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

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 talk about diode limiters.

Â In our previous lesson, we introduced an application of rectifiers.

Â The conversion of an AC sinusoid to a DC voltage that could be used to

Â power our electronic devices.

Â The objectives for today's lesson are to introduce limiters,

Â examine their behavior for sinusoidal inputs and to analyze limiter circuits.

Â A limiter, also known as a clipper,

Â is a non linear device that limits the output voltage to a particular level.

Â Let's look at how a limiter effects the shape of an input sinusoidal voltage

Â Here I have the block diagram that represents the limiter,

Â here's our input voltage.

Â Let's look at how the output voltage of this limiter is related to

Â the input voltage.

Â You can see that as the input voltage is increased past some level,

Â some voltage level, which I'm calling the V plus limit,

Â the output voltage is limited, or restricted to that voltage level.

Â As the input voltage is decreased below some voltage level, which I'm calling V

Â minus limit, the output voltage is restricted to that level.

Â But, if the input voltage is between these two levels, or

Â these two limits, the output voltage is equal to the input voltage.

Â Now this is the, an output of a general limiter where we have two limits,

Â both a positive limit and a negative limit.

Â But it's possible to design a limiter such that we only have a positive limit or

Â only have a negative limit.

Â Let's look at how a limiter is defined in terms of rules that act on

Â the input to produce the output.

Â If the input is greater than or

Â equal to the positive limit, the output is set equal to the positive limit.

Â If the input is less than or

Â equal to the negative limit, then the output is set equal to the negative limit.

Â Then, otherwise, if the input is between these two limits,

Â then the output is equal to the input.

Â Here's the voltage transfer characteristic of the limiter we've been talking about.

Â We can see that, if the input voltage, let me draw two lines here,

Â to represent the range of input voltages that are between the V plus limit and

Â the V minus limit.

Â If the input voltage is within this range, the output is equal to the input and

Â our voltage transfer characteristic, or

Â our plot of output voltage versus input voltage, has a slope of one.

Â If the input voltage is greater than the positive limit,

Â then the output voltage is set equal to the positive voltage limit.

Â And if the input voltage is less than the negative voltage limit,

Â the output is equal to the negative voltage limit.

Â Now you can see from this voltage transfer characteristic that to implement a circuit

Â that has this voltage transfer characteristic requires three states.

Â One state in this region, one state in this region, and one state in this region.

Â Two trans, two corners on the voltage transfer characteristic,

Â which represent two transitions between states.

Â Here I've drawn a schematic of a circuit that can be used to implement

Â a positive voltage limiter.

Â We can see that the output voltage is taken across this branch,

Â the series combination of V1 and diode D1.

Â An ideal diode.

Â The input voltage is on the left side.

Â In this circuit are two DC voltage supplies.

Â A voltage supply, V in, that attempts to push current around the loop in

Â this direction because of its polarity.

Â The current flows from the positive side to the negative side.

Â And the voltage supply V1 that attempts to

Â push current around the loop in this direction.

Â Now we can see that because of the diode D1,

Â current is only allowed to flow around the loop in this direction.

Â Now the net direction of current depends on the voltage difference between V

Â in and V1.

Â If V in is greater than V1 then current will flow clockwise around the loop and

Â the diode D1, because of its direction, is forward biased and

Â the output voltage across this branch would be equal to V1.

Â So we can write that for the case where V in

Â is greater than V1, D1 is on.

Â And the output voltage, V out, is equal to V1.

Â Then for the case where V1 is greater than V in,

Â the case where current should flow in this direction,.

Â Counter clockwise around the loop but cannot because of the direction of D1.

Â D1 is off a reverse biased, so no current would flow through this branch, and

Â the output voltage taken across here would be exactly equal to the input voltage.

Â Because with zero current, there can be no voltage drop across the resistor R.

Â So we can write for the case where V in is less than V1,

Â D1 is off and

Â the output voltage V out is equal to the input voltage V in.

Â We can combine these two equations to form our voltage transfer characteristic, and

Â let me just quickly sketch it out from the equations down here.

Â Our V out axis and our V in axis.

Â And let me label V1 and V1.

Â Now when V N is less than V1,

Â we can see that the output voltage is equal to the input voltage.

Â So on our characteristic curve, you would have a line with slope of 1 volt per volt.

Â Then when the input voltage is greater than V1,

Â we can see that the output is exactly equal to V1.

Â And we would have the voltage transfer characteristic for

Â a positive voltage limiter.

Â Here, I've increased the complexity of the circuit by adding an additional branch.

Â This portion of this circuit is exactly the same as the positive limiter that we

Â analyzed on the previous slide.

Â I've added this branch.

Â Another series combination of a diode and a voltage source.

Â But you notice that the direction or

Â polarity of this diode is opposite that of the diode D1.

Â I'm assuming the diodes here are ideal.

Â And I'm also assuming that the voltage V1 is greater than the voltage V2.

Â Letâ€™s draw a,

Â a number line that represents relative voltages in this circuit.

Â So we have two voltages, V1 and V2, that

Â indicate where transitions in state for this circuit will occur.

Â And we know that V1 is more positive than V2.

Â Let's first assume that the input voltage is a voltage that lies between the two

Â voltages V1 and V2.

Â In that case, VN is less than V1, so the voltage at

Â the anode of D1 would be less than the voltage at the cathode of D1.

Â So D1 would be off.

Â VN is greater than V2.

Â In that case, the voltage at the cathode of D2

Â is greater than the voltage of the anode of D2.

Â So D2 would also be off.

Â So for input voltages in this range, we know that both D1 and D2 are off.

Â So we can write that for this range here.

Â With D1 and

Â D2 both off, the output voltage would be equal to the input voltage.

Â Now lets increase the input voltage such that we're in this range, greater than V1.

Â If our voltage at the input is greater than D1,

Â then D1 would now be forward biased, but D2 would still be reverse biased or

Â off, because the voltage here is bigger than the voltage here.

Â So when D1 is on,

Â we can replace it by a short circuit, and the output voltage measured from this node

Â to this node would be measured directly across the voltage V1.

Â So we can write that for this range here, Vout is equal to V1.

Â Then finally, let's decrease the voltage such that,

Â decrease the input voltage such that it is less than the voltage V2.

Â If this voltage here is less than V2, diode D1 would be reverse biased,

Â because this voltage is less than this voltage.

Â But, D2 now is forward biased,

Â because the voltage here at its anode is greater than the voltage at the cathode.

Â So D2 is forward biased in this region, and

Â the output voltage is measured directly across the voltage D2.

Â So, for this region here, we can write

Â that V out is equal to the voltage V2.

Â So, this circuit has three states of operation.

Â In this region here, both diodes D1 and D2 are off.

Â In this region here, diode D1 is on, and in this region here diode D2 is on.

Â Let's look at the voltage transfer characteristic for this circuit.

Â We can see that this characteristic is the characteristic that I

Â showed you earlier in the lesson for the bipolar voltage limiter.

Â When the input voltage is between V1 and

Â V2, in this region here, the output is equal to the input so

Â that Vtc would have a slope of one volt per volt.

Â If the input voltage is greater than V1,

Â then the output voltage is equal to V1 volts.

Â If the input voltage is less than V2 volts,

Â then the output voltage is equal to V2 volts.

Â And here it's apparent that the circuit has three states.

Â One state here, one state here, one state here.

Â This state occurs when both diodes are off.

Â This state occurs when diode D1 is on, and this state occurs when diode D2 is on.

Â Let's look at the relationship between halfway rectifiers and limiters.

Â Let's say that we have a negative voltage limiter that has a negative limit of

Â zero volts.

Â As I've drawn here.

Â When the input voltage is positive,

Â the output voltage is equal to the input voltage.

Â But, when the input voltage goes less than the limit of zero volts,

Â the output is equal to zero volts.

Â You can see that this negative limiter, with a limit of zero volts, has exactly

Â the same voltage transfer characteristic as a positive half wave rectifier.

Â We can draw for

Â a sinusoidal input, the output of a circuit that has this characteristic.

Â So when the input is positive, the output is equal to the input.

Â But when the input is negative the output is equal to 0.

Â Let's say instead,

Â we had a positive limiter that had a voltage limit of zero volts.

Â It would have a characteristic that looks like this.

Â This positive limiter, with a limit of zero volts,

Â has the same transfer characteristic as a negative half-wave rectifier.

Â When the input is negative, the output is equal to the input but

Â when the input is positive, the output is equal to zero.

Â So for this characteristic, with the sinusoidal input,

Â the output voltage for the same sinusoidal input would look like this.

Â When the input is positive the output is zero,

Â when the input is negative the output is equal to the input.

Â So in summary, during this lesson we have looked at limiter operation and

Â we looked at some circuits that could be used to implement limiters.

Â In our next lesson we going to look at circuits known as voltage regulators.

Â So thank you and until next time.

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