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

So welcome back to Electronics.

Â This is Dr. Robinson.

Â In this lesson we're going to look at what are called

Â Voltage Transfer Characteristics.

Â In your previous lessons, we introduced half-wave and full-wave rectifiers, and we

Â analyzed example circuits that were used to implement these types of rectifiers.

Â In today's lesson we're going to introduce voltage transfer characteristics.

Â We're then going to use VTCs to determine a circuit's output for a given input.

Â And we're also going to, from given input and

Â output plots, see if we can determine a circuit's voltage transfer characteristic.

Â A voltage transfer characteristic can be thought of as a graphical description of

Â the behavior of a nonlinear circuit.

Â It's simply a plot of the output voltage versus the input voltage for a circuit.

Â And let's look at how the voltage transfer characteristic can be obtained

Â from an equation.

Â If you remember this equation, we've found it when we analyzed the positive half-wave

Â rectifier circuit in a previous lesson.

Â For a positive half-wave rectifier, the output is equal to the input,

Â when the input is positive, and the output is equal to 0 when the input is negative.

Â Here I have two axes.

Â An output voltage axis and an input voltage axis.

Â For the region on this graph,

Â where the input voltage is positive, to the right of the V-out axis.

Â We know that the output must be equal to the input.

Â So I've drawn a line with the slope of 1 volt per volt.

Â When the input voltage is 1, the output voltage is 1.

Â When the input voltage is 2, the output voltage is 2.

Â Now, in the region of the graph where the input voltage is negative,

Â we know that the output voltage must be equal to 0, so

Â I've drawn a horizontal line here at V-out equals 0 volts.

Â Now, if you remember,

Â the circuit that we analyzed to obtain this equation had a single diode in it.

Â And that diode could be either off or on.

Â So this circuit had two states, as indicated by this equation.

Â One state, one state.

Â Now it's immediately apparent from the voltage transfer characteristic that

Â the circuit has two states.

Â Here is state one, here is state two.

Â And on a voltage transfer characteristic,

Â every corner on the graph indicates a transition between circuit states.

Â Now let's look at an example where we use a voltage transfer characteristic

Â to determine the output of a circuit for a given input.

Â Here's a voltage transfer characteristic that describes the behavior of

Â our circuit.

Â And here's the voltage wave form.

Â Now we can see that when the input voltage waveform is positive,

Â the output voltage is exactly equal to the input voltage.

Â So on this curve where the input voltage is positive,

Â we know the output voltage is exactly equal to the input voltage.

Â So I can draw the output voltage as tracking the input voltage in this region.

Â Now, from the VTC, we see that when the input voltage is negative, the output

Â voltage is obtained by multiplying the input voltage by minus one.

Â So, when the input voltage is -1, the output voltage is 1.

Â When the input voltage is -2 the output voltage is 2.

Â So on our time domain wave form, when the input voltage is negative,

Â we know to obtain the output voltage, we multiply each one of these voltages by -1.

Â In other words we flip this portion of the curve about the x axis.

Â To obtain this curve here.

Â V-in is positive, so the output is exactly equal to the input.

Â V-in is negative, so the output is equal to the input times -1.

Â And we obtain a curve that looks like this.

Â In other words, this is the voltage transfer characteristic for

Â a positive full-wave rectifier.

Â And this v-shaped curve is characteristic of a full-wave rectifier or

Â absolute value circuit.

Â Now let's look at how we can obtain a voltage transfer characteristic for

Â a circuit from given input and output voltage waveforms versus time.

Â On this graph I'm showing both the input voltage and the output voltage.

Â The input voltage is a sinusoidally varying waveform.

Â The output voltage here is shown in red,

Â and it's known as a center clipped sine wave.

Â The center of the sine wave has been clipped to be 0 volts.

Â Now let me draw some marks on this graph to help us better obtain the VTC.

Â Now, we can see that if the input voltage is less than this value of 3 volts,

Â where the input voltage is less than 3 volts, the output is equal to 0.

Â But when the input voltage is greater than this 3 volt threshold, we obtain the put

Â put voltage by subtracting 3 volts from every point on the input wave form.

Â Now, down here, we can see that when the input voltage is between 0

Â volts and -3 volts, the output voltage is 0.

Â But when the input voltage becomes less than -3 volts,

Â we obtain the output voltage by adding.

Â Three volts to every value on the waveform.

Â So let's go over our voltage transfer characteristic.

Â Remember, this is the V-out axis.

Â And this is the V-in axis.

Â And we know things are happening at a threshold of -3 and plus 3 volts.

Â If we're in the region where our input is between -3 and plus 3 volts,

Â from our voltage versus time graph, we know the output is exactly equal to 0.

Â Now when our voltage is less than -3 volts in this region, we know we obtain

Â the output voltage by adding 3 volts to every value of the input voltage.

Â So I draw a line that represents the equation V-out is equal to V-in plus 3.

Â Now, when the input voltage is greater than 3 volts in this region here,

Â we know we obtain the output but

Â subtracting 3 volts from every one of our input voltages.

Â So I draw a line that represents the equation.

Â V-out is equal to V-in minus 3 volts.

Â So we have slopes here of +1 and here of +1 and

Â our transition points are at -3 volts and +3 volts.

Â So you can see that this circuit immediately from the VTC,

Â has three states.

Â One state, two states, three states.

Â And two corners in the VTC that represent the transition between the states.

Â So a better picture is shown here of the actual VTC obtained from this plot.

Â Now let's look at how we can use a voltage transfer characteristic

Â to aid us in designing a circuit containing diodes and resistors and

Â implements a particular function.

Â Now, the function we're going to look at here is that the output voltage on this

Â axis is equal to the log base 10 of the input voltage shown on this axis.

Â We're going to use this graph to determine

Â the type of circuit we need to build to approximate this curve.

Â Now a simple approximation would be to draw one point here, one point here and

Â draw a straight line between them.

Â Now this approximation requires no diodes to implement, just resistors, but

Â is not a very good approximation.

Â So another possibility would be to divide this curve into,

Â piecewise linear segments.

Â So say we drew one segment here,

Â one segment here, one segment here, and one segment here.

Â Now, these piecewise linear segments form the voltage transfer characteristic for

Â a circuit it could be used to approximate this log base 10 function.

Â So, from this graph we can see the transition points that must occur in

Â the circuit between the states.

Â And we can see that there are one, two,

Â three, four required states in this circuit.

Â So from the graph, we could read off transition point locations.

Â And the required output voltage versus the input voltage relationship for each state.

Â So during this lesson we looked at voltage transfer characteristics and found that

Â they're simply plots of output voltage versus input voltage for a circuit.

Â A big advantage to a voltage transfer characteristic

Â is it allows one to quickly determine the behavior of a nonlinear circuit.

Â And in our next lesson,

Â we're going to look at an application of rectifiers, AC to DC conversion.

Â Thank you, and until next time.

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