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

Diodes Part 2

Learning Objectives: 1. Examine additional applications of the diode. 2. Make use of voltage transfer characteristics to analyze diode circuit behavior.

- 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.

Â This is Dr. Robinson.

Â In this lesson, we're going to look at voltage regulators.

Â In our previous lesson, we introduced diode limiters and

Â we examined circuits that could be used as limiters.

Â Our objectives for today's lesson are to introduce voltage regulation.

Â And to examine diode regulator circuits.

Â Here I have a definition of an ideal dc voltage regulator.

Â It's a device that maintains a constant dc output voltage,

Â regardless of variations in the input voltage or the load.

Â Lets examine the behavior of a regulator in blocked diagram form.

Â Here I have the block representing the regulator.

Â On the left side is the input voltage,

Â a DC voltage applied to supply the power to the regulator.

Â Now if this were an ideal voltage regulator, this output voltage here

Â would not vary with changes in the value of the input voltage.

Â Now, typically this input voltage is a voltage greater than the output voltage of

Â the regulator.

Â So say this were a five volt voltage regulator.

Â Then it would produce at its output five volts.

Â A reasonable input voltage to that regulator would be, say, 15 volts.

Â Now, if we have a circuit that uses, in different parts,

Â different power supply voltages we could use different regulators or

Â multiple regulators to supply those voltages.

Â So from this single input 15 volts supply we could add one regulator that

Â produces five volts for one portion of the circuit.

Â And we could have another regulator that produces say an output voltage of 12 volts

Â for different portions of the circuit.

Â Now, let's add a load to our regulator.

Â So here I'm drawing a resistor of value RL for load resistor.

Â Now this doesn't have to be an actual resistor.

Â This resistance can represent the input to some circuit of interest to us.

Â Like say, an amplifier, a cell phone,

Â something that we need to power with a voltage supply.

Â As, this resistance value changes for this output voltage to

Â remain constant we know that by Ohms law the current here must change.

Â So if, say, I made this resistance smaller and smaller and

Â smaller, for the output voltage across the regulator to be constant,

Â this current must continue to increase, to keep the equation V out

Â equals I L, the load current times R L, true.

Â Now, in a practical regulator, there's some limit to the amount of

Â current that can be put out of its output terminal.

Â So, that at some point as this resistance becomes smaller and

Â smaller, we will lose the regulation capability.

Â Now, there are two figures of merit that are typically used to

Â describe a regulator.

Â These are the line regulation and the load regulation.

Â The line regulation is the ratio of the change in

Â output voltage to a change in input voltage.

Â Now, remember, an ideal regulator, if we change the input voltage,

Â the output voltage would not change at all.

Â So for an ideal regulator, the line regulation would be zero.

Â Now for load regulation, that indicates how well the regulator can

Â maintain the output voltages for changes in the value of this load resistance.

Â So, as I make this load resistance smaller, the current increases.

Â If that current changes by some amount, ideally for

Â a perfect regulator, the output voltage would not change at all.

Â So, the load regulation for an ideal regulator would also be equal to zero.

Â Lets examine the concept of loading by looking at ideal and real voltage sources.

Â Here I have an ideal voltage source.

Â If I put a load across this ideal source.

Â Of value RL, we know that because of the properties of an ideal voltage source,

Â no matter what the value of this resistance is,

Â the voltage across this resistor will always be equal to V in.

Â However, a real voltage source one that we can build in practice has associated with

Â it some sort of internal resistance which I have modeled here as a resistor RS.

Â So this source it's output terminals are between here and here.

Â Now if we attach a load to this real voltage source.

Â [NOISE] We can see that we have completed the loop

Â around the circuit, current can now flow.

Â And the voltage here at the output of the source does not have to

Â be equal to the input voltage.

Â Because we can have a drop across this source resistor.

Â If RL were an infinite resistance such that no current could flow, or

Â in other words an open circuit.

Â Then the voltage we measure across the output terminals of the real voltage

Â source would be equal to V in, just as it would be for the ideal voltage source.

Â We can write [NOISE] a relationship between the output voltage and

Â the input voltage for this real voltage source using voltage division.

Â The out, is equal to, again by voltage division,

Â RL over RS plus RL times Vn.

Â We can see that as the source resistance approaches zero, the output voltage

Â approaches the input voltage and the real voltage source becomes more ideal.

Â Now this concept of the output voltage

Â changing with changes in load resistance is known as loading.

Â We say that we have loaded the source,

Â because the output voltage is no longer equal to the open circuit output voltage.

Â Let's look at how we can take advantage of the characteristics of

Â a diode to implement a voltage regulator.

Â Here I've drawn the characteristic I-V curve for a diode.

Â You can see the expected exponential relationship between the current through

Â the diode and the voltage across the diode.

Â Now lets say this diode is introduced into a circuit and

Â biased such that the current through it places it in this portion of the curve.

Â This steep portion where, for

Â large changes in current there's small changes in voltage.

Â Now we can see for this particular diode I can vary the current through the diode

Â from say 500 milliamps to one amp, and as that current is varied the voltage

Â across the diode, changes only by approximately 50 mV.

Â Now, as the current through the diode becomes less and less, we can see that,

Â say, if we were operating in this region, then for the same amount of current change

Â in the diode we can get a much larger change in voltage across the diode.

Â Now it's the steepness of this curve that we can take advantage of

Â to implement a voltage regulator.

Â If for example this curve were a perfectly vertical line then we can see that for

Â any current through the diode, the voltage across the diode will not change.

Â Here I've drawn a circuit that can be used to implement a diode regulator.

Â This portion of the circuit is the regulator,

Â this resistor RL is the load resistor attached to the output of the regulator.

Â Now, say initially that there's no load resistance.

Â Or the load resistance is at infinite value.

Â We can calculate the current that flows around this loop.

Â We know that, the forward voltage drop across a diode, when it's forward biased,

Â is approximately 0.7 volts.

Â So this current, I, can be written as,

Â I is equal to 12 volts minus 0.7 volts, divided by R1, 100 ohms.

Â Is equal to 11.3 over 100,

Â is equal to 113 milliamps.

Â Now this current is

Â chosen to bias the diode in the steep portion of its characteristic curve.

Â Now, let's say we add the load resistor to the regulator.

Â When the resister is present some of that current that 113 milliamps can

Â now flow through this branch.

Â But because the diode is biased on the steep portion of its curve even though

Â the current through the diode has decreased the voltage across the diode

Â remains relatively constant.

Â Now as this resistor gets smaller and smaller and smaller and we take more and

Â more of the diode current away, so that it can flow through this branch.

Â We eventually reach a point where the steep part of

Â the characteristic curve is left and we lose the regulation.

Â So this circuit can be thought of as a approximately a 0.7 volt regulator.

Â Where that 0.7 volts is due to one forward voltage drop across this diode.

Â Now lets look at how that circuit actually behaves in

Â terms of it's line regulation and it's load regulation.

Â On this graph I've plotted the output voltage of

Â the regulator versus the load resistance value.

Â You can see that for a load resistance value of 500 ohms.

Â We have an output voltage of approximately 0.75 volts,

Â one forward diode voltage drop.

Â And you can see that the output voltage is maintained for values of 400,

Â 300, 200, and 100 ohms of load resistance.

Â Now, once we decrease below 100 ohms of load resistance, the current through

Â the load resistor is such that we are pulling more current away from the diode.

Â And it's leaving that steep portion of that characteristic curve.

Â Now for this particular curve,

Â it turns out that a load resistance value of approximately 45 ohms,

Â results in a decrease in the output voltage by approximately 1%.

Â So, at this point here, the output voltage has decreased by 1% from

Â the voltage it is in this region here.

Â Lets look at how the output voltage varies with changes in input voltage.

Â In other words, what is its line regulation.

Â Now nominally, that circuit we're looking at has an input of 12 volts.

Â This is a plot of output voltage of the regulator versus input voltage to

Â the regulator.

Â And this point here the input voltage is 12 volts.

Â We can see that the output voltage is 0.753 millivolts for that input voltage.

Â Now say for some reason the input voltage is changed to 15 volts.

Â Well, if that's done, the output voltage is equal to 0.768 volts.

Â So there's a very small change in output voltage for

Â this relatively large change of 3 volts in the input voltage.

Â And, if we decrease the nominal value of 12 volts to 9 volts for some reason.

Â We can see that the output voltage is still 0.735 volts,

Â still a relatively small change in output voltage.

Â So we can see that this circuit for

Â that six volt swing the output voltage is relatively stable around 0.753 volts.

Â Now the voltage regulator that we've been examining has a regulated output voltage

Â of approximately 14 diode voltage drop of 0.7 volts.

Â How can we build a diode regulator that has an output voltage greater than

Â this voltage?

Â Now here I don't mind at all if you want to pause the video and

Â think about the answer to this question before I give it to you.

Â The answer is we add more diodes.

Â You can see here that I placed in series three diodes,

Â across the output of the regulator.

Â Each one introduces a forward diode voltage drop of approximately 0.7 volts.

Â So, the total output voltage of this regulator would be 0.7 plus 0.7 plus 0.7,

Â or approximately 2.1 volts regulated output voltage.

Â Now let's look at how we can use a diode regulator circuit as a component of

Â a DC power supply, to improve its performance.

Â You may remember from a previous lesson.

Â The process involved converting an AC voltage to a DC voltage.

Â Remember we start with an AC input voltage, possible from our wall outlet.

Â We apply that to a rectifier.

Â And let's assume the rectifier is a full wave rectifier.

Â We then filter this combined AC and

Â DC voltage to obtain an output voltage that is.

Â Mostly a DC voltage, but still has an AC component.

Â Now, we can apply this voltage as the input to a voltage regulator.

Â [NOISE] And we know that a good regulator will

Â produce a constant output voltage.

Â Even when the input voltage has variations.

Â So the effect of adding this regulator to our power supply is to

Â smooth the output DC voltage of the regulator.

Â So we can draw the output voltage of the regulator as ideally a straight line.

Â The variations in the input voltage have been removed

Â by the functionality of the regulator, its line regulation.

Â So in summary, during this lesson we introduced voltage regulation and

Â we examined a diode regulator circuit.

Â In the next lesson, we are going to look at the demodulation of AM signals, or

Â in other words, you can think of this as a good start to building an AM radio.

Â Thank you and until next time.

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