How Is the Third State Achieved in a 3-State Circuit Design?

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Discussion Overview

The discussion revolves around understanding how the third state, referred to as "disconnected" or "high-impedance," is achieved in a three-state circuit design, particularly focusing on the role of MOSFETs and their gate voltages. The context includes theoretical explanations and clarifications related to circuit design and operation.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how the third state is achieved and expresses uncertainty about the concept.
  • Another participant prompts for the initial thoughts of the inquirer to guide the discussion.
  • A participant explains that when the enable input is low, the transistors are turned off due to zero gate voltage, resulting in high impedance and minimal current flow, effectively disconnecting the output.
  • The inquirer seeks clarification on the term "zero gate voltage" and its implications for the circuit operation.
  • A participant provides a detailed explanation of the source of NMOS and PMOS transistors, clarifying the concept of gate to source voltage (Vgs) and its role in turning the transistors on or off.

Areas of Agreement / Disagreement

Participants appear to agree on the mechanism by which the third state is achieved, specifically the role of gate voltage in controlling the transistors. However, there remains some uncertainty regarding the terminology and specific details of the operation, as indicated by the inquirer's requests for further explanation.

Contextual Notes

The discussion includes assumptions about the participants' familiarity with transistor operation and terminology, which may not be universally understood. The explanation of gate voltage and its effects on transistor states is dependent on the specific types of MOSFETs being discussed.

Who May Find This Useful

This discussion may be useful for individuals studying circuit design, particularly those interested in the operation of MOSFETs and three-state logic circuits.

mmmboh
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[PLAIN]http://img99.imageshack.us/img99/1046/fig9.jpg

This isn't actually homework so I'm not sure it should be posted here but I think it fits. In the notes the teacher wrote that in this figure to think about how the third state (disconnected) is achieved, and well I've thought about it but aren't sure. I have a quiz coming up and this is bothering me can someone help me please?

Thanks!
 
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Hey mmmboh. Let's start by hearing what you think might be happening.

- Warren
 
What's the truth table?
 
mmmboh, see, I figured you were really close to the right answer already. You've got it right. When the enable input is low, the two transistors are turned off by giving them zero gate voltage. (If you don't know what I mean, let me know.) When the transistors are turned off, their impedance is very high. That means very little current can flow through either of them, which effectively makes the output "disconnected." Usually, it's called a "high-impedance" state.

- Warren
 
So basically the MOSFETs are turned off by giving them zero gate voltage, and at this point the impedance is very high so their is little current which is like an open...
But yeah, I'm not EXACTLY sure what you mean by zero gate voltage, I'd be thankful for a bit of an explanation.
 
Sure, mmmboh. First, a definition. The "source" of a transistor is where its carriers come from. NMOS transistors, the ones on the bottom, conduct electrons, which come from the negative supply. So the source of the NMOS transistors is the ground rail at the bottom of the circuit. Everything's backwards for PMOS transistors, so their sources are the supply rail at the top of the circuit.

When I said "zero gate voltage," I implicitly meant "zero volts on the gate, relative to the source." The proper term is "gate to source" voltage, or Vgs.

When the gate is driven to the same voltage as the source -- like when the enable input is low -- the transistor has "zero gate voltage," and turns off. It takes about Vgs >= 0.6 or 0.7V to turn on common NMOS MOSFETs. (And Vgs <= 0.6 or 0.7V for PMOS.)

- Warren
 

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