We have already disclosed the basic idea of the historically first RTL gate
https://www.researchgate.net/post/How_were_basic_Boolean_functions_implemented_in_electronics_What_were_the_general_idea_structure_and_operation_of_the_first_logic_gates
It was surprising but true that the RTL NOR gate was an analog device implemented by humble resistors while “true digital” logic gates were (are) implemented by electronic switching elements – diodes (DL, DTL and TTL gates), bipolar transistors (ECL gates) and MOS transistors (NMOS, PMOS and CMOS logic gates):
https://www.researchgate.net/post/What_is_the_basic_idea_of_the_input_logical_part_of_transistor-transistor_TTL_diode-transistor_DTL_and_diode_DL_logic_gates_Are_they_related
Although RTL stays away from all these logic families, it is still interesting to see if there is some connection between them... some common general idea... Let’s try to find it...
Remember that in RTL we summed voltages by converting them to currents. But this circuit was sensitive to the magnitudes of the voltages and resistances; in addition, the number of inputs was limited. It seems we can sum, besides voltages and currents, why not resistances as well? Here is the implementation.
The input logic variables turn on (at logic "1") or turn off (at logical "0") equal reference resistances (conductances). They are summed by an analog summer again; their sum is converted to voltage and compared by a threshold device (voltage comparator) whose threshold is lower than one reference. So it is sufficient that only one reference is turned on and the output is set at logic state "1".
This idea is taken to the extreme in the classic DL, DTL, TTL, MOS and CMOS circuits where the reference resistances are increased up to infinity. In practice, they are implemented by diode or transistor switches operated by the logic input variables. They are connected in series to sum the switch resistances or in parallel to sum their conductances (DL, DTL and TTL use only a parallel connection).
Because the sum of the included resistances/conductances is infinite (even if only one element is connected), the comparator can have an any threshold within the supply voltage. In this situation, it is sufficient only one switch in series/parallel to be open/closed so that the total resistance/conductance becomes infinitely large, and the threshold element switches. This element can even be absent if the thresholds of the next electrically operated switches are used (MOS and CMOS logic elements exploit this idea).
Depending on the way of connection (in series/parallel), the correlation between the values of the logical input variables and the state of the switches, as well as on the presence of an inverter at the output, OR (NOR) or AND (NAND) elements can be obtained. For example, in a DTL circuit the diode switches are connected in parallel, the correlation is “logic ‘0’ – open switch” and “logic ‘1’ - closed switch ); as a result, a NAND logic gate is obtained.
As a conclusion, my extravagant idea about the essence of electronic logic gates is:
Digital logic gates (all kinds) are implemented in electronics by cascading two devices – a “summer” (a pure analog device) and a “comparator” (a mixed analog-to-digital device); the discrete (binary) Boolean logic functions OR/AND are implemented by the arithmetical summation.
Long ago we had lotz of analog component especially BJTs. A single transistor with load/ bias resistors made it a switch subject to the frequency of the driving signal applied at the base. As far as I understand :
1. At high frequency signals never mind digital/analog, FET acts as a set of R,C components. These capacitors are presented at every junction (p-n) hence takes finite the charging and discharging times. In total the signal frequency makes the capacitance become visible and depending on how they appear (parallel or serial ) makes the time- constant variability (switching speed of the device). More can be found in old books : Millman and Halkias ; Pucknell and High speed circuit designs R.Baker and analytical models of high - frequency and low frequency analysis.
2. The logic-0/1 is also implied when the signals are actually 0V and 1.8/3.3(MOS)/5 (TTL) levels. If the output falls below these again combination (CMOS) as drivers came about to have clean logics (0V and 5V) as the case maybe.
3. Power , performance and area are main consideration for IC design and these have trade-offs so I guess the layouts of logic realizable circuits vary depending on what the designer wants !
The central thesis in the question - that a logic gate essentially consists of a summing circuit followed by a comparator - is an attractive generalisation.
In the circuit sketch supplied with the question (a load resistor and two mosfets in series), one might ask 'Where is the comparator?' As you say, in this case, the comparator is implemented using the threshold voltage at the inputs of the next gate.
I would like to add this observation: all logic circuits require gain in the signal path to preserve the distinct '0' and '1' levels, and the function of the comparator element in any logic circuit is to provide gain.
Logic gates can be implemented using the basic analog components, resistors, diodes, bipolar junction transistors and field effect transistors. All digital logic gates OR, AND, NAND and NOR can be implemented using switches however the behavior of these gates also can be achieved from the basic components. Logic gates can be performed using RTL, DTL, TTL, MOS and CMOS circuits.
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