An idea becomes real
February 20, 2018
Let's say, we want to add two decimal numbers. (A and B)
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)
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.
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:
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:
The next step is to put all parts 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.
- The vertical resistors are pull-down ones, to ground the Gate, if the input is zero.
- Source is connected to ground.
- Drain with a current limiter to Vcc.
In conclusion, the value of the resistors depends on the attached circuits, wanted switching speed, current consumption, and so on.