[Test-Equipment] Transistor Circuit Design Question

[David]

I think you have the right idea.  What you suggest is how I would
initially do it.

The 3870 CMOS microprocessor output ports are rated to sink 1.8mA at
0.4V (it looks like they wanted to be able to drive a single TTL load
if only barely) so the first 10k resistor limits the current to about
440uA which is reasonable and a 2N3904 will have no trouble with that
however . . .

That 440uA is going to barely pull the collector load of 10k down to
ground so the 2nd 2N3904 is barely going to turn off; there is little
to no margin.  I would halve the value of the emitter resistor to 4.7k
doubling the emitter current to 1mA and remove the resistor in series
with the base of the 2nd 2N3904.  Note that a 4.7k pull-up resistor is
typical for TTL so they probably had that in mind when they designed
the 3870.

As an alternative, double the collector load resistor to 22k (and
remove the series resistor); this will limit drive to the 2nd
transistor but it should still be larger than needed by a factor of 4
to drive a 1k pull-up to +5V.

If the signal needs to be sped up, then a small amount of capacitance,
like 100pF, could be added directly between the I/O pin and the base
of the second transistor but the switching should be checked on an
oscilloscope to make sure it does not glitch or oscillate.

On Mon, 25 May 2015 21:21:26 -0400 (EDT), you wrote:

>Yes, I've seen that one too.
>
>Here's a sketch of the circuit I'm using.
>
>https://n4buq.shutterfly.com/pictures/41
>
>Thanks,
>Barry - N4BUQ
>
>----- Original Message -----
>> From: "KA4INM" <ka4inm at gmail.com>
>> To: "Discussion of Electronic Test Equipment" <test-equipment at mailman.qth.net>
>> Sent: Monday, May 25, 2015 7:54:43 PM
>> Subject: Re: [Test-Equipment] Transistor Circuit Design Question
>> 
>> On 05/25/2015 08:33 PM, Barry wrote:
>> 
>> > I have a situation where I have strobe and data signals originating from a
>> > 3870 CMOS microprocessor (part of a Fluke 8050A) that I need to convert to
>> > make them compatible with the display/latch I'm using to retrofit a
>> > different display.
>> 
>>    http://vondervotteimittiss.com/belfry/?p=180
>> 
>> --
>>    Ron  KA4INM - Youvan's corollary:
>>                  Every action results in unwanted side effects.
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