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Can I ask why digital BJTs (pre-biased) BJTs are called digital?

There is almost no source that describes why this product is so named.

JRE
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Ahmed Sabry
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    Because marketing. –  Sep 28 '20 at 15:50
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    Does this answer your question? [Pre-biased Bipolar Transistor (BJT) Tradeoffs](https://electronics.stackexchange.com/questions/294315/pre-biased-bipolar-transistor-bjt-tradeoffs) – Chris Stratton Sep 28 '20 at 15:51

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Because it contains internal base resistors, it can be directly driven with a digital logic signal, without external resistors that are needed with standard BJT transistors when they are driven with digital logic signal.

Justme
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I'd argue that Digital BJT does not exist. A BJT on it's own is an analog device, regardless of biasing.

The predominant issue with Transistor to Transistor Logic (TTL) is the connection of transistors to create a input threshold voltage. 1.4V for AS family TTL. Above this threshold (2.0V) input is considered high by transistor array. Below (0.8V), input is low. 2V and 0.8 are 0.6V up and down from 1.4V.

Now create output that follows threshold convention and output can drive inputs, which is binary logic or Transistor to Transistor Logic.

From TI's Advanced Schottky (ALS and AS) Logic Families Application Report

$$V_{Th} = V_{BE_{Q2}} + V_{BE_{Q3}} + V_{BE_{Q4}} - (V_{BE_{Q1A}}\ or\ V_{BE_{Q1B}})$$

Or approximately 1.4V for Si.

74AS00 Schematic

So keying off Brian Drummond's comment, it is a bit of marketing, but it has more to do with clearly identifying a new type of binary devices derived from analog devices.

StainlessSteelRat
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  • I’d argue that all logic IC’s are analog on the interface ;) … and the 4000 series gates even make cheap linear amplifiers with gain up to 1k with video bandwidth when AC coupled with self biased R feedback which can be used with R ratio gains. – Tony Stewart EE75 Jun 11 '21 at 23:58
  • @TonyStewartEE75 I'm not disagreeing, but the key for TTL was the thresholds provided by the BE diodes. Below 0.8V and above 2.2V. Once crossed the actual input was irrelevant. – StainlessSteelRat Jun 12 '21 at 00:18
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    Yes the actual threshold was a skill testing question I used based for interviewing Test Engineers to see if they had ever measured a floating TTL input. Which I’m sure you know. – Tony Stewart EE75 Jun 12 '21 at 00:20
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Rohm Semi invented the term “digital transistors” for logic-level controlled current sinks decades ago which come pre-biased.

They are small , come in clusters or tiny in singles and are cost-effective solutions.

Unlike logic IC’s which are analog voltage controlled voltage switches with analog noise margins to guarantee switching with worst case tolerances for input and out voltage.

Tony Stewart EE75
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