# Role of the two diodes in this circuit ?

#### James_1

Joined Aug 15, 2020
4

So this is the circuit that I have . I managed to define what his function is. The last question that I had to answer is what role does the 2 diodes Dz and Dh have in our circuit ?

I'm not quite sure whether they do have a specific function in this circuit or should I just give a general definition like these diodes allow the current to pass in one direction and blocks the flow of current in the other ?

#### dl324

Joined Mar 30, 2015
16,118
Welcome to AAC!

Is this a homework?

#### scorbin1

Joined Dec 24, 2019
103
Don't mean to change the subject, but im curious about the 5 pin transistor. I have honestly never come across such a thing. Given the function of the circuit im guessing the transistor is basically an and gate. Is this really an actual transistor or like one of those super secret proprietary epoxy encased ICs? If it is indeed a transistor do you happen to have a part number that i could dig up a data sheet for?

#### dl324

Joined Mar 30, 2015
16,118
im curious about the 5 pin transistor. I have honestly never come across such a thing.
They only exist in integrated circuits.

#### James_1

Joined Aug 15, 2020
4
Welcome to AAC!

Is this a homework?

Actually no I'm learning alone these kind of circuits because I'm going to study them for the next semester . So I found this exercice on the internet and I wanted to solve it .

#### dl324

Joined Mar 30, 2015
16,118
Actually no I'm learning alone these kind of circuits because I'm going to study them for the next semester . So I found this exercice on the internet and I wanted to solve it .
OK. Tell us what you think the diodes are doing.

#### James_1

Joined Aug 15, 2020
4
Well they allow the current to pass in one direction and blocks the flow of current in the other way around but in some states the current may not flow when the Cathode and Anode have the same voltage which means it will be blocked and will look for another to alternative for the current to flow. That's what I think about them .

#### crutschow

Joined Mar 14, 2008
33,339
they allow the current to pass in one direction and blocks the flow of current in the other way around
Basically, yes.
A typical silicon junction diode (which these are) requires a forward bias of about 0.6-0.7V for significant current to flow.

As to their function in the circuit, one acts to block and pass current depending upon the inputs. but the other is just to provide a small forward bias voltage as it's never reversed biased.
I'll leave the reason for the needed bias voltage as a exercise for you.

#### slackguy

Joined Feb 11, 2016
76
There are many NAND circuit diagrams to Google, depending on number of inputs (two is typical, any other would imply an interpretation of which order and what arrived first, since NAND is a binary function).

The "+Us" is not a symbol in my "standard engineering diagram" book (which uses V+), and your diagram does not label if "+Us" is a biased output to Q or if Q pairs with Ground (which, could be labeled in the diagram, isn't, often assumed). There should be a circle on each input, output: but there is not. The symbol apparently meaning ground is not the right symbol for ground.

Instead of being told what inputs and outputs are, we have to read diagram and see OE (whatever that is), I1, I2 (usually currents but not this case) and Q (usually charge, but not in this case) are the ABC->D

The circuit above does NOTHING without specifying the component values - and not all NAND input circuits have the same layout. On this forum it can often be found that "promiscious circuits" are often published which, when assembled even with specified values don't work (ie, because the stated values are not indicative of the tolerance of real components - extra components are then required to adjust). SO - DO WE KNOW if this circuit works?

My main worry about the design is that the base of the NPN transistors, without doing allot of equations to check for sure, could "get zapped". That is, in that design if there is a "wrong state" in the "flops", then higher voltage flows to the base of one of the transistors: there is no clear input chain and output chain, they are mixed, one depends on the other (if one acts wrong, perhaps it burns one or more transistors). I might say "ok, Vout cannot be greater the VbaseMax since the bases aren't isolated from fluctuations", but then I already have circuit values and must insure there are other parts to match it "that can be bought". I then have to consider if Dh Dz are above or below VBaseMax, leading to loop of doing the same thing in that scenario, and I'm stuck in another goose chase.

My other concern is this added "+Us" where we have 2 outputs, one going somewhere unstated, and whether ground is part of the output or if the output is biased output and we are just supposed to know that "somehow, because some school book said everyone knows that". what is U? what is Us? from there i can figure +Us

If I were you and I had a book and teacher resolute to pose me with bogus challenges I'd find something else. My intro to electronic engineering book does exactly this: It introduces engineering layout standards then some circuits. When it gets to logic it gives a simple symbol, the digital chart, simple diagram well labeled, discussion on how it all works without silly questions about what if i add another input can i move a diode around: AND MOVES ON TO OTHER IMPORTANT TOPICS (meaning, there is allot to cover, no time for "what ifs").

In this book, DTL NAND (diode transistor logic Not And) requires three diodes, four resistors, two npn transistors. My book has a long explanation of component interaction I'd type it in but your book supposedly has a similar explination: it's long, 3 paragraphs, and includes things like calling Q2 a phase-splitter. It does not include a discussion of what D1 D2 D3 does except to say they do not allow reverse bias. It, however, takes 3 paragraphs of explaining it: and if you've done that I have to wonder why you do not know what they are for.

You said you'd verified the logic chart: I'm quite certain the logic cannot be verified by simple logic if diodes are not known to prevent reverse bias while making the output diagram.

Last edited:

#### MrAl

Joined Jun 17, 2014
10,891
They only exist in integrated circuits.
There are double gate MOSFETs you can buy.

#### Papabravo

Joined Feb 24, 2006
20,600
There are many NAND circuit diagrams to Google, depending on number of inputs (two is typical, any other would imply an interpretation of which order and what arrived first, since NAND is a binary function).

The "+Us" is not a symbol in my "standard engineering diagram" book (which uses V+), and your diagram does not label if "+Us" is a biased output to Q or if Q pairs with Ground (which, could be labeled in the diagram, isn't, often assumed). There should be a circle on each input, output: but there is not. The symbol apparently meaning ground is not the right symbol for ground.

Instead of being told what inputs and outputs are, we have to read diagram and see OE (whatever that is), I1, I2 (usually currents but not this case) and Q (usually charge, but not in this case) are the ABC->D

The circuit above does NOTHING without specifying the component values - and not all NAND input circuits have the same layout. On this forum it can often be found that "promiscious circuits" are often published which, when assembled even with specified values don't work (ie, because the stated values are not indicative of the tolerance of real components - extra components are then required to adjust). SO - DO WE KNOW if this circuit works?

My main worry about the design is that the base of the NPN transistors, without doing allot of equations to check for sure, could "get zapped". That is, in that design if there is a "wrong state" in the "flops", then higher voltage flows to the base of one of the transistors: there is no clear input chain and output chain, they are mixed, one depends on the other (if one acts wrong, perhaps it burns one or more transistors). I might say "ok, Vout cannot be greater the VbaseMax since the bases aren't isolated from fluctuations", but then I already have circuit values and must insure there are other parts to match it "that can be bought". I then have to consider if Dh Dz are above or below VBaseMax, leading to loop of doing the same thing in that scenario, and I'm stuck in another goose chase.

My other concern is this added "+Us" where we have 2 outputs, one going somewhere unstated, and whether ground is part of the output or if the output is biased output and we are just supposed to know that "somehow, because some school book said everyone knows that". what is U? what is Us? from there i can figure +Us

If I were you and I had a book and teacher resolute to pose me with bogus challenges I'd find something else. My intro to electronic engineering book does exactly this: It introduces engineering layout standards then some circuits. When it gets to logic it gives a simple symbol, the digital chart, simple diagram well labeled, discussion on how it all works without silly questions about what if i add another input can i move a diode around: AND MOVES ON TO OTHER IMPORTANT TOPICS (meaning, there is allot to cover, no time for "what ifs").

In this book, DTL NAND (diode transistor logic Not And) requires three diodes, four resistors, two npn transistors. My book has a long explanation of component interaction I'd type it in but your book supposedly has a similar explination: it's long, 3 paragraphs, and includes things like calling Q2 a phase-splitter. It does not include a discussion of what D1 D2 D3 does except to say they do not allow reverse bias. It, however, takes 3 paragraphs of explaining it: and if you've done that I have to wonder why you do not know what they are for.

You said you'd verified the logic chart: I'm quite certain the logic cannot be verified by simple logic if diodes are not known to prevent reverse bias while making the output diagram.
U is a German symbol for voltage. Can you really be that chauvinistic?
In German "difference" is "Unterschied"
So +Us is just a positive voltage with a subscript.

#### DickCappels

Joined Aug 21, 2008
10,104
You said you'd verified the logic chart: I'm quite certain the logic cannot be verified by simple logic if diodes are not known to prevent reverse bias while making the output diagram.
This is why electrical characteristics are specified in the datasheets.

#### Picbuster

Joined Dec 2, 2013
1,045
View attachment 214808
So this is the circuit that I have . I managed to define what his function is. The last question that I had to answer is what role does the 2 diodes Dz and Dh have in our circuit ?

I'm not quite sure whether they do have a specific function in this circuit or should I just give a general definition like these diodes allow the current to pass in one direction and blocks the flow of current in the other ?
What happens when you shortcut them?
currents and voltages in the circuit in both cases ( diodes in and in a shortcut)?

Picbuster

#### MrAl

Joined Jun 17, 2014
10,891
View attachment 214808
So this is the circuit that I have . I managed to define what his function is. The last question that I had to answer is what role does the 2 diodes Dz and Dh have in our circuit ?

I'm not quite sure whether they do have a specific function in this circuit or should I just give a general definition like these diodes allow the current to pass in one direction and blocks the flow of current in the other ?
Did anyone mention this yet...
Look at the circuit and try to figure out what changes when you either open each diode or just replace them with a wire, but do one diode at a time.

#### James_1

Joined Aug 15, 2020
4
Did anyone mention this yet...
Look at the circuit and try to figure out what changes when you either open each diode or just replace them with a wire, but do one diode at a time.

Thank you . I think I got it when I replaced the diodes with wires there will be a huge problem in some states . Assuming I1 = 0 , I2 = 0 and OE = 1 (5V) then in this case the base of the transistor T4 will have 5V as a voltage coming towards him . (Because OE is 5V ) so in this case the transistor will be melt and broken since he's becoming a huge amount of voltage ( As I read in some documentations the transistor needs between 0.6 and 0.7V to turn on . When the voltage at the base is greater than 0.7V , the transistor starts saturating and looks like a short circuit between collector and emitter. )

So to conclude , the diodes are used to prevent any kind of infinite current flow into the base or an
infinite power loss that leads to a broken transistor . In other words it will protect the transistor . Am I correct ?

#### MrAl

Joined Jun 17, 2014
10,891
View attachment 214978
Thank you . I think I got it when I replaced the diodes with wires there will be a huge problem in some states . Assuming I1 = 0 , I2 = 0 and OE = 1 (5V) then in this case the base of the transistor T4 will have 5V as a voltage coming towards him . (Because OE is 5V ) so in this case the transistor will be melt and broken since he's becoming a huge amount of voltage ( As I read in some documentations the transistor needs between 0.6 and 0.7V to turn on . When the voltage at the base is greater than 0.7V , the transistor starts saturating and looks like a short circuit between collector and emitter. )

So to conclude , the diodes are used to prevent any kind of infinite current flow into the base or an
infinite power loss that leads to a broken transistor . In other words it will protect the transistor . Am I correct ?

I am not sure what you are doing there.
If you want to be sure what you should do is draw the circuit four different ways:
diode 1 open and diode 2 open
diode 1 open and diode 2 at forward voltage 0.7 volts
diode 1 at forward voltage 0.7 volts and diode 2 open
diode 1 at forward voltage 0.7 volts and diode 2 at forward voltage 0.7 volts

That covers all four possibilities.
Then you can look and see first which cases are actually possible.
Then you can you figure out what difference it makes to the circuit for each valid case.

For example, when diode DH is 0.7v it causes a voltage drop rather than provide no drop if the diode was shorted out. The question to ask then is why would they want a voltage drop there rather than just a short.