What is full-swing dc voltage? - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: January 31, 2023, 06:25:27 pm
content: 1. Here What is meant by full-swing dc voltage?2. what is the significance of using the word cooperative here?

author: wasedadoc
author_rank: Super Contributor
timestamp: January 31, 2023, 06:40:32 pm
content: "full swing" in that context means changing between the highest and lowest voltages. 0 and +10 Volts in that circuit."cooperative" is not a term I would have used there. It isn't a technical term. I guess the author just meant that the FET "would not object" or "could accept" such voltages. Unlike a bipolar junction transistor which certainly would not like 10 volts applied to its base emitter junction.

author: BillyO
author_rank: Super Contributor
timestamp: January 31, 2023, 09:22:25 pm
content: I think cooperative in this case means "operating simultaneously" or "operating together". In other words the gate is subject to the DC voltage swing. If I had written that, I would just have left the word cooperative out. Then it works just fine.

author: khatus
author_rank: Regular Contributor
timestamp: February 02, 2023, 01:46:26 pm
content: OK. I am having difficulty understanding the meaning of these lines1)Why is it called a saturated transistor switch??2)"Without squandering excessive power......................."In this line where do we squander excessive power?? is it on the input side??3) Why this analog switch can not be done with a bipolar transistor ??4)I can't understand the line "considering worst-case minimum hFE combined with the lamp's cold resistance "?hFE = IC/IB ,if hFe to be minimum IB has to be maximum

author: wasedadoc
author_rank: Super Contributor
timestamp: February 02, 2023, 03:55:46 pm
content: Saturated. Collector voltage cannot go any lower and collector current cannot go any higher even with more base current. Because voltage across the resistor (its resistance value multiplied by the collector current) leaves only a small voltage (perhaps about 0.2 Volts) at the collector.The FET can act as variable resistor. The gate does not inject or take away any current that flows between drain and source. That is not the case with the base of a bipolar transistor.The resistance of the filament of an incandescent lamp increases greatly when hot. It takes maximum current at switch on from cold. So that determines the collector current. The specs of the transistor should state what the minimum hfe of the transistor is. (Transistors have tolerances so not all samples of the same model number have the same hfe. The specs should state a guaranteed minimum.) From the collector current and the minimum hfe we can calculate the maximum base current that needs to be provided. No need to provide more than that.From the excerpts you have shown, I don't like the author's style.

author: TimFox
author_rank: Super Contributor
timestamp: February 02, 2023, 04:46:44 pm
content: I agree about the textbook author's style being confusing.An NPN BJT saturates when the collector voltage goes below the base voltage (with respect to emitter) but not quite down to the emitter voltage.At that point, the collector current is limited by the supply voltage and load resistance, and increasing the base current cannot increase it further.However, if you reduce the load resistance, then you can achieve a higher collector current by increasing the base current.To ensure saturation (which results in relatively low collector power dissipation, a good thing), you must make sure that the base current is more than sufficient, and the "DC beta"hFE=IC/IB, the simple ratio of DC currents (not the derivative) is the relevant parameter and the manufacturer should specify a minimum value so that the designer knows how much base current is required to guarantee saturation.