Related Concepts: 03 BJT as NOT Gate & Inverter Operation | 04 NPN Transistor Basics & DTL Logic | 06 CMOS Logic & Logic Gate Characteristics

4.05 TTL Logic Families & Open Collector Bus Systems

Concept Overview: TTL Logic

Transistor-Transistor Logic (TTL) replaced DTL by utilizing multi-emitter transistors at the input stage, achieving much higher switching speeds.

Standard TTL uses a Totem-Pole output for active pull-up and pull-down. Open-Collector TTL removes the active pull-up to allow outputs to be safely tied together for common bus systems.

graph TD
    subgraph TTL NAND Totem-Pole Stages
        InputStage[Input Stage: Multi-Emitter Transistor T1] --> PhaseSplitter[Phase Splitter: Transistor T2]
        PhaseSplitter -->|Inverted Collector Output| PullUp[Active Pull-Up: Transistor T4 + Diode D]
        PhaseSplitter -->|Direct Emitter Output| PullDown[Active Pull-Down: Transistor T3]
        PullUp & PullDown --> OutputNode[Output Y]
    end

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1. Standard TTL NAND Gate Working Principle

• Inputs: Multi-emitter transistor $T_1$.

• Case 1: ANY Input is LOW (0V)

  • Emitter-base junction of $T_1$ is forward-biased $⟹$ current flows out of input pin.
  • Base-collector junction of $T_1$ is reverse-biased $⟹T_2$ turns OFF.
  • $T_2$ OFF $⟹$ pull-down transistor $T_3$ turns OFF.
  • Collector of $T_2$ rises toward $V_{CC}$, driving base of $T_4⟹$ pull-up transistor $T_4$ turns ON.
  • Result: Output pulled HIGH to $V_{CC}$ ($Y=1$).

• Case 2: ALL Inputs are HIGH (5V)

  • Emitter-base junctions of $T_1$ are reverse-biased.
  • Base-collector junction of $T_1$ becomes forward-biased, steering base current into $T_2⟹T_2$ turns ON.
  • $T_2$ ON $⟹$ emitter current drives $T_3$ ON into deep saturation.
  • Collector of $T_2$ drops low $⟹$ pull-up transistor $T_4$ turns OFF (diode $D$ ensures $T_4$ stays OFF).
  • Result: Output pulled LOW to Ground through $T_3$ ($Y=0$).

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2. Open Collector Gates & Wired-AND Logic

Connecting standard Totem-Pole outputs directly together causes destructive short-circuit currents when one gate outputs HIGH while another outputs LOW.

Open Collector TTL Solution:

• The active pull-up transistor ($T_4$) and diode ($D$) are removed.
• The collector of $T_3$ is left unconnected internally. An external Pull-Up Resistor ($R_L$) is attached between the node and $V_{CC}$.

graph LR
    Gate1[NAND Gate 1 Output] & Gate2[NAND Gate 2 Output] --> TiedNode[Common Tied Node Y]
    TiedNode --> PullUp[External Pull-up Resistor RL to VCC]
    TiedNode --> Function[Function: Y = AB ' • CD ']

Major Exam Proof: Wired-AND Justification (2015 - 11 Marks)

Statement: “Open collector TTL gates tied together form a Wired-AND logic.”

Proof:

1. If Gate 1 outputs LOW, its $T_3$ pulls node $Y$ to Ground, regardless of Gate 2.
2. If Gate 2 outputs LOW, its $T_3$ pulls node $Y$ to Ground, regardless of Gate 1.
3. Node $Y$ is HIGH only if BOTH Gate 1 AND Gate 2 outputs are HIGH.
4. Mathematical function: $Y=(AB{)}^{\prime }\cdot (CD{)}^{\prime }=(AB+CD{)}^{\prime }\text{(AND-OR-INVERT Logic)}■$

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Past Year Questions (PYQs)

• [PYQ 2015]: Wired-AND logic justification for open collector gates. (11 Marks)
• [PYQ 2019]: Common bus system operation using open collector gates. (07 Marks)
• [PYQ 2023]: TTL AND gate operation and truth table. (09 Marks)
• [PYQ 2025]: TTL NAND gate working principle and circuit schematic. (08 Marks)