Op-Amps & Mosfets

1. OP-AMPs

There are two different inputs which are called inverting(-) and non-inverting(+) input. If the non inverting input is higher than inverting input the output will follow the V+ of voltage rail. On the other hand, if the inverting input is higher the output will go the V- of voltage rail. Therefore, I can make various circuits with different tendency of output values from different input voltages.
reference by http://moodle.unitec.ac.nz/file.php/1309/Theory/Lesson_6_Opamps/Lesson6_Operational_Amplifiers.pdf


-  Non inverting OP-AMP
Vout= ((Rf/Rs)+1) x Vin

The non-inverting OP-AMP will need a path for DC to ground. If the signal source might not give this, or if that source requires a given load impedance, the circuit will require another resistor  from input to ground. In either case, the ideal value for the feedback resistors (to give minimum offset voltage) will be such that the two resistances in parallel roughly equal the resistance to ground at the non-inverting input pin.
- Inverting OP-AMP
Vout= ((Rf/Rs) x -Vin
The output voltage changes in an opposite direction to the input voltage

- Applications

· audio- and video-frequency pre amplifiers and buffers
· differential amplifiers 
· differentiators and integrators 

· filters 

· precision rectifiers
· precision peak detectors 
· voltage and current requlators 
· analog calculators
· analog to digital converters 
· digital to analog convrters 
· voltage clamps 
· oscillators and waveform generators 

reference by http://en.wikipedia.org/wiki/Operational_amplifier


2. Mosfets(Metal Oxide Semiconductor Field Effect Transistor)
 A mosfet operates device with its switching function which is similar to transistors with simiconductor material. The transistor's switching is controlled by the current, however, mosfets are voltage controlled device.
 -  Comparison

reference by http://moodle.unitec.ac.nz/file.php/1309/Theory/Lesson_5_MOSFET/Lesson_5_MOSFET.pdf


- Types
Enhancement mode :
 The transistor requires a Gate-Source voltage, to switch the device "ON". The enhancement mode MOSFET is equivalent to a "Normally Open" switch.
N-channel type

There are three different terminals in a N-channel type. When the certain voltage gains in a gate, the drain will connect the source(-). Electron flows from negative to positive. This type is similar to PNP transistor.


P-channel type

The diode direction is only changed. When the gate voltage is at 0v, this mosfet turns on so the between Drain and source will be connected. This type is similar to NPN transistor.


Depletion mode :
 The transistor requires the Gate-Source voltage,  to switch the device "OFF". The depletion mode MOSFET is equivalent to a "Normally Closed" switch.


- Transistor / MOSFET tutorial

reference by http://www.youtube.com/watch?v=Te5YYVZiOKs

- Resistors in series circuit





 When the resistors are connected in series the Vout (available voltage) will be different values due to different resistor values. Therefore, I can make the various circuits from the output voltages, such as an O2 sensor indicator. 
Formula Vout1 =(R2+R3)/RT x Vin, Vout2 =R3/RT x Vin
If I know the Vout1 and Vout2  the R1, R2 and R3 of the resistor values would be determined.
Chose any one resistor value.
Voltage drop of R1=Vin-Vout1,
I=V/R, (calculate the amp value)
Voltage drop of R2=Vout1-Vout2 
R2=Vd2/I
 Voltage drop of R3=R3-0
R3=Vd3/I