Re: Very basic transistor usage

From: CFoley1064 (cfoley1064_at_aol.com)
Date: 10/11/04 

Date: 11 Oct 2004 14:23:38 GMT

>Subject: Re: Very basic transistor usage
>From: tatto0_2000@yahoo.com (Wong)
>Date: 10/11/04 3:00 AM Central Daylight Time
>Message-id: <509bfe22.0410110000.67a506fe@posting.google.com>
>

<snip>
>> Here's one possible way to do this (view in fixed font or M$ Notepad):
>>
>> VCC
>> + VCC VCC VCC
>> | + + +
>> .-. | | |
>> 2.2K| | | | .-.
>> | | | | | |2.2K
>> '-' | | | |
>> ___ | |/ | '-'
>> .--|___|-o-| 2N3906 >| | ___
>> | 2.2K |< 2N3906 |-o-|___|--.
>> | | /| 2.2K |
>> | | | |
>> | .-. .-. |
>> | 75 ohm| | | | |
>> | | | | |120 ohm |
>> | '-' '-' |
>> | | | |
>> | Blue V ~ Red V ~ |
>> | - ~ - ~ |
>> | | | |
>> | | | |
>> | '---------o--------' |
>> | | |
>> Netwk|\ | === |
>> -| >O--' GND |
>> |/ |
>> 2/8 74HC244 |
>> |\ |
>> -| >O-----------------------------------------------'
>> HD |/
>> created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
>>
<snip>
>Hi Chris,
> Something that I don't understand. Why is there a "voltage divider"
>between the node of a 2.2K resistor and the transistor base?
> Or it just the pullup resistor because of 74HC244 ?
> Think that you wont get 2mA when Netwk is "1" and 244 output is "0",
>I think you will get less than that, right ?
>
>

Hi, Jason. It's common practice when using a transistor as a switch to have a
pulldown (NPN) or pullup (PNP) resistor from the base to the emitter to keep
things well-behaved. On is really on, and off is really off. In fact,
"digital transistors" are made these days which have the series resistor and
pullup/pulldown from base to emitter built in, and I'll usually use one of
those when using a discrete transistor as a switch. Fewer components to place,
less hassle. But if you're driving the transistors from HCMOS, which has logic
levels pretty much at the supply rails (unless you're driving some current),
the pullups technically wouldn't be necessary. Back in days of yore you'd need
these extra resistors because TTL logic levels would lead to fuzzy turn-on and
turn-off states. Times have changed. Good observation.

Good luck with your project
Chris

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