# Find the Q-points for the diodes in the four circuits in Fig. P3.68 using (a) the ideal diode model

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#### thevien2507

Joined May 1, 2023
1
Find the Q-points for the diodes in the four circuits in Fig. P3.68 using (a) the ideal diode
model and (b) the constant voltage drop model with Von =

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#### ericgibbs

Joined Jan 29, 2010
17,981
Hi Thev,
Welcome to AAC.
As this is Homework you have to show your best attempt at answering, we can then help.
E

#### WBahn

Joined Mar 31, 2012
29,149
So... what's your question? Where are you getting stuck?

The best way for use to help you is for you to show your best effort so that we can see where you are going wrong.

#### MrAl

Joined Jun 17, 2014
10,588
Find the Q-points for the diodes in the four circuits in Fig. P3.68 using (a) the ideal diode
model and (b) the constant voltage drop model with Von =
View attachment 293229
Hello,

At the very least finish your sentences and drawings

#### Ramussons

Joined May 3, 2013
1,354
Find the Q-points for the diodes in the four circuits in Fig. P3.68 using (a) the ideal diode
model and (b) the constant voltage drop model with Von =
View attachment 293229
What is a Q point?

#### WBahn

Joined Mar 31, 2012
29,149
What is a Q point?
The Q point is the quiescent operating point, also known as the DC bias point.

#### crutschow

Joined Mar 14, 2008
32,848
You show only one circuit.
What about the other three?

#### WBahn

Joined Mar 31, 2012
29,149
You show only one circuit.
What about the other three?
Presumably, they are only asking about one of them.

Hopefully they don't ask about the others in this same thread -- then we will have discussions about four different problems all intermingled. That's why we ask for one-problem-per-thread (and one-thread-per-problem for other, but related, reasons).

#### Ramussons

Joined May 3, 2013
1,354
The Q point is the quiescent operating point, also known as the DC bias point.
Thanks.

#### MrAl

Joined Jun 17, 2014
10,588
What is a Q point?
I think a Q point for a diode is a little more unusual than for something like a transistor. This implies that there is some kind of analog action going on that depends on the voltage/current curve of the diode. Once the diode is 'biased' it will have a certain dynamic resistance and that would be tantamount to understanding how the circuit operates in the specific application.

#### WBahn

Joined Mar 31, 2012
29,149
I think a Q point for a diode is a little more unusual than for something like a transistor. This implies that there is some kind of analog action going on that depends on the voltage/current curve of the diode. Once the diode is 'biased' it will have a certain dynamic resistance and that would be tantamount to understanding how the circuit operates in the specific application.
How is that any more unusual than for a transistor, which is the exact same things -- the dynamic behavior about the operating point depends on the operating point in both cases.

#### MrAl

Joined Jun 17, 2014
10,588
How is that any more unusual than for a transistor, which is the exact same things -- the dynamic behavior about the operating point depends on the operating point in both cases.
Hello there,

Mostly because diodes are rarely biased, they are usually just rectifying something.
I'm not saying that they are never biased, that's for certain.

In fact, i biased one a long time ago to try to get more sensitive reception on an AM receiver. The diodes used for that have some threshold where they don't conduct until the voltage reaches a certain point, so by applying a small DC bias the diode starts to conduct sooner, making the radio more sensitive to lower level transmissions.

There is also the voltage controlled AC attenuator. The diode is biased with DC while an AC low level signal is coupled to the diode and when the DC bias is changed the AC signal level changes, increasing or decreasing, depending on the DC bias.

I've probably seen 500 rectifying circuits to every 1 diode biased circuit, while we talk about transistor amplifiers quite often around here and they are almost always biased. Hence the term, "unusual". Yes the transistor can be used as a switch and thus no bias to speak of, but it seems that the diode appears in switching circuits more than in analog circuits.

If you feel the need, you can quote more diode biased application circuits.

#### WBahn

Joined Mar 31, 2012
29,149
If "unusual" is based on diodes being used more in switching circuits than in small-signal operation, then transistors operated in small-signal are probably unusual, too, since my guess is that they are used in switching applications far more often, as well -- and that's not even considering MOS transistors that are overwhelmingly used as switches, especially if you include integrated transistor circuits.

But it's really neither here nor there. The Q-point of neither is "unusual" since the Q-point doesn't care how the device is being used. It is simply the quiescent operating point of the device in that circuit, whether it be an amplifier, a receiver, a rectifier, or whatever.

#### MrAl

Joined Jun 17, 2014
10,588
If "unusual" is based on diodes being used more in switching circuits than in small-signal operation, then transistors operated in small-signal are probably unusual, too, since my guess is that they are used in switching applications far more often, as well -- and that's not even considering MOS transistors that are overwhelmingly used as switches, especially if you include integrated transistor circuits.

But it's really neither here nor there. The Q-point of neither is "unusual" since the Q-point doesn't care how the device is being used. It is simply the quiescent operating point of the device in that circuit, whether it be an amplifier, a receiver, a rectifier, or whatever.
Hello again,

Thanks for your explanation.

The view on this is completely subjective and therefore there is no scientifically based right answer.
It's fine if you disagree, that actually makes life a little more interesting.

For me, it's unusual. That would be because in the course of a 50 year span i've seen a lot less analog diode circuits than other types such as switching and rectifying. Even on this site here are more switching than analog circuits using diodes. Questions about diode switching circuits come up all the time while we rarely talk about a circuit where the diode has to be biased to some particular voltage or current. To me, that makes it unusual.

To a person who was born and lived on a desert island all his life a visitor from another place in the world for the first time would be unusual. Visitors to other places in the world are as common as fried eggs, but to him it's not only unusual it's very unusual.

#### WBahn

Joined Mar 31, 2012
29,149
I don't see what the application has to do with it. Every diode has an operating point in every circuit, just as every transistor has an operating point in every circuit. If it seems unusual to you that diodes have an operating point, fine.

#### RoofSheep

Joined Mar 7, 2023
36
Find the Q-points for the diodes in the four circuits in Fig. P3.68 using (a) the ideal diode
model and (b) the constant voltage drop model with Von =
View attachment 293229
The first question is what the textbook means by an ideal diode model. It can be either of the following:
1. The diode acts like an open circuit when it is reverse biased and a a short circuit when it is forward biased.
2. The diode is characterized by the Shockley equation.

In case 1 the problem is really simple and you should be able to solve it easily.
In case 2 you have to solve a system containing nonlinear equations which can probably only be done numerically.

Please show us your best attempt.

#### WBahn

Joined Mar 31, 2012
29,149
The first question is what the textbook means by an ideal diode model. It can be either of the following:
1. The diode acts like an open circuit when it is reverse biased and a a short circuit when it is forward biased.
2. The diode is characterized by the Shockley equation.

In case 1 the problem is really simple and you should be able to solve it easily.
In case 2 you have to solve a system containing nonlinear equations which can probably only be done numerically.

Please show us your best attempt.
The "ideal diode" model virtually always refers to (1) and on the few occasions when it doesn't, almost always refers to what is more commonly known as the constant-voltage drop model. In this case, the latter is ruled out in this case because the question explicitly asks for that as the second part of the question.

#### MrAl

Joined Jun 17, 2014
10,588
I don't see what the application has to do with it. Every diode has an operating point in every circuit, just as every transistor has an operating point in every circuit. If it seems unusual to you that diodes have an operating point, fine.
Hi again,

Well, that sounds too general to me. When is the last time you considered the operating point of a rectifier diode in an AC to DC rectifier circuit.
On the other hand, if you were to design a diode AC attenuator it would be imperative to know the operating point because that is a major, major factor in the operation of the circuit.

Thanks for the reply, always good to hear about other's observations.

#### WBahn

Joined Mar 31, 2012
29,149
Whether or not it's important to know what the operating point is is completely beside the point. That has no effect on the fact that it HAS an operating point.

And I consider the operating point of a rectifier diode in nearly any circuit I design, since the operating point provides the information needed to determine whether the diode I am considering is adequate. If the operating point when forward biased is going to be about 1 V at 10 A, there are lots of diodes I would not consider because they are not suitable for use at that operating point.

#### MrAl

Joined Jun 17, 2014
10,588
Whether or not it's important to know what the operating point is is completely beside the point. That has no effect on the fact that it HAS an operating point.

And I consider the operating point of a rectifier diode in nearly any circuit I design, since the operating point provides the information needed to determine whether the diode I am considering is adequate. If the operating point when forward biased is going to be about 1 V at 10 A, there are lots of diodes I would not consider because they are not suitable for use at that operating point.
Hello,

Ok i guess we have different interpretations of what an operating point is and how it can differ.

I'd like to hear how you design a regular rectifier circuit by operating point of the diodes, as to what the forward voltage drop is so you can determine if the diode is adequate or not.
Then we have the switching regulators such as the buck. The diode is usually chosen by the RMS current rating (as well as the max voltage) and if you want to call that the 'operating point' i guess that's up to you, but it is clearly different than an operating point for a linear transistor amplifier circuit.

The best i can agree with is that there are different types of "operating points". Analog circuits are completely different than switching circuits.

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