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[DPRG] increased output drive by CMOS and TTL devices

Subject: [DPRG] increased output drive by CMOS and TTL devices
From: R. Bickle rbickle at swbell.net
Date: Wed Jun 27 11:15:47 CDT 2001


Most logic families today use CMOS rather than BJT's. With the exception of
LS, AS, and F, all other logic families that come to mind use CMOS. All of
the newer technologies such as LS, LSTTL, HC, HCT and so forth are CMOS
devices. Some of these (the T and TTL versions) have adjusted the voltage
switching threshold in order to behave like TTL devices. (TTL swiches at
about 1.3 above ground while CMOS switches from 1/3 to 2/3 of VCC.) The
great advantage of CMOS is higher input impedance, better fanout, and much
lower power consumption.

You are right that BJT's in parallel will current hog, but I don't think
that's the case in most logic gates. I do agree that the best solution to
the problem is to use a separate driver transistor.


-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org]On Behalf
Of Tom Gralewicz
Sent: Tuesday, June 26, 2001 2:18 PM
To: dprglist at dprg.org
Subject: RE: [DPRG] increased output drive by CMOS and TTL devices

I think the problem with paralleling TTL is that you have bipolar
transistors.  You can't directly parallel these since variables in voltage
drop across different transistors can make one pull the full current while
the other does nothing.  Usually when paralleling bipolar transistors you
put a small resistor in series with output of each one to allow current
sharing.  With TTL it is generally a bad idea to parallel them, you are
better off using a driver chip.

At 10:10 AM 6/26/01 -0500, you wrote:
>I've seen this done in a number of cases. The only problem you might run
>into is that the two devices are not going to switch at exactly the same
>time, possibly resulting in a very short rail to rail connection. The end
>result of this is a spike on your 5V rail. For hobby projects and such, you
>are probably fine. I would not do it however on a production unit.
>-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org]On Behalf
>Of Ben Strednak
>Sent: Monday, June 25, 2001 4:58 PM
>To: dprglist at dprg.org
>Subject: [DPRG] increased output drive by CMOS and TTL devices
>Hi folks.
>According to Forrest Mims, in his Engineer's Notebook, you can tie together
>the outputs of a CMOS IC to increase the amount of current the IC can
>and sink.  For example, Mims uses a 4011 (quad nand gate), and connects all
>of the inputs of two gates to one another.  He then connects the two
>to each other.  Can you do the same thing with TTL ICs and get the same
>effect?  More specifically, a 74LS04?
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Tom Gralewicz
mot at ieee.org

Midwest Computer Recyclers
(414) 541-1716

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