This course introduces students to the basic components of electronics: diodes, transistors, and op amps. It covers the basic operation and some common applications.

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From the course by Georgia Institute of Technology

Introduction to Electronics

325 ratings

Georgia Institute of Technology

325 ratings

This course introduces students to the basic components of electronics: diodes, transistors, and op amps. It covers the basic operation and some common applications.

From the lesson

MOSFETs

Learning Objectives: 1. Develop an understanding of the MOSFET and its applications. 2. Develop an ability to analyze MOSFET circuits.

- Dr. Bonnie H. FerriProfessor

Electrical and Computer Engineering - Dr. Robert Allen Robinson, Jr.Academic Professional

School of Electrical and Computer Engineering

Welcome back to Electronics, this is Dr. Robinson.

Â In this lesson, we'll look at MOSFET characteristics.

Â In your previous lesson, you were introduced to CMOS logic gates.

Â Our objectives for this lesson are to introduce MOSFET characteristic curves,

Â and to introduce DC biasing.

Â Let's look at some characteristic curves for an in-channel enhancement mode MOSFET.

Â Now in a previous lesson, you saw how the underlying physics of the MOSFET results

Â in a set of characteristic curves known as the MOSFET output characteristics,

Â a plot of the MOSFET drain current versus the drain to source voltage for

Â different values of gate to source voltage.

Â Now implicit in this set of curves,

Â is another characteristic curve known as the MOSFET transfer characteristic.

Â A plot of the MOSFET drain current versus the gate-to-source voltage.

Â Now remember, the parameter that's changed to generate each one of

Â the curves on the output characteristic is VGS, and in this plot I varied VGS.

Â From 1.5 up to a value of four,

Â in steps of 0.5 Volts.

Â So this bottom curve is for VGS = 1.5 or less, and the top curve is VGS = 0.5.

Â So to generate a transfer characteristic from an output characteristic,

Â we choose a VDS.

Â And in this case, you can see that I chose VDS equal to 7 volts.

Â So I draw a vertical line at VDS equals 7 volts, and

Â then work my way up the line reading off the VGS versus ID pairs so

Â here we have a drain current of zero at a VGS of 1.5.

Â So we come to a VGS of 1.5 and we have a drain current of zero.

Â We then move to the next curve, a VGS of 2 and

Â we have some slightly higher drain current.

Â And we continue that to generate the transfer characteristic curve.

Â Now it's apparent from looking at this curve,

Â the Transfer Characteristic Curve, that the relationship between Drain Current and

Â Gate to Source voltage is not a linear relationship.

Â You can also see that here on the Output Characteristics Curve.

Â Between each one of these curves is a VGS step of 0.5 but as the VGS increases

Â the distance between each consecutive curve increases.

Â So on the Transfer Characteristic, in moving from 1.5 to 2 VGS, we

Â get this small change in ID, but moving that same distance in voltage,

Â from say 3.5 to 4, we get a much larger change in drain current.

Â Now let me draw on this set of MOSFET Output Characteristics

Â curves wherein channel enhance mode MOSFET a boundary line and

Â this boundary is define by the equation VDS=VGS-VTO.

Â Now MOSFET's quiescent point or Q point or

Â bias point is defined by the relationship of it's drain current,

Â it's drain to source voltage and it's gate to source voltage.

Â If this key point lies to the left of this boundary line,

Â we say that the MOSFET is operating in it's saturation region.

Â So for example, if it had a,

Â the MOSFET had a drain to source voltage of five volts and a drain current of three

Â milliamps, that keypoint would place the MOSFET in it's saturation region.

Â Now, if those three quantities combine such that the q-point is to the left of

Â this boundary, we say that the MOSFET is operating in it's linear region.

Â Now associated with each MOSFET is a parameter known as the threshold

Â voltage or turn on voltage, VTO.

Â This is the minimum value of VGS for which current flows through the MOSFET.

Â So if VGS is less than this threshold voltage value ID is equal to zero and

Â the MOSFET is off or it's set to be operating at it's cut off region.

Â So on the output characteristics curves this

Â line at the bottom where ID is equal to zero would define the cut off region.

Â Where VGS is less than VTO.

Â Now let's define the MOSFET regions of operation,

Â that we examine graphically in the previous slide, in terms of equations.

Â Remember the MOSFET operates in its cutoff region

Â when it's gate to source voltage is less than the threshold voltage, and

Â in that region, no current flows through the MOSFET.

Â It's drain current is equal to 0.

Â Now, if the gate to source voltage is greater than VTO and current is flowing,

Â then the region of operation is determined by the value of VDS.

Â If VDS is such that we lie to the left of that boundary on the previous slide,

Â or in other words, if VDS is less than VGS- VTO,

Â then the drain current is related to the gate to source voltage in a linear way.

Â If VGS is greater than VTO but the drain to source voltage is such that we lie to

Â the right of that boundary on the previous slide,

Â VDS is greater than VGS- VTO then you can see that the drain current is

Â related to the gate to source voltage as a square law relationship.

Â It's proportional to VGS squared.

Â Now in these equations for ID in the linear regions and ID

Â in the saturation region, you can see that we have two intrinsic MOSFET parameters.

Â K, known as the transconductance parameter, which has units of amps per

Â volts squared and VTO, the thresh hold voltage or turn on voltage.

Â Now let's look at how the different regions of operation affect the shape

Â of the MOSFET transfer characteristics.

Â Here we're examining two sets of transfer characteristics.

Â One where VDS is equal to seven volts and one where VDS is equal to one volt.

Â For a MOSFET having a threshold voltage of 1.5 volts.

Â Now, we can see that when VGS is less than VTO of 1.5 volts.

Â There's no drain current and the MOSFET is off in its cutoff region.

Â Now, let's examine this point here where VGS is equal to two volts.

Â Now we know that the region of operation is determined by the relationship of VDS

Â to VGS- VTO.

Â So where VGS is equal to two volts that implies

Â that VGS- VTO is equal to zero point five volts.

Â So for both this condition, where VDS is equal to one volt and

Â this condition where VDS is equal to seven volts, both seven and

Â one are greater than zero point five.

Â So the MOSFET is operating in the saturation region,

Â For a VGS of 2.

Â But let's look at a VGS of 2.5.

Â Here we have VGS minus VTO is equal to 1.

Â And the MOSFET that has a VDS of 7 is still operating in

Â its saturation region because 7 is greater than 1.

Â But the MOSFET that has a VDS of 1 volt

Â is operating at the transition between the saturation region and the linear region.

Â Because 1 is equal to 1, or VDS is equal to VGS minus VTO.

Â Then as we continue to increase the gate to source voltage moving in this

Â direction, the MOSFET with a VDS of 1 will continue to be in its linear region.

Â And you can see that in this region ID is related to VGS in a linear way.

Â But for the MOSFET that has a VDS of 7 as VGS continues to increase,

Â it remains in its saturation region where ID is related to VGS in a parabolic way.

Â Or ID is related to the square of VGS.

Â Now when a MOSFET is operating in it's saturation region,

Â it can be used as an amplifier.

Â When a MOSFET is operating in it's liner region it's used as a voltage controlled

Â resistor where the resistance is controlled by the gate to source voltage

Â So, in summary, during this lesson we introduced MOSFET characteristics, and

Â we introduced DC biasing.

Â In our next lesson, we'll look at the common source amplifier,

Â a particular type of circuit that uses a MOSFET,

Â and we'll concentrate on the DC analysis of this amplifier.

Â So thank you, and until next time.

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