What is the role of Q3 in this LS TTL schematic? - Page 1
Forum section: electronics / projects

author: VinzC
author_rank: Frequent Contributor
timestamp: August 12, 2024, 11:17:07 am
content: Hi again.Out of curiosity I wanted to check again the internals of Low Power Schottky TTL circuits and I stumbled upon this schematic:I think I overlooked that but now I wonder: what is Q3 doing here? There is a resistor in standard TTL instead of this transistor, so what is its role, exactly?Thanks in advance for any hint/info.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: August 12, 2024, 01:23:39 pm
content: Good Morning Vinz, I am curious what xistor Q1 is, that has 2 emitters

author: edavid
author_rank: Super Contributor
timestamp: August 12, 2024, 02:43:10 pm
content: Q3 is an active pulldown that turns Q6 off more quickly than a resistor pulldown.

author: moffy
author_rank: Super Contributor
timestamp: August 12, 2024, 02:58:09 pm
content: Quote from: BILLPOD on August 12, 2024, 01:23:39 pmBipolar transistors can have multiple emitters, it's often used for current mirrors if say one wants twice the current out for that coming in, a matched transistor with two emitters will often be used. You could think of the analogy as two transistors with C-C and B-B but the emitters are separate.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2024, 06:52:20 pm
content: Quote from: BILLPOD on August 12, 2024, 01:23:39 pmIt's equivalent to many transistors with their base and collector terminals joined. In some cases the transistors will be different sizes i.e.take up different amounts of room on the chip.

author: bson
author_rank: Unknown
timestamp: August 12, 2024, 09:11:48 pm
content: Note also the Schottky transistors (with an S shape diode symbol).

author: TimFox
author_rank: Super Contributor
timestamp: August 12, 2024, 09:16:50 pm
content: The multiple-emitter input transistors are why this type of circuit is called "TTL" (transistor-transistor logic), as opposed to "DTL" (diode-transistor logic).Many BJT logic ICs are called TTL, when they are actually DTL with Schottky diodes.A generic name for this style of IC is "current-sinking logic", where the current to be sunk is either diode current or emitter current, and the sink is a grounded-emitter NPN output (usually with active pull-up circuit).The important feature of current-sinking logic is that the input-low input current is much higher than the input-high input current, as opposed to CMOS which is more balanced.

author: VinzC
author_rank: Frequent Contributor
timestamp: August 13, 2024, 09:13:20 am
content: Quote from: BILLPOD on August 12, 2024, 01:23:39 pmSorry for being late, indeed that's a classic configuration encountered in transistor gate logic, as explained earlier.Quote from: edavid on August 12, 2024, 02:43:10 pmThat is interestingI don't remember ever seeing such a configuration. I only needed that answer to finally open the gates to literature on this topic (e.g.https://electronics.stackexchange.com/questions/676573), so thanks a bunch for the tip!EDIT: If I were to give that setup a name I'd go for "active turn-off" as the symmetric version for a PNP transistor would be an active pullup instead (I guess).