[Test-Equipment] Transistor Circuit Design Question

[Barry]

Hi David,

Okay - thanks so much for the analysis.  I'm not sure if I'll stick with shifting the voltages or just run the entire thing between -5V and 0V as the other David suggests.  I may try your suggestions just to prove it out for my own satisfaction.

Thanks again,
Barry - N4BUQ

----- Original Message -----
> From: "David" <davidwhess at gmail.com>
> To: "Discussion of Electronic Test Equipment" <test-equipment at mailman.qth.net>
> Sent: Monday, May 25, 2015 10:37:34 PM
> Subject: Re: [Test-Equipment] Transistor Circuit Design Question
> 
> 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.
> >> ______________________________________________________________
> >> Test-Equipment mailing list
> >> Home: http://mailman.qth.net/mailman/listinfo/test-equipment
> >> Help: http://mailman.qth.net/mmfaq.htm
> >> Post: mailto:Test-Equipment at mailman.qth.net
> >> 
> >> This list hosted by: http://www.qsl.net
> >> Please help support this email list: http://www.qsl.net/donate.html
> >> 
> >______________________________________________________________
> >Test-Equipment mailing list
> >Home: http://mailman.qth.net/mailman/listinfo/test-equipment
> >Help: http://mailman.qth.net/mmfaq.htm
> >Post: mailto:Test-Equipment at mailman.qth.net
> >
> >This list hosted by: http://www.qsl.net
> >Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> Test-Equipment mailing list
> Home: http://mailman.qth.net/mailman/listinfo/test-equipment
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Test-Equipment at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>