Welcome back to electronics. This is Dr. Robinson. In this lesson, we're going to talk about diode limiters. In our previous lesson, we introduced an application of rectifiers. The conversion of an AC sinusoid to a DC voltage that could be used to power our electronic devices. The objectives for today's lesson are to introduce limiters, examine their behavior for sinusoidal inputs and to analyze limiter circuits. A limiter, also known as a clipper, is a non linear device that limits the output voltage to a particular level. Let's look at how a limiter effects the shape of an input sinusoidal voltage Here I have the block diagram that represents the limiter, here's our input voltage. Let's look at how the output voltage of this limiter is related to the input voltage. You can see that as the input voltage is increased past some level, some voltage level, which I'm calling the V plus limit, the output voltage is limited, or restricted to that voltage level. As the input voltage is decreased below some voltage level, which I'm calling V minus limit, the output voltage is restricted to that level. But, if the input voltage is between these two levels, or these two limits, the output voltage is equal to the input voltage. Now this is the, an output of a general limiter where we have two limits, both a positive limit and a negative limit. But it's possible to design a limiter such that we only have a positive limit or only have a negative limit. Let's look at how a limiter is defined in terms of rules that act on the input to produce the output. If the input is greater than or equal to the positive limit, the output is set equal to the positive limit. If the input is less than or equal to the negative limit, then the output is set equal to the negative limit. Then, otherwise, if the input is between these two limits, then the output is equal to the input. Here's the voltage transfer characteristic of the limiter we've been talking about. We can see that, if the input voltage, let me draw two lines here, to represent the range of input voltages that are between the V plus limit and the V minus limit. If the input voltage is within this range, the output is equal to the input and our voltage transfer characteristic, or our plot of output voltage versus input voltage, has a slope of one. If the input voltage is greater than the positive limit, then the output voltage is set equal to the positive voltage limit. And if the input voltage is less than the negative voltage limit, the output is equal to the negative voltage limit. Now you can see from this voltage transfer characteristic that to implement a circuit that has this voltage transfer characteristic requires three states. One state in this region, one state in this region, and one state in this region. Two trans, two corners on the voltage transfer characteristic, which represent two transitions between states. Here I've drawn a schematic of a circuit that can be used to implement a positive voltage limiter. We can see that the output voltage is taken across this branch, the series combination of V1 and diode D1. An ideal diode. The input voltage is on the left side. In this circuit are two DC voltage supplies. A voltage supply, V in, that attempts to push current around the loop in this direction because of its polarity. The current flows from the positive side to the negative side. And the voltage supply V1 that attempts to push current around the loop in this direction. Now we can see that because of the diode D1, current is only allowed to flow around the loop in this direction. Now the net direction of current depends on the voltage difference between V in and V1. If V in is greater than V1 then current will flow clockwise around the loop and the diode D1, because of its direction, is forward biased and the output voltage across this branch would be equal to V1. So we can write that for the case where V in is greater than V1, D1 is on. And the output voltage, V out, is equal to V1. Then for the case where V1 is greater than V in, the case where current should flow in this direction,. Counter clockwise around the loop but cannot because of the direction of D1. D1 is off a reverse biased, so no current would flow through this branch, and the output voltage taken across here would be exactly equal to the input voltage. Because with zero current, there can be no voltage drop across the resistor R. So we can write for the case where V in is less than V1, D1 is off and the output voltage V out is equal to the input voltage V in. We can combine these two equations to form our voltage transfer characteristic, and let me just quickly sketch it out from the equations down here. Our V out axis and our V in axis. And let me label V1 and V1. Now when V N is less than V1, we can see that the output voltage is equal to the input voltage. So on our characteristic curve, you would have a line with slope of 1 volt per volt. Then when the input voltage is greater than V1, we can see that the output is exactly equal to V1. And we would have the voltage transfer characteristic for a positive voltage limiter. Here, I've increased the complexity of the circuit by adding an additional branch. This portion of this circuit is exactly the same as the positive limiter that we analyzed on the previous slide. I've added this branch. Another series combination of a diode and a voltage source. But you notice that the direction or polarity of this diode is opposite that of the diode D1. I'm assuming the diodes here are ideal. And I'm also assuming that the voltage V1 is greater than the voltage V2. Let’s draw a, a number line that represents relative voltages in this circuit. So we have two voltages, V1 and V2, that indicate where transitions in state for this circuit will occur. And we know that V1 is more positive than V2. Let's first assume that the input voltage is a voltage that lies between the two voltages V1 and V2. In that case, VN is less than V1, so the voltage at the anode of D1 would be less than the voltage at the cathode of D1. So D1 would be off. VN is greater than V2. In that case, the voltage at the cathode of D2 is greater than the voltage of the anode of D2. So D2 would also be off. So for input voltages in this range, we know that both D1 and D2 are off. So we can write that for this range here. With D1 and D2 both off, the output voltage would be equal to the input voltage. Now lets increase the input voltage such that we're in this range, greater than V1. If our voltage at the input is greater than D1, then D1 would now be forward biased, but D2 would still be reverse biased or off, because the voltage here is bigger than the voltage here. So when D1 is on, we can replace it by a short circuit, and the output voltage measured from this node to this node would be measured directly across the voltage V1. So we can write that for this range here, Vout is equal to V1. Then finally, let's decrease the voltage such that, decrease the input voltage such that it is less than the voltage V2. If this voltage here is less than V2, diode D1 would be reverse biased, because this voltage is less than this voltage. But, D2 now is forward biased, because the voltage here at its anode is greater than the voltage at the cathode. So D2 is forward biased in this region, and the output voltage is measured directly across the voltage D2. So, for this region here, we can write that V out is equal to the voltage V2. So, this circuit has three states of operation. In this region here, both diodes D1 and D2 are off. In this region here, diode D1 is on, and in this region here diode D2 is on. Let's look at the voltage transfer characteristic for this circuit. We can see that this characteristic is the characteristic that I showed you earlier in the lesson for the bipolar voltage limiter. When the input voltage is between V1 and V2, in this region here, the output is equal to the input so that Vtc would have a slope of one volt per volt. If the input voltage is greater than V1, then the output voltage is equal to V1 volts. If the input voltage is less than V2 volts, then the output voltage is equal to V2 volts. And here it's apparent that the circuit has three states. One state here, one state here, one state here. This state occurs when both diodes are off. This state occurs when diode D1 is on, and this state occurs when diode D2 is on. Let's look at the relationship between halfway rectifiers and limiters. Let's say that we have a negative voltage limiter that has a negative limit of zero volts. As I've drawn here. When the input voltage is positive, the output voltage is equal to the input voltage. But, when the input voltage goes less than the limit of zero volts, the output is equal to zero volts. You can see that this negative limiter, with a limit of zero volts, has exactly the same voltage transfer characteristic as a positive half wave rectifier. We can draw for a sinusoidal input, the output of a circuit that has this characteristic. So when the input is positive, the output is equal to the input. But when the input is negative the output is equal to 0. Let's say instead, we had a positive limiter that had a voltage limit of zero volts. It would have a characteristic that looks like this. This positive limiter, with a limit of zero volts, has the same transfer characteristic as a negative half-wave rectifier. When the input is negative, the output is equal to the input but when the input is positive, the output is equal to zero. So for this characteristic, with the sinusoidal input, the output voltage for the same sinusoidal input would look like this. When the input is positive the output is zero, when the input is negative the output is equal to the input. So in summary, during this lesson we have looked at limiter operation and we looked at some circuits that could be used to implement limiters. In our next lesson we going to look at circuits known as voltage regulators. So thank you and until next time.
