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

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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

Bipolar Junction Transistors

Learning Objectives: 1. Develop an understanding of the NPN BJT and its applications. 2. Develop an ability to analyze BJT 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're going to look at the BJT terminal characteristics.

Â In our previous lesson, we introduced the Bipolar Junction Transistor or BJT.

Â And our objectives for this lesson are to examine the BJT terminal characteristics.

Â And the curves and equations that represent these characteristics.

Â I told you earlier that we were going to characterize the BJT,

Â based on its external behavior, it's behavior at its terminals.

Â And you can think of characterizing the BJT in that way,

Â as performing an experiment with this being the experimental set up.

Â I've connected the BJT in this way, so

Â that I have a voltage supply, a DC battery between emitter and base.

Â And I also have a battery connected between collector and emitter and

Â what I want to do is fix this voltage at 12 volts.

Â And vary this voltage between zero and one volts and

Â as I do that, I want to measure the (uuu) current Into the transistor Ic.

Â If I do that, remember fixing this at 12 and

Â sweeping this from zero volts to one volt.

Â We get a characteristic curve that looks like this,

Â known as the transfer characteristic curve.

Â A plot of Ic versus VBE for

Â a constant VCE and you can see that this curve has exactly the same form.

Â As does the relationship between current and

Â voltage for a p-n junction diode, except that, remember.

Â The voltage on this axis is the voltage between these two terminals.

Â While the current on this axis is the current between collector and emitter.

Â So again we're controlling a separate current with the VBE voltage,

Â the base emitter voltage.

Â Let's relate this curve to the regions of operation that we talked about.

Â You can see that if we're less,

Â have a Vb of less than about .5 volts the transistor is off, Ic is equal to zero.

Â So this region, we can label as the cutoff region.

Â And this region where VBE is greater than about 0.5 volts,

Â you can see the curve begins to leave the zero volt asymptote.

Â And the transistor turns on, allowing current to flow from collector to emitter,

Â ao in this region, this could be either the active- or the saturation region.

Â Because in both of these regions, current flows from collector to emitter.

Â Now remember, when we analyze the p and

Â junction diode, we assume that there was a forward voltage drop.

Â Across the diode of approximately 0.7 volts and for the same reason,

Â we can assume that when the transistor is on.

Â That we can approximate this very steep curve by a straight line of

Â approximately 0.7 volts.

Â So, the intersection of this approximation

Â with the X axis is about point seven volts.

Â So when the transistor is on,

Â we can assume that the DBE is about point seven volts.

Â We can obtain a second set of characteristic curves for

Â the NPN BJT, using this setup.

Â So here, I've replaced the voltage between base and a meter by A current supply.

Â And I still had a voltage connected between collector and

Â emitter, so to form this set of curves known as the output characteristic curves.

Â I hold the base current at some consent values, so

Â I set this to some consent value.

Â And then, I sweep VCE from zero to 12 volts,

Â while measuring the collector current I see.

Â So just like the transfer characteristic code,

Â the y axis here is the collector current.

Â So I say, initially said, IB equal to zero and

Â sweep DCE from zero volts to twelve volts.

Â I will get this curve here and

Â because IB is equal to zero, IC is equal to zero no matter what DCE is.

Â So this is the IB equals zero,

Â microamps curve.

Â I then increased Ib to 20 microamps and make my sweep again and

Â I get this green curve, so this is the 20 microamp curve.

Â And I can continue that 20,

Â 40 ,60 ,80, 100 microamps to get this family of curves.

Â And again, the difference between curves is the value of IB and

Â the output characteristic of curves.

Â Is the plot of IC versus VCE for constant IB.

Â Now, just like we did for the transfer characteristic curves,

Â we can identify the regions of operation.

Â On this set of curves, this red line where IC is equal to

Â zero would be the cut off region.

Â This region here?

Â Where the relationship between IC and VCE is linear,

Â see these straight lines but IC is greater than zero, is the active region.

Â And finally, this region here, this very narrow region.

Â Let's see if I can label that.

Â This region here is the saturation region, let me just put it the SAT.

Â And we can see that a characteristic of the saturation region

Â is that VCE is approximately a constant of about 0.2 volts.

Â Zero point two volts, so when in saturation,

Â we can assume that VCE is zero point two volts and get a pretty accurate result.

Â If VCE is greater than zero point two volts and

Â IC is greater than zero, we're in the active region.

Â Here I've summarized the information we obtained from the characteristic curves

Â for each of these three regions.

Â Cutoff active and saturation.

Â We know from the transfer characteristics curve

Â that when we're in the cutoff region the transistor when it's off.

Â We have a VBE of less than or equal to 0.5V and

Â we also know in that region that the base current.

Â The collector current and the emitter current are all equal to zero.

Â In the active region, we know from the transfer characteristic curve that

Â VBE can be approximated at 0.7 volts.

Â We know from the output characteristic curve, that VCE is greater than 0.2 volts.

Â We know the transistor is on, so the base current is greater than zero and

Â we didn't obtain this equation from the characteristic curve.

Â But I am telling you that in the active region, IC, IB, and

Â IE can be related in this way.

Â Where beta is the base to collector current gain with a typical value of 100.

Â And alpha is the emitter to collector current gain with a typical value of .99.

Â You can consider these both to be parameters of the transistor.

Â In the saturation region, we know that again,

Â from the transfer characteristic curve that VBE is approximately 0.7.

Â We know from the output characteristic curve that VCE is equal to approximately

Â 0.2 volts, the transistor is on.

Â But in the saturation region, the transistor is saturated,

Â which means that maximum current into the collector is flowing.

Â So, the collector current is actually less than what we'd expected to be

Â from the beta IB product.

Â So if we're in the active region, IC would be equal to beta IB but

Â because we've reached the maximum value of collector current.

Â As we further increase the base current, this number is less than this number.

Â Now, it's possible to write equations that define the behavior of the BJT and

Â all regions of operation.

Â But if we know we're operating in the active reason,

Â the amplifier region, we can make approximations to those equations.

Â And I've shown these approximation here.

Â Here we have the relationship between IC and VBE,

Â you can see it's an exponential relationship.

Â Just as it was for the PN junction dot iode.

Â In this equation, IS is known as the saturation current and

Â VT is the thermal voltage.

Â The thermal voltage is given by kT over q,

Â where k is Boltzmann's constant, q is the charge on an electron.

Â And T is typically assumed to be 300 Degrees kelvin to give a typical

Â value use in calculation of 0.0259 Volts or 25.9 Millivolts.

Â So if we plot this curve, this IC versus DVE curve,

Â using this parameter of the transistor and this constant.

Â We would get the transfer characteristic curve In the active region.

Â Now this equation, Ic is equal to beta Ib.

Â If I make this substitution for the value of beta,

Â then we have a single equation that relates Ic to both Ib and Bce.

Â We have a linear relationship if Ib is considered to be a constant and

Â we get those straight lines.

Â That we saw in the active region of the output characteristics curve.

Â Now ,Beta naught

Â can be considered an intrinsic transistor parameter along with ISO.

Â And this quantity VA, which is known as the early voltage, so

Â if you looked at the data sheet of a transistor.

Â These quantities, beta naught, IS0 and VA,

Â would be on there somewhere and those three parameters.

Â Give us the values to plug in to these equations and

Â these equations determine the operation of the transistor in the active region.

Â Now, beta naught is known as a zero bias base to collect current gain.

Â There's a typical value of 100, which indicates that there's a current

Â gain from base to collector, so a small value of base current.

Â Because this is large, can result in a large collector current,

Â which is why this transistor can be operated as an amplifier.

Â Alpha is related to Beta in this way, Beta over Beta plus one, so

Â if beta is 100, our beta null is 100.

Â Alpha has a typical value of 0.99, the early voltage Va,

Â has a typical value of 150 volts.

Â And the saturation current is a very small number,

Â typically about one times 10 to the minus 15 Amps.

Â So in summary,

Â during this lesson, we examined the terminal characteristics of the BJT.

Â In our next lesson,

Â we'll continue to look more at the parameters of the BJT, Beta, IS0, and VA.

Â And we'll look at how those parameters affect the characteristic curves.

Â So thank you and until next time.

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