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24 February 2009
Author: Giorgos Lazaridis
BJT Transistor theory

Transistor Circuit Essentials

Before we start talking about the different types of transistor connections, we first need to declare some characteristic sizes and symbols that will be used from now on.

IE is the Emitter current, IC is the Collector current and IB is the Base current. The direction of each current has to do with the type of the transistor (PNP or NPN). The voltage across two leads will be symbolizes by the letter V, with the two letters of the corresponding leads of the transistor as pointers. The second letter will always be the one that also characterizes the connection type of the transistor. So, for example, in Common Base connection, the voltage across the emitter and the base is VEB, and the voltage across the collector and the base is VCB. Similarly, in Common Emitter connection, VBE is the voltage across the base and the emitter and VCE is the voltage across the collector and the emitter. We will symbolize the power supplies of the leads with the letter V followed by the letter of the corresponding lead, twice. The symbol VEE is for the emitter supply, VCC for the collector supply and VBB for the base supply.

Choosing the right connection

There are 3 methods that a transistor can be connected, each one having advantages and disadvantages and specific application uses. So it is very important before you start designing your circuit, to be able to choose the proper connection according to your application requirements. First we will see these 3 connections in a glance, and then we will discuss each one thoroughly.

The Common Base (CB) Connection in a glance

A transistor is connected with Common Base, when the emitter-base diode is forward biased and the collector-base diode is reverse-biased, the input signal is applied in the emitter, and the output is taken from the collector. It is called "Common Base" because the input and output circuits share the base in common.

The Common Base connection is probably the most rarely used type, due to the strange behavior that it has. As we saw in the operation of a transistor in the previous page, the emitter current is the strongest current of all within a transistor (IE=IB+IC). What this means is that the input of this circuit must be able to provide enough current to source the output. Moreover, the output current (IC) will be slightly less than the input current (IE). So this connection type is absolutely improper for a current amplifier, since the current gain is slightly less than unity (0.9 < hfb<1), in other words it acts as a current attenuator rather than a current amplifier.

On the other hand, it does provide a small voltage amplification. The output signal is in phase with the input signal, so we can say that this is a non-inverting amplifier. But here comes another strange behavior: The voltage amplification ratio of this circuit is very difficult to be calculated, because it depends on some operational characteristics of the transistor that are difficult to be measured directly. The emitter-base internal resistance of the transistor for example and the amount of DC bias of the input signal play a major role in the final amplification ratio, but these are not the only ones. The current that flows within the emitter changes the internal emitter-base resistance which eventually changes the amplification ratio. This connection type has a unique advantage. Due to the fact that the base of the transistor is connected to the ground of the circuit, it performs a very effective grounded screen between the input and the output. Therefore, it is most unlikely that the output signal will be fed back into the input circuit, especially in high frequency applications. So, this circuit is widely used in VHF and UHF amplifiers.

The Common Collector Connection (CC) in a glance (Emitter Follower)

A transistor is connected with Common Collector, when the base-collector and emitter-collector diodes are forward biased, the input signal is applied in the base, and the output is taken from the emitter. It is called "Common Collector" because the input and output circuits share the collector in common.

The common collector connection is used in applications where large current amplification is required, without voltage amplification. As a matter of fact, this circuit has the highest current gain factor. Remember that hfb=IC/IE (current gain in CB), hfe=IC/IB (current gain in CE) and hfc=IE/IB (current gain in CC). Taking into account that IE>IC (IE=IB+IC) and that IB is the smallest current, from the previous 3 formulas we can easily conclude that hfc>hfe>hfb. The current that this circuit can provide at its output is indeed the highest, and it it is the sum of the IC current plus the IB current from the input.

The most distinctive characteristic though that this connection type has, is that the output voltage is almost equal to the input voltage. As a matter of fact, the output voltage will be equal to the input voltage, slightly shifted towards ground (VE=VB-VBE). This voltage drop depends on the material that the transistor is made of. A Germanium transistor has VBE=0.3V and a Silicon transistor has VBE=0.7 volts. So, the output signal on a silicon transistor will be exactly the same as the input signal, only that it will be shifted by 0.7 volts. This is why this connection is also called "Emitter Follower". The output signal is in phase with the input signal, thus we say that this is a non-inverting amplifier setup.

Basic voltage regulator circuit

This is a very efficient circuit to match impedance between two circuits, because this mode has high input impedance and low output impedance. It is widely used for example to drive the speakers in an audio amplifier, since the speakers have usually very low impedance. It is also used as a current amplifier in applications where the maximum current is required, such as driving solenoids, motors etc. This feature makes this type also perfect for designing Darlington pair transistors, since the maximum current amplification is the requirement.

It is also a very effective circuit to make voltage regulators with high current supply. A Zener diode for example at the base of the transistor will provide a fixed voltage, and the output will be 0.7 (or 0.3) volts less than the Zener diode's regulation voltage, since it will follow the input, no matter how much current it provides.

The Common Emitter (CE) Connection in a glance

A transistor is connected with Common Emitter connection, when the base-emitter and emitter-collector diodes are forward biased, the input signal is applied in the base, and the output is taken from the collector. It is called "Common Emitter" because the input and output circuits share the emitter in common.

This is the most common transistor connection used. The reason is because it can achieve high current amplification as well as voltage amplification. This results in very high power amplifications gain (P = V x I). Although -in maths- the current amplification of a Common Collector circuit is larger than a Common Emitter circuit, typically we can safely say that they both have almost the same gain:

hfc = IE / IB
hfe = IC / IB (1)
IE = IB + IC (2)
(1)(2)=> hfe = (IE - IB) / IB = IE / IB - IB / IB = hfc - 1 => hfe = hfc - 1

Suppose that a transistor has hfc=100 (in CC). From the above analysis we see that if we connect this transistor with common emitter, it will have hfe=99 (hfc-1), which is not a significant decrement. Additionally, the output voltage can also be predictably amplified. This is what makes this circuit so widely used. We will discuss this specific connection extensively with different biasing techniques.

This mode is used in several applications, such as audio amplifiers, small signal amplification, load switching and more. A distinctive characteristic for this connection is that the output signal has 180 degrees phase difference from the input signal, thus we call it inverting amplifier.

General Connection Characteristics

Here is a table with the characteristic sizes of the 3 different transistor connections, so that you can directly compare them

Common Base Common Emitter Common Collector
Input Impedance Low (about 50 Ω) Medium (1-5 KΩ) High (300-500 KΩ)
Output Impedance High (500KΩ-1MΩ) Medium (about 50KΩ) Low (up to 300 Ω)
Current Gain Low (<1) High (50 - 800) High (50-800)
Voltage Gain Low (about 20) High (about 200) Low (<1)
Power Gain Low (about 20) High (up to 10000) Medium (about 50)

Relative pages
  • Basic transistor circuits
  • 555 theory of operation
  • Learn how to interface ICs
  • Learn how dimmers work
  • The TRIAC theory
  • Learn about the most popular PC Cooling methods
  • Dr.Calculus: Checking transistor functionality
  • Op-Amp IC Pinouts

  • Comments


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  • At 25 October 2015, 17:44:11 user Mohammad Irfan wrote:   [reply @ Mohammad Irfan]
    • Hey keep it up! nice work

  • At 28 August 2015, 21:23:03 user tomaeh wrote:   [reply @ tomaeh]
    • to get more information about <a href="http://911electronic.com/tunnel-diode-characteristic-symbol-definition/">tunnel diode</a> click hotlink. I found this site yesterday and i think there is a lot information about diodes.

  • At 17 June 2015, 15:13:52 user Vahhab wrote:   [reply @ Vahhab]
    • Hi,
      Would you please tell that why the mentioned curvature happens? I have this problem in my TFT (Thin Flim transistor) and in the low voltage of D-S, D-S current does not like a diode curve and it has a curvature like you mentioned.

  • At 6 April 2015, 23:20:04 user Billy Keith wrote:   [reply @ Billy Keith]
    • These are perfect explanations. Thank you for a great reference source.

  • At 19 February 2015, 8:23:36 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @lee smith Sure. It is here:

  • At 18 February 2015, 16:20:16 user lee smith wrote:   [reply @ lee smith]
    • Thanks George!

      Just went through your transistor theory pages. Do you have a pdf/doc package with all the pages? I have some serious studying to do.

  • At 5 January 2015, 16:56:28 user sadusaisaandeep wrote:   [reply @ sadusaisaandeep]
    • how to dc power changing ac power?

  • At 13 October 2014, 18:29:02 user pranav wrote:   [reply @ pranav]
    • wow you have explained all in very detailed form . thank you very very much.

  • At 26 June 2014, 5:39:22 user tharaka wrote:   [reply @ tharaka]
    • what is the anode voltage if the cathode is grounded in a diode?

      A-|>---(grnd) what is the voltage value of A if the diode is silicon ?

  • At 25 May 2014, 19:48:48 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Nash Its in this link:

  • At 25 May 2014, 15:51:22 user Niyas wrote:   [reply @ Niyas]
    • where can I find pdf version of this article?

  • At 9 January 2014, 14:04:31 user Nash wrote:   [reply @ Nash]
    • @Giorgos Lazaridis Your Diagrams for the current flow for a NPN connection are wrong. BIG MISTAKE Fix it please http://pcbheaven.com/wikipages/images/trans_theory_1317761009.png & http://pcbheaven.com/wikipages/images/trans_theory_1317761285.png

  • At 7 May 2013, 22:45:13 user mangyi wrote:   [reply @ mangyi]
    • your explanation is far better than my course book, thank you very much

  • At 15 April 2013, 20:29:39 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Ganesh Nadar No, the biasing must be DC

  • At 15 April 2013, 14:16:25 user Ganesh Nadar wrote:   [reply @ Ganesh Nadar]
    • Can Vcc of transistor be AC 12V???

  • At 9 December 2012, 10:05:26 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @peter boltink in this example, VC is Vre because only Vre is there to change Vc

  • At 22 November 2012, 4:12:45 user peter boltink wrote:   [reply @ peter boltink]
    • I think there is a mistake
      VC is Vce VE = 7.76V
      and the voltage at the collector resistor is (1.93 x 2200) = 4.24v

      Vc is not the same as Vre


  • At 30 October 2012, 14:21:18 user ravi shankar wrote:   [reply @ ravi shankar]
    • working of bjt tansistor {clearly}

  • At 16 July 2012, 17:59:37 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Mint Electronics Sorry for 2 reasons: first for the loooong delay (i thought that i had posted the answer immediately), and sorry for not explaining this in the article. I will re-read the whole theory when i finish it and fix some issues like that. I though that it was not so important to explain it, but maybe i will put some spoilers with the proof. Anyway:

      Question 1:
      Vcc = Vb Vbe Ve => Vcc - Vbe = Ve Vb

      But we can approximate that Ve = Ic x Re (since Ic almost = Ie)
      Also, Vb = Ib x Rb => Vb = (Ic x Rb) / hfe

      From the above:

      Vcc - Vbe = (Ic x Re) [(Ic x Rb) / hfe] => (We get Ic in common)
      Vcc - Vbe = Ic x [Re (Rb / hfe)] => (divide both sides with term)
      (Vcc - Vbe) / [Re (Rb / hfe)] = Ic

      Question 2:
      The voltage divider current will always be bigger than the base current, since it is composed by the voltage divider current PLUS the base current.

  • At 9 July 2012, 1:58:04 user Mint Electronics wrote:   [reply @ Mint Electronics]
    • Can somebody please explain to me how the following formulas were constructed?

      IC = (VCC - VBE) / (RE (RB / hfe)) (pg. 3)

      How is the voltage divider current bigger than the base current? Isn't it the other way around? (pg. 4)

      VCE = VCC - IC x RC - IE x RE = VCC - IC (RC RE) (pg. 4)

      IVD > 10 x IB => RVD < 0.1 x %u03B2dc x RE (pg. 4)

  • At 3 July 2012, 8:18:54 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Mint Electronics Although i try to keep the math as simple as possible, the way you want me to re-arrange it would be more like a math tutorial rather than a transistor tutorial. I keep it simple but not that simple, it would be tiring and confusing for those who want to learn transistors.

      As for the "arrow", it is not an arrow, it is the Greek letter %u03B2 (Beta) which probably you cannot see due to your browser's encoding used. It is good to know that there are people who cannot follow this encoding. This %u03B2 letter is the same as the hfe. In formulas we use %u03B2 rather than hfe for short. I think i have to find something else to show this....

      In the meanwhile try to change your encoding and the correct letter will reveal.

  • At 3 July 2012, 2:26:34 user Mint Electronics wrote:   [reply @ Mint Electronics]
    • Hey!
      This is a good tutorial, however I am a bit lost in the math by the way you write it.
      Would it be possible to rearrange it and have the formula go down the page e.g.:
      5*(2 2)

      Also can you please explain to me the arrow that you use in the formulas, what does it mean? e.g.: IE = %u03B2 x IB => IB = IE / %u03B2 = 1mA / 30 => IB = 33uA

      - Cheers,
      Mint Electronics.

  • At 17 April 2012, 7:40:29 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @john I know what you mean, but i do not show the conventional current flow but the electron flow (which is the reverse). I think i have to explain this somehow in a short paragraph.

  • At 14 April 2012, 9:19:47 user john wrote:   [reply @ john]
    • I think the current flow in your explanation is for PNP transistor, because for NPN transistor current flow going in to basis not going out.

  • At 23 February 2012, 4:14:02 user fulton g.w. wrote:   [reply @ fulton g.w.]
    • The use holes to explain any part of a transistor function is confusing, a positive charge does not move because of the mass of the positively charged nucleus.[ie; Protons, so called holes.]The flow of electrons is convincingly demonstrated by the cathode ray tube and other experiments carried out a hundred years ago. Electron deficiency and excess better explain the attraction or repulsion which is used for a transistor to function

  • At 16 January 2012, 20:09:23 user Kammenos wrote:   [reply @ Kammenos]
    • @almalo you're right, the typo is obvious. I used the minus sign used to show the reverse current directin, as an algebric sign. I had to use ABS numbers for the comparison. Common collector has the maximum current amplification. hfc>hfe>hfb

      I write -hfc = IE / IB but i should write instead |-hfc| = IE / IB. The result with this change is:
      hfe = hfc - 1

      I tripple check this editorial because i do not want to make such mistakes, sometimes i fail to locate them though. Thank you for noticing.

  • At 16 January 2012, 11:07:08 user almalo wrote:   [reply @ almalo]
    • Sorry I'am a bit confused:
      First you wrote: hfc>hfe>hfb.
      2nd: hfe = hfc + 1
      So what is the truth?

  • At 21 November 2011, 9:18:21 user Kammenos wrote:   [reply @ Kammenos]
    • @Russ i took a quick search around but i could not find one with more details about the biasing history.

  • At 20 October 2011, 1:01:32 user Russ wrote:   [reply @ Russ]
    • Can you recommend a book that goes into the history of transistor biasing and the development of other basic circuits? I am interested in how this developed. From hindsight it seems so clear and I wonder how difficult it really was.

  • At 16 July 2011, 0:55:40 user Juan Carlos wrote:   [reply @ Juan Carlos]
    • The two best invention of mankind are the transistor and the Condon

  • At 12 July 2011, 13:51:05 user ramzal wrote:   [reply @ ramzal]
    • it is very effective to understand basics of transistors!

  • At 13 June 2011, 17:09:57 user Kammenos wrote:   [reply @ Kammenos]
    • @Fung which circuit are you referring to?

  • At 13 June 2011, 15:40:12 user Fung wrote:   [reply @ Fung]
    • The L7805 1A voltage regulator, there are 2 situations when a switch is open and close.

      When open, only an LED and a 74HC00 IC are working, the total current may be about 30mA.

      When closed, at least 5 ICs will be in operation, with an 7-segment dual display, the total current may increase to about 150mA. Then the regulator heats up in a short time.

      The transistor is also heat up because of the voltage regulator.

      Will the problem be solved by moving it away from the voltage regulator?

  • At 22 April 2011, 5:29:31 user Kammenos wrote:   [reply @ Kammenos]
    • @Fung yet is varies. but the situation you describe does not sound normal to me. which voltage regulator you use, and how much current this is supposed to provide?

  • At 21 April 2011, 7:46:10 user Fung wrote:   [reply @ Fung]
    • The resistance of a transistor varies as its temperature changes, am I right?

      I have a circuit which has a +5V voltage regulator, a transistor is used to amplify the signal of a PIEZO sounder and it is placed nearby the voltage regulator because of the routing of tracks. However, due to the quite-high current, the regulator heats up when it is in operation, heatsink is not setup yet, not only itself, the tracks under the board are also heated up.

      Due to the case stated above, the transistor is heated up later on, it changes the frequency to the PIEZO sounder (ie unable to keep the original frequency) because of the change of its internal resistance. As I need to add the heatsink on the regulator, what things should I do in order to keep the frequency (that is to keep the internal resistance of the unit) of the amplifying circuit?

  • At 25 September 2010, 4:28:00 user pradeep wrote:   [reply @ pradeep]
    • motor invetor

  • At 2 July 2010, 16:52:58 user suguna wrote:   [reply @ suguna]
    • why is the fixed bias designed so?

  • At 18 May 2010, 23:49:41 user Raldey wrote:   [reply @ Raldey]
    • Thankz..it really helps me a lot.. now i understand my lesson..!! ^_^

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