The hybrid parameters [h]
The hybrid parameters are values that characterize the operation of a transistor, such as the amplification factor, the resistance and others. They are used to calculate and properly use the transistor in a circuit. Most of the the hybrid parameter values are given in the datasheet by the manufacturer. You do not need to learn everything about hybrid parameters to design a tranisstor circuit, but it is good to know that they exist.

1.The hybrid parameters for Common Emitter (CE) connection
Here is the first set of hybrid parameters for a transistor connected with Common Emitter. For now you do not have to worry about the type of connection. We will discuss them thoroughly in the next post.

       1] h_ie - input impedance
The first hybrid parameter that we will see is the h_ie. This parameter is defined by the result of the division of the V_BE with the I_B.
       h_ie = V_BE / I_B
This parameters defines the input resistance of a transistor, when the output is short-circuited (V_CE=0)
       2] h_fe - Current Gain
This is the most important parameter and is extensively used when calculating a transistor amplifier. This is actually the only parameter you need to know to begin designing amplifiers. The equation for this parameter is the following:
     h_fe = I_C / I_B
When we have the output of the transistor short-circuited (V_CE=0), h_fe defines the current gain of the transistor in common emitter (CE) connection. Using this parameter we can calculate the output current (I_C) from the input current (I_B)
    I_C = I_B x h_fe 
This explains why this parameter is so useful. A typical BJT transistor has typical current amplification from 30 to 800, while a Darlington pair transistor can have an amplification factor of 10.000 or more. Another symbol for the h_fe is the Greek letter β (Beta).

      3]  h_re - Dynamic transfer ratio reverse voltage
This parameter is calculated with this equation
    h_re = V_BE / V_CE
If the input of the transistor is open (I_B=0) then this parameter gives the voltage gain when the transistor is connected with common emitter (CE)
  
      4]  h_oe - Output Conductivity
This parameter is defined with the input open (I_B=0) and the transistor connected in common emitter (CE) connection. The equation is
        h_oe = I_C / V_CE
With the above conditions, this parameter defines the conductivity of the output. So, the impedance of the output can be defined as follows
  ro = 1 / h_oe = V_CE / I_C


2.The hybrid parameters for Common Base(CB) connection

   1] h_fb - Current Gain
Like in Common Emitter connection, in Common Base connection there is a current gain ratio which is defined by the manufacturer with the h_fb parameter. In this type of connection, the current amplification is almost 1 which means that no practical amplification occurs. h_fb is also symbolized with the Greek letter α (Alpha).
  0.9 < α < 1
 The formula to calculate this parameter is the following:
 -h_fb = I_C / I_E 


3.The hybrid parameters for Common Collector(CC) connection

   1]h_fc - Current Gain
As you understand, the current gain is the most important parameter in every type of connection. Same applies for the common collector connection. The equation is as follows:
-h_fc = I_E / I_B 
An alternative symbol for h_fc is the Greek letter γ (Gama).

How BJT transistors work

A BJT (Bipolar Junction Transistor) transistor has inside two similar semiconductive materials, and between them there is a third semiconductive material of different type. So, if the two similar materials are P and the middle one is N, then we have a P-N-P or PNP transistor. Similarly, if the two materials are N and the middle one is P, then we have a N-P-N material or NPN.

Each transistor has 3 leads which we call base, collector and emitter, and we use the symbols b, c and e respectively. Each lead is connected to one of the 3 materials inside, with the base being connected to the middle one. The symbol of the transistor has an arrow on the emitter. If the transistor is a PNP, then the arrow points to the base of the transistor, otherwise it points to the output. You can always remember that the arrow points at the N material. 

Transistor operation

I will now explain the operation for the transistor, using an NPN type. The same operation applies for the PNP transistors as well, but with currents and voltage sources reversed.

With no power applied to the transistor areas, there are two depletion zones between the two P-N contacts. Suppose now that we connect a power source between the base and the collector in reverse-bias, with the positive of the source connected to the collector and the negative to the base. The depletion zone of the P-N contact between the base and the collector will be widened. Moreover, a slight current will flow withing this contact (due to impurities). This current is the reverse contact current and we will use the symbol I_CBO:

Now suppose that we connect another voltage supply between the emitter and the base in forward bias, with the positive of the source connected to the base and the negative connected to the emitter. The depletion zone between the emitter and the base will be shortened, and current (electrons) will flow when the voltage exceeds a specific level. This level depends on the material that the transistor is made of. Germanium (Ge) is the material that was originally used to make transistor, and later Silicon (Si) was used. For Germanium, the voltage is around 0.3 volts (0.27 @ 25^oC), and for Silicon the voltage is around 0.7 volts (0.71 @ 25^oC). Some of the electrons that go through the e-b depletion zone, will re-connect with holes in the base. This is the base current and we will use the I_B symbol for reference. In real life, this current is at the scale of micro-amperes (μA or uA):

But most of the electrons will flow through the base (due to spilling) and will be directed to the collector. When these electrons reach the depletion area between the base and the collector, they will experience a force from the electric field which exists in this zone, and the electrons will pass through the depletion zone. The electrons will then re-connect with holes in the collector. The re-connected holes will be replaced with holes coming from the base-collector power supply (V_CC). The movement of these holes equals to a movement of electrons in the opposite direction, from the collector to the supply. In other words, the current that flows to the emitter will be divided into the small base current and the larger collector current: 

I_E = I_B + I_C

Generally, the number of electrons that arrive at the collector is the 99% of the total electrons, and the rest 1% causes the base current.At the collector, except the electrons that come from the emitter, there is also the reverse current from the base-collector contact that we saw before. Both currents flow at the same direction, so they are added:

I_C' = I_C + I_CBO 

What is Mechatronics ?

The name Mechatronics stems from mechanical and electronics and is a relatively new approach to product design and development, merging the principles of electrical, mechanical, computer and industrial engineering. It addresses the four interconnected disciplines used for all complex modern devices. Mechatronic systems are typically composed of traditional mechanical and electrical components but are referred to as "smart" devices or systems because of the incorporation of sensors, actuators and computer control systems. Over the years, the term "mechatronics" has come to mean the integrated methodology for designing products that exhibit fast, precise performance.

Mechatronics is an emerging field of engineering that integrates electrical engineering, mechanical engineering, computer science, control engineering and information technology. In layman's terms, mechatronics combines these areas of engineering to allow the design, development and application of "smart devices" in an integrated, cross-disciplinary manner. The mechatronics concept establishes basic principles for a contemporary engineering design methodology. In this methodology, engineering products and processes have components that require manipulation and control of dynamic (moving) constructions to the required high degree of accuracy. Also, the design process requires integrating enabling technologies such as information technology and control engineering. A key factor for the design process involves integrating modern microelectronics and the engineering of software into mechanical and electromechanical systems.

mechatronic themes