An idea becomes real
February 20, 2018
topic pic
Let's say, we want to add two decimal numbers. (A and B)
back box
Now, we convert the numbers into binary. (a10 -> a2)
The picture is a decimal to binary decoder. It is very simple and only needs some diodes. (Red lines)
Vcc could be 5V and the pull-down resistors could be about 10kΩ. (or for example a transistor like the BC547)
diode grid
Next step is the arithmetic part.
In our case, we need a 4 bit adder which consists out of 1 half-adder and 3 full-adders.
HalfAdder and FullAdder
Diagram of the HalfAdder and FullAdder
The next problem is that a real AND-gate creates a voltage drop, depending on which component you use. A simple 1N4148 (Diode) dropes around 0.7 V. LEDs range from 1,4 to 4V.
(The "forward voltage" is the needed external electrical field to overcome the inner electrical field. Similar to the breakdown voltage. This inner field is created at the p-n junction, when the Al in the Si-grid has 3 electrons and the P atom has 5 electrons. )
That's why we convert the logic-gates into NAND, NOR, NOT – gates. A NOT-gate could be made out of a NAND-gate too, but this would need one more transistor/ mosfet.
NMOS: The use of NAND-gates is a standard for ICs.
CMOS: Nowadays, ICs are made with CMOS logic. Which means both types of MOSFETs are used. (N- and P- channel)
PMOS: But it's possible to design a circuit with P-channel MOSFETs only.
Finally, the common types of MOSFETs:
mosfet types
But be aware:
If you use NMOS, the Source is connected to ground. Different to PMOS, the Drain is connected to ground.
I use the enhancment type of NMOS which is very common.
Logic gate conversion:
logic gate conversion
The next step is to put all parts together:
put all together
Afterwards, we convert the logic-gates into the transistor level. That could look like this:
To leave it simple, we only use N-Channel MOSFETs (enhancement-type). And the Body diode, capacitors and a fast gate driver are omitted.
mosfet gates
In conclusion, the value of the resistors depends on the attached circuits, wanted switching speed, current consumption, and so on.