How Does a Two Diode AND Gate Work?

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Discussion Overview

The discussion centers around the functioning of a diode-resistor AND gate as presented in a specific online resource. Participants explore the operational principles of the circuit, including the behavior of diodes in relation to input states and output results. The conversation includes technical explanations, clarifications, and challenges regarding the circuit's design and logic.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants express confusion about how the diode-resistor AND gate operates, particularly regarding the reversed diodes and their effect on input signals.
  • Others clarify that for the output to be high, both inputs must be in a high state, while any low input will pull the output low.
  • There is a discussion about the concept of grounding in relation to the circuit, with some participants questioning whether the output is grounded and how this affects the circuit's operation.
  • One participant notes the difference in behavior between diode-based and transistor-based logic gates, particularly regarding how outputs are pulled high or low in floating input states.
  • Concerns are raised about the forward voltage drop of diodes and its implications for defining logic states, with some suggesting a specific voltage range for logic levels.
  • Participants discuss the possibility of disconnecting the input ends of the diodes to prevent the output from being pulled down, seeking clarification on the implications of such an action.

Areas of Agreement / Disagreement

Participants generally agree on the basic operational principles of the AND gate but express differing views on specific aspects, such as grounding and the implications of diode behavior. The discussion remains unresolved regarding the nuances of how these components interact in practice.

Contextual Notes

Some participants highlight the importance of understanding the voltage ranges for logic states and the potential for errors introduced by diode characteristics. There is also mention of the undefined voltage region in logic circuits, which is not universally agreed upon.

Who May Find This Useful

This discussion may be useful for individuals interested in electronics, particularly those exploring digital logic design and the functioning of basic logic gates using diodes and resistors.

extremepilot
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Hi, I am new to EE.
On this page - http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/diodgate.html#c1" - the very first circuit shows a diode-resistor AND gate. How does that work? I understand the OR gate shown right below it, but... the two inputs on this AND gate are connected to reversed diodes meaning charges can't flow. Doesn't this mean the input is useless? How does this work?
 
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With both A and B in a low state, you would need both A and B in a high state to get a high state output. As long a one input is low, that diode will be conducting. A diode drop in voltage is ~0.6v hence a low state.

Welcome to PF
 
Thanks!

The first part is basically the definition of an and gate, but could you please elaborate on

"As long a one input is low, that diode will be conducting. A diode drop in voltage is ~0.6v hence a low state." ?

If one diode is conducting it would be conducting towards the input (since it can't conduct the opposite way), turning the input into an output. That's all I understand from this circuit. Hope this clears up my understanding of it so you can see what I'm not understanding, heh.
 
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extremepilot said:
Hi, I am new to EE.
On this page - http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/diodgate.html#c1" - the very first circuit shows a diode-resistor AND gate. How does that work? I understand the OR gate shown right below it, but... the two inputs on this AND gate are connected to reversed diodes meaning charges can't flow. Doesn't this mean the input is useless? How does this work?

extremepilot said:
Thanks!

The first part is basically the definition of an and gate, but could you please elaborate on

"As long a one input is low, that diode will be conducting. A diode drop in voltage is ~0.6v hence a low state." ?

If one diode is conducting it would be conducting towards the input (since it can't conduct the opposite way), turning the input into an output. That's all I understand from this circuit. Hope this clears up my understanding of it so you can see what I'm not understanding, heh.

No, the inputs are still inputs. Either input low will pull the output low. So both inputs have to be high for the output pullup resistor to be able to pull the output high. That's an AND function from inputs to output.
 
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berkeman said:
No, the inputs are still inputs. Either input low will pull the output low. So both inputs have to be high for the output pullup resistor to be able to pull the output high. That's an AND function from inputs to output.

Ok I actually understand - if any of the diodes are grounded they conduct charge through them and away from the output preventing a 1 - then is the output also grounded? if not which part of the circuit gets grounded?
 
extremepilot said:
Ok I actually understand - if any of the diodes are grounded they conduct charge through them and away from the output preventing a 1 - then is the output also grounded? if not which part of the circuit gets grounded?

In logic, you inputs and outputs get "pulled" high and low, either by transistors, diodes or resistors. So you aren't really "grounding" anything, just pulling to logic high or low.

So if you had two of these AND gates in a row, with the output of the first one driving one of the inputs of the second one, do you see how the low and high logic states can propagate along?
 
I think I actually understand and don't know why this didn't occur to me before. But in the linked schematic the output gets connected to ground, correct? I am working with a breadboard and understand that you can't just have (lets say 5v) power connected with no ground connected whatsoever, so going off this fact alone... output gets grounded correct? I'm extra unsure since the schematic on that site shows ground just floating by itself, not connected to anything.
 
extremepilot said:
I think I actually understand and don't know why this didn't occur to me before. But in the linked schematic the output gets connected to ground, correct? I am working with a breadboard and understand that you can't just have (lets say 5v) power connected with no ground connected whatsoever, so going off this fact alone... output gets grounded correct? I'm extra unsure since the schematic on that site shows ground just floating by itself, not connected to anything.

I think I see what is confusing you. For each of the circuits shown, they show how power and ground are used by the circuit. For the diode-based AND circuit, no actual ground connection is needed, since the inputs being pulled low will pull the output low,without any actual in-gate connection to ground. If it were TTL totem-pole logic, then yes, there are transistors connected to power and to ground, to be able to actively pull the output high and low.
 
Also note what a logical 0 or 1 voltage range is.

In both of these gates, we have made the assumption that the diodes do not introduce any errors or losses into the circuit. This is not really the case; a silicon diode will experience a forward voltage drop of about 0.65v to 0.7v while conducting. But we can get around this very nicely by specifying that any voltage above +3.5 volts shall be logic 1, and any voltage below +1.5 volts shall be logic 0. It is illegal in this system for an output voltage to be between +1.5 and +3.5 volts; this is the undefined voltage region.

http://www.play-hookey.com/digital/electronics/dl_gates.html"
 
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  • #10
hi guys,

ok I thought I understood this till I looked further into that link and saw this diag
showing the transistor and the diode version side by side.

what puzzles me now is, in a floating input state, the transistor version the output is pulled low via the 4k7 resistor but the diode version is pulled hi by the resistor to +V so they are opposite.
That is correct isn't it ?

Inferring that one would have to take that into consideration if making discrete component gates?

Dave
 

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  • #11
davenn said:
hi guys,

ok I thought I understood this till I looked further into that link and saw this diag
showing the transistor and the diode version side by side.

what puzzles me now is, in a floating input state, the transistor version the output is pulled low via the 4k7 resistor but the diode version is pulled hi by the resistor to +V so they are opposite.
That is correct isn't it ?

Inferring that one would have to take that into consideration if making discrete component gates?

Dave

There are both pull-up and pull-down resistors in logic gates, but the logic is the same. The only difference in the states are the voltage ranges for the high and low states.
 
  • #12
Thank you

One more stab at it ... the output is pulled low if either or both of the inputs are grounded (low/'0'). If both inputs are high (connected to voltage over +3.5v) the output is not 'pulled down' and is high. Right? Also can you simply disconnect the input end of both diodes to prevent the output from being pulled down?

http://www.allaboutcircuits.com/vol_3/chpt_3/10.html" made more sense than the first link
 
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  • #13
extremepilot said:
Thank you

One more stab at it ... the output is pulled low if either or both of the inputs are grounded (low/'0'). If both inputs are high (connected to voltage over +3.5v) the output is not 'pulled down' and is high. Right? Also can you simply disconnect the input end of both diodes to prevent the output from being pulled down?

http://www.allaboutcircuits.com/vol_3/chpt_3/10.html" made more sense than the first link

You got it. Good job!
 
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  • #14
Thank you VERY much
 
  • #15
berkeman said:
I think I see what is confusing you. For each of the circuits shown, they show how power and ground are used by the circuit. For the diode-based AND circuit, no actual ground connection is needed, since the inputs being pulled low will pull the output low,without any actual in-gate connection to ground. If it were TTL totem-pole logic, then yes, there are transistors connected to power and to ground, to be able to actively pull the output high and low.

is it correct that in AND gate the output is taken across the diodes or parallel to diode.so when diode conduct we measure the voltage drop of 0.6v.
thanks
 
  • #16
amaresh92 said:
is it correct that in AND gate the output is taken across the diodes or parallel to diode.so when diode conduct we measure the voltage drop of 0.6v.
thanks

No. In real AND gates, diode logic is not used. The example is for teaching purposes only.
 

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