am aware that several web-based tutorials (and even some printed textbooks) contain this error.
    Summary: The well-known formula Ic=beta*Ib is correct and can be used during the design – however, it says nothing about the physical relationship (cause and result).

    • C

      You’ve made a statement without any arguments. I’m curious why you say that not the Ib is the one which controls the Ic.


      • L

        Sorry for the late reply.
        If we remember how the pn diode is working, we recognize that it is, of course, the VOLTAGE across the depletion zone that determines the width of this area and- thus – the current through it.
        The same applies, of course, to the depletion layer of a BJT.
        Remember: EACH transistor design starts with an assumption regarding the B-E voltage, which “opens” the transistor and allows a certain dc current Ic. Why do you think that a slight variation of this current Ic could be CAUSED by a change in Ib ?
        The physical cause of both – change in Ic and Ib – is the VOLTAGE across the pn junction, which means: Vbe.
        Ask Winfield Hill (Co-author of the “Art of Electronics”).

        General remark: As I have mentioned earlier – from the energy point of view it is impossible that a smaller quantity (like Ib) DIRECTLY can control a larger quantity (like Ic).

        • Wayne Storr

          So what you are saying is that if a voltage is applied to a transistors Base so that no current enters the Base then a big current will still flow from Collector to Emitter, (assuming NPN transistor). Nice thought.

          • L

            Quote Wayne Storr: “So what you are saying is that if a voltage is applied to a transistors Base so that no current enters the Base then a big current will still flow from Collector to Emitter, (assuming NPN transistor). Nice thought.”

            No – I didn´t say this.
            I repeat: The physical cause of both – change in Ic and Ib – is the VOLTAGE across the pn junction, which means: Vbe.
            Of course, there is a base current Ib. But why do you think that Ib CONTROLC Ic? Just because the relation Ic=beta*Ib looks so nice and simple?
            There are many wittnesses supporting this view (it´s more than a view”, it is a physical fact!): The great Barry Gilbert, Winfield Hill (Art of Electronics), several University papers.
            The current Ib is something we cannot avoid but this current is NOT the controlling quantity. It is simply a more or less fixed small percentage of Ic. It is really unfortunate that the ration Ic/Ib>1 was called “current gain” because this wording indeed implies that Ib would be an input quantity which is amplified. It would be much better to say Ib/Ic<1 is an "efficiency factor" or something like that.
            (By the way: Do you know that some manufacturers use the term "current gain" even for FET´s?).

  • Shehan

    Why the Collector terminal of the transistor in the above common collector circuit diagram is not really common for the input and output ?

  • r
    rujra bhatt

    i would like to know about designing of BJT, how to design BJT RC coupled Common Emitter amplifier?

  • n

    Guys, I’m a starter so help me out….the tutorial assumes knowledge of the relationship alpha=beta/(beta+1) , but I am not aware of this relationship…How is it derived??

    • Wayne Storr

      A transistor is an amplifying device so we need to define the amount of possible DC current amplification it can produce. Beta (b) indicates the DC current gain and is the ratio of output current Ic to its input current Ib. (b = Ic/Ib).

      Alpha (a) is the ratio of collector current, Ic to emitter current, Ie given as (a = Ic/Ie). Alpha is more a measure of the quality of the transistors construction with regards to its “ON” condition and voltage drop. The higher its alpha value the better the transistor conducts as Ie equals Ic and less power is wasted in heat.

      Then as all three current values, Ie, Ic and Ib are used, the relationship between alpha (a) and beta (b) is therefore a = b/(1+b) or b = a/(1-a) as described above.

      • J
        Julio Baez

        Is this statement only valid for Common Emitter Configuration?

        • Wayne Storr

          No, you can find a transistors current gain using Beta (Ic/Ib) = alpha/(1-alpha), Alpha (Ic/Ie) = beta/(beta+1), or Gamma (Ib/Ic) = (beta+1), for common emitter, common base or common collector configurations respectively.

    • D
      Dinesh kannaa

      alpha is the current gain .
      alpha->a , beta->b
      a=output/input i.e.,ic/ie b=ic/ib
      but ie=ic+ib
      so, a=ic/(ic+ib)
      =1 + (1/b)
      =(b + 1)/b.

  • D


  • a

    in common emmiter bjt npn emmiter &collector are n-type .but how they connected in forward & reverse biased. we know in forward p-type is connected to + end of supply &n-type is connected to – end of supply.in reverse u know .how it operates when one n-type is connected to + end & another n-type connected to – end of supply.how can we say this is forward or reverse biased.

  • O
    Ojas Kadu

    In the table, why is the current gain for CE configuration shown to be “Medium” and for CC configuration shown to be “High”? You have mentioned earlier that current gain for CE is highest amongst all three configurations.

  • v

    Good explanation.

  • A

    I Luv this site.

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