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TOPIC: Who right

Re: Who right 7 years 7 months ago #10062

  • jmessina
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What is RE and why doesn't Burt have one in his circuit. Is something else acting on it's behalf?

In that circuit, RE is the 330 ohm resistor and the LED

Re: Who right 7 years 7 months ago #10063

  • Jon G
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That's what it looks like... So it can be anything? It sounds as though RE should be something particular based on what someone is doing/wanting. Or will just simply swapping in the relay in place of the resistor accomplish the task?

I'll just read up on it I guess.

Re: Who right 7 years 7 months ago #10064

  • MMcLaren
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You've seen the emitter-follower configuration before, haven't you? We use NPN source drivers in the emitter-follower configuration for column drivers on Charlieplexed common anode displays because it follows the active high column driver signal requirement. Likewise, we use PNP emitter-follower drivers with active low inputs as sinking drivers for command cathode Charlieplexed displays.
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Re: Who right 7 years 7 months ago #10065

  • MMcLaren
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However, let's think for a moment about the facts here, disregarding calculations. As near as I can tell from some research, the BJT has been around since 1948, giving people some 62 years to experiment with them. I guess I didn't suppose nobody had ever thought of trying to place an NPN on the high side, given how convenient that arrangement would be for a great many circuits. This leads someone like myself to wonder "why?" Then I see someone post saying that it's possible to do that. I thought maybe I'd just overlooked something since I hadn't seen that configuration yet.

The common-collector or emitter-follower circuit actually has roots going back long before the transistor in the form of the "cathode-follower" vacuum tube circuit configuration...

Re: Who right 7 years 7 months ago #10066

  • Jon Chandler
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We use NPN source drivers in the emitter-follower configuration for column drivers on Charlieplexed common anode displays because it follows the active high column driver signal requirement.

Mike, what's the advantage of doing this vs. a "conventional" PNP high-side switch?

Re: Who right 7 years 7 months ago #10067

  • MMcLaren
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We use NPN source drivers in the emitter-follower configuration for column drivers on Charlieplexed common anode displays because it follows the active high column driver signal requirement.

Mike, what's the advantage of doing this vs. a "conventional" PNP high-side switch?

The Charlieplexing signal rules (for CA display) require that only a single pin is driven high to select a column and the remaining pins are either hi-z (segment off) or driven low (cathode segment on) so you need a source driver with an active high input (the NPN emitter-follower). Likewise, for Charlieplexed CC displays, you need a sinking driver with an active low input (PNP emitter-follower) to turn on a column and the remaining pins are either hi-z (segment off) or driven high (anode segment on).

If I was driving a traditional multiplexed display I would use PNP or PFET high side drivers or NPN or NFET low side drivers...
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