Help on current flow/direction (DTL circuit)

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Discussion Overview

The discussion revolves around understanding current flow and direction in a DTL (Diode-Transistor Logic) circuit, specifically focusing on the behavior of transistors T2 and T3 under certain input conditions. Participants are analyzing a circuit diagram and its implications for output logic states.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why the "blue arrow" indicating current flow only goes through path 1 and not through paths 1, 2, and 3.
  • Another participant suggests that the diagram may lack context, indicating that T3 is "ON" and should conduct current via path 2, while T2 is "OFF," implying no current flows through path 3.
  • A participant expresses confusion about why T2 is OFF, seeking clarification on the conditions required for T2 to be ON.
  • Further analysis is provided regarding the necessary conditions for producing a logic 0 at the output, detailing the voltage requirements for T1 and T2 based on the states of T3 and the associated voltages.
  • It is noted that for T2 to be ON, its base voltage must exceed a calculated threshold, but since it is at 1V, T2 must be OFF.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the operation of T2 and T3, with some agreeing on the implications of T3 being ON and T2 being OFF, while others seek further clarification on these points. The discussion remains unresolved regarding the initial question about the current flow paths.

Contextual Notes

The discussion highlights potential limitations in the provided diagram and the assumptions made about the circuit's operation, particularly regarding the states of the transistors and their interactions.

frank1
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Homework Statement


Hello everyone,

In the circuit below I don't understand why the "blue arrow" only goes through path 1, instead of path 1, 2 and 3?

Homework Equations


The Attempt at a Solution



dtllogic.jpg
 
Last edited:
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It looks as though the diagram is taken out of some context where the "blue arrows" are part of a description of the circuit's operation, and no doubt referred to in that text.

The diagram indicates that transistor T3 is "ON", so it will conduct current via path 2 although it's not shown, probably because its not relevant to the point the descriptive text is making. T2, on the other hand is indicated to be "OFF", so no current should flow in path 3.
 
Hi gneill.

It really got too much out of context. It is like you said, it is a text that describes a circuit using transistors and diodes and what happens if both inputs (e1 and e2) are '1' (5V).

I don't understand why T2 is OFF.
 
frank1 said:
I don't understand why T2 is OFF.

Start by working out what is needed to produce a logic 0 at the output...

T1 must be ON pulling the output down so the Base of T1 must be about Vbe = 0.7V.

Current must be flowing...
a) through the resistor connected between the base of T1 and ground.
b) A little into the base of T1.

That current must come from the emitter of T3 so T3 must be ON. If T3 is ON then it's collector voltage will be about 0.3V (Vcesat) above the emitter. The emitter is at the same voltage (0.7V) as the base of T1 so the collector of T3 is about 0.7 + 0.3 = 1V.

Now look at T2. For T2 to be ON the base of T2 must be greater than about...

Vcesat of T1 + Vd + Vbe = 0.3 + 0.7 + 0.7 = 1.7V say

However the base of T2 is at same voltage as the collector of T3 which is 1V so T2 must be OFF.

Good job T2 is OFF when T1 is ON or the smoke will get out.
 

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