10 October 2016
n WITH KRISTEN McINTRYE
• How to Build a Gate
• Battery Polarity NOT Important
• Help with a Set of Archer Catalog
How to Build a Gate
QI have been looking for a logic gate design for quite a while. I have read the book ‘Code’ and am very motivated to build some of the more complex logic circuits they explain. The
foundation of these circuits is simple logic gates and I
cannot find anything that is not overly complex. Some
You Tube videos have gates that use capacitors, but they
seem way more complex than necessary. Is there any way
to build a quick and dirty gate out of two cheap transistors
and the fewest possible resistors? I am looking for simple.
AThat’s a great question, Bert, and something I recently included in a talk I gave about ransistor design, titled The Mighty Transistor. Unfortunately, simple designs with transistors
are becoming a bit of a lost art. Let’s see if we can revive
The simplest designs are typically called RTL, or
Resistor Transistor Logic. A simple transistor in a saturated
common emitter configuration can be used to create an
inverter (see Figure 1). What we’re basically doing in this
NPN example is amplifying the input logic signal to the
point where the transistor turns fully on when a logic 1 or
high voltage is input, and turning it off completely when a
logic 0 or low is input. In response, the collector falls when
it’s on and rises when it’s off.
It’s not a perfect inverter for a few reasons. It’s slow
and it can’t output much current when the output is
high, but it often serves just fine. You can speed it up a
bit by placing something like a few hundred picofarad
capacitors across the base resistor. That will help push and
pull the charge through the base, causing the transistor to
change states more quickly. It’s also possible to use a Field
Effect Transistor (or FET) in this circuit. It might be faster,
depending on the gate capacitance.
Now, how do we make a gate out of that? The simplest
Q & A
In this column, Kristen answers questions
about all aspects of electronics, including
computer hardware, software, circuits,
electronic theory, troubleshooting, and
anything else of interest to the hobbyist.
Feel free to participate with your questions,
comments, or suggestions. Send all questions
and comments to: Q&A@nutsvolts.com.
Hello to everyone who is reading the Q & A column today. This is my first one, so
please bear with me as I settle in to try to answer your questions as best I can. I have
to say that I’m honored by the trust the editors (and by proxy, the readers) have placed
in me. I will do my best to earn that trust. Here in Silicon Valley where I live, I end up
answering quite a few questions on the air on amateur radio. I hope to be able to do
the same here. So, without further ado, let’s dive in!
n FIGURE 1.