# Transistors

 - [directory](http://www.ittc.ku.edu/~jstiles/412/handouts/) with a lot of slides about transistors
 - [notes](http://aries.ucsd.edu/najmabadi/CLASS/ECE60L/02-S/NOTES/) about transistors

Have three pins named **base/gate**, **collector/source** and **emitter/drain**

MOSFET is voltage controlled. While BJT is current controlled,

 - The collector must be more positive than the emitter
 - the base-emitter and base-collector circuits behave like diodes
 - \(I_C=h_{FE}I_B=\beta I_B\) where \(\beta\) is tipically about 100

the \(\beta\) parameter is highly dependent on temperature, current, etc..
so **a circuit dependents on it is a bad circuit**.

## BJT

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.

 - [Bipolar Transistor](http://www.electronics-tutorials.ws/transistor/tran_1.html)
 - [NPN](http://www.electronics-tutorials.ws/transistor/tran_2.html)
 - [Tutorial](http://blog.oscarliang.net/bjt-bipolar-junction-transistor-beginner-tutorial/)
 - [Slides with transfer function calculations](http://aries.ucsd.edu/NAJMABADI/CLASS/ECE65/12-W/Slides/ECE65_W12-BJT.pdf)
 - [Slides from a course](http://cc.ee.ntu.edu.tw/~lhlu/eecourses/Electronics1/Electronics_Ch4.pdf)

### Q-point

The Q stands for quiescent which means still, quiet or stable. The Q-point is
on the DC load line and represents the current flowing in output circuit and
voltage across it. In our case, they are IC and VCE. The value of current and
voltage at Q-point are written as (VCEQ, ICQ).

The position of Q-point is selected according to the application of transistor.

 - If transistor is to be used as switch, Q-point is in cut-off region for open switch and in saturation region for closed switch.
 - If transistor is to be operated as amplifier, Q-point is placed exactly in
   the middle of the DC load line. It is preferred at midpoint of DC load line
so that it does not enter in other regions due change in temperature, βdc etc.

 - [Biasing calculation](http://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/biasing-calculations/)

### Configuration

Transistors comprise of three terminals i.e. Base, emitter and collector but
when we connected it in a circuit, we require four terminals; two for the input
and two for the output. To overcome this problem we make one terminal of the
transistor common to both input and output terminals. The input is fed between
the common terminal and one of the other two terminals while the output is
obtained between the common terminal and the remaining terminal. Based on this,
transistor is configured in the circuit in the following three ways:

 - Common emitter (with emitter degeneracy when a resistor is used between emitter and ground)
 - Common base
 - Common collector

### Junction capacitance

 - [slide](http://transport.ece.illinois.edu/ece340f12-lectures/ece340lecture27-junccap.pdf)
 - [slide](http://www.ittc.ku.edu/~jstiles/412/handouts/5.8%20BJT%20Internal%20Capacitances%20and%20high%20frequency%20model/section%205_8%20BJT%20Internal%20Capacitances%20lecture.pdf) about modelling BJT Internal Capacitances

## MOSFET

Variable resistor controlled by voltage: you can vary the resistence between drain and source
varing the voltage applied between gate and source. The output current is not a multiplier
of the input current (remind that MOSFET are all about voltage).

 - $R_{DS}(on)$
  - $V_{GS}TH$: Voltage threasold between gate and source (if it's less than 5 volts then is logic level)

It's important to calculate the power (and of conseguence the heat) consumed by the formula

$$
P = R_{DS}\cdot I^2
$$

and with the parameter $R_{\theta JA}$ (Junction to ambient coefficient) and the following
formula that calculate the power dissipated at ambient temperature $T_A$ when the
transistor has a maximum junction temperature $T_J$

$$
P_D = {\max(T_J) - T_A\over R_{\theta JA}}
$$

If $P\leq P_D$ then you don't need a dissipator.

Here an example of usage

![Example for N Channel MOSFET](Images/n-mosfet-example.png)

## Links

 - [EE 320/320L Electronics I](http://whites.sdsmt.edu/classes/ee320/)
 - [Tutorial by Sparkfun](https://learn.sparkfun.com/tutorials/transistors)
 - http://electronics.stackexchange.com/questions/19233/when-to-use-what-transistor
 - http://blog.oscarliang.net/how-to-use-mosfet-beginner-tutorial/
 - [AO #10 BJTs as Transistor Switches](https://www.youtube.com/watch?v=sRVvUkK0U80)
 - [AO #11 MOSFETs and How to Use Them](https://www.youtube.com/watch?v=GrvvkYTW_0k)
 - [Transistor biasing](http://www.electronics-tutorials.ws/amplifier/transistor-biasing.html)
 - https://rheingoldheavy.com/building-things-npn-transistors/
 - #281: Bipolar Transistor Switching Time Measurement [video](https://www.youtube.com/watch?v=XgSKLsWAWGs)