Question

Which of the following options comes under the non-saturated logic family?

• A. Emitter -- coupled Logic
• B. High-Threshold Logic
• C. Integrated -- injection Logic
• D. Diode-Transistor Logic

Hint:

- ECL Key Facts: It is the fastest logic family, uses non-saturated BJTs, has high power consumption, features low noise immunity, and uses a differential amplifier layout. - Schottky TTL is another non-saturated choice. It uses a built-in Schottky diode across the base-collector junction to clamp the voltage and prevent the transistor from saturating.

Answer 1

The Correct Option is A

Understanding the Concept: Digital bipolar logic families are divided into two main categories based on the operating regions of their internal transistors:
• Saturated Logic Families: Internal bipolar junction transistors (BJTs) are driven hard into saturation when conducting (representing a logic state) and cut off completely when non-conducting. Examples include TTL, DTL, RTL, HTL, and $I^2L$.
• Non-Saturated Logic Families: Transistors are carefully kept out of the saturation region during conduction, operating only within their active and cut-off zones. This prevents excess minority charge carriers from accumulating in the base region.

Step 1: Understanding the Storage Delay Bottleneck
When a BJT is driven into saturation, the base-emitter and base-collector junctions both become forward-biased. This causes an excess storage of minority carriers in the base region. When the gate tries to switch states, the transistor cannot turn off immediately; it must first clear out these stored charges. This creates a delay called storage time ($t_s$), which sets a hard limit on the switching speed of saturated logic circuits.

Step 2: How Emitter-Coupled Logic (ECL) Solves This
Emitter-Coupled Logic (ECL) uses a differential amplifier configuration as its core switching element. The current through the circuit is steered between two parallel transistor paths depending on the input logic levels.
• The circuit is designed so that the transistors never enter saturation.
• Because the transistors stay in their active region, storage time delay is completely eliminated ($t_s = 0$). This makes ECL the fastest available bipolar logic family, offering exceptionally short propagation delays (often below $1$ nanosecond).

Step 3: Evaluating alternative options

• High-Threshold Logic (HTL): A modified, high-voltage version of DTL designed for industrial noise immunity. It drives transistors deep into saturation and has relatively slow switching speeds.
• Integrated-Injection Logic (\(I^2L\)): A high-density bipolar configuration that relies entirely on saturated multi-collector transistors.
• Diode-Transistor Logic (DTL): An early logic family using diodes at the input and a saturated common-emitter transistor stage at the output.