TTL NAND and AND gates

Description: This schematic represents a real circuit, though it is not referred to as a "two-input inverter." An analysis will reveal the circuit's logic function and its appropriate designation. Similar to the inverter and buffer, the "steering" diode cluster labeled "Q1" is configured like a transistor, despite not being utilized for amplification. A simple NPN transistor structure is insufficient to replicate the three PN junctions required in this diode network; therefore, a different transistor (and symbol) is necessary. This transistor features one collector, one base, and two emitters. In the single-input (inverter) circuit, grounding the input resulted in an output that assumed the "high" (1) state. In the case of the open-collector output configuration, this "high" state was merely "floating." Connecting the input to Vcc or allowing it to float caused the output to be grounded, representing the "low" (0) state. Consequently, a 1 input produced a 0 output, and vice versa. Given the close resemblance of this circuit to a simple inverter, with the only difference being a second input terminal connected similarly to the base of transistor Q2, it can be inferred that each input will affect the output in the same manner. Specifically, if either input is grounded, transistor Q2 will enter a cutoff state, turning off Q3 and allowing the output to float (output goes "high"). The subsequent series of illustrations demonstrates this for three input states (00, 01, and 10): In any situation where there is a grounded ("low") input, the output will remain floating ("high"). Conversely, the output will only go "low" if transistor Q3 is activated, which requires transistor Q2 to be turned on (saturated), meaning neither input can divert R1 current away from the base of Q2. The only condition that meets this requirement is when both inputs are "high" (1). In the previous discussion on NAND gates, this type of gate was formed by taking an AND gate and enhancing its complexity by incorporating an inverter (NOT gate) at the output. However, upon examining this circuit, it becomes evident that the NAND function is the most straightforward and natural mode of operation for this TTL design. To create an AND function using TTL circuitry, the complexity of this circuit must be increased by adding an inverter stage to the output, similar to how an additional transistor stage was added to the TTL inverter circuit to convert it into a buffer. It should be noted that both NAND and AND gate circuits can be designed with totem-pole output stages rather than open-collector outputs. The open-collector versions are presented here for simplicity.

This circuit schematic illustrates a two-input NAND gate configuration utilizing a transistor-based design. The circuit features a diode cluster, labeled as "Q1," which mimics a transistor structure. The unique arrangement involves a transistor with one collector, one base, and two emitters, allowing for the necessary three PN junctions to be formed within the diode network. The operation is based on the principle that grounding either input will lead to a high output state, while both inputs must be high for the output to transition to a low state.

In this design, the output is characterized as open-collector, meaning it can float high when neither input is grounded. The design allows for flexibility in input states, with the output remaining high unless both inputs are activated, which would turn on transistor Q2 and subsequently activate transistor Q3. This behavior is consistent with the properties of NAND gates, where the output is low only when all inputs are high.

The circuit can be analyzed through various input combinations, demonstrating that the output state changes in response to the inputs as expected for a NAND gate. When both inputs are high, the circuit enters a saturated state, allowing current to flow through R1 into the base of Q2, which activates Q3 and pulls the output low. Conversely, any grounded input state guarantees a high output, showcasing the fundamental logic operation of the NAND configuration.

In summary, this schematic serves as a practical representation of a NAND gate using TTL technology, emphasizing its operational simplicity and the inherent characteristics of open-collector outputs. The design can be further adapted to include totem-pole output stages for applications requiring more robust output drive capabilities, although the current open-collector configuration maintains clarity in demonstrating the logic function of the circuit.This schematic illustrates a real circuit, but it isn`t called a "two-input inverter. " Through analysis we will discover what this circuit`s logic function is and correspondingly what it should be designated as. Just as in the case of the inverter and buffer, the "steering" diode cluster marked "Q1" is actually formed like a transistor, even thoug

h it isn`t used in any amplifying capacity. Unfortunately, a simple NPN transistor structure is inadequate to simulate the three PN junctions necessary in this diode network, so a different transistor (and symbol) is needed. This transistor has one collector, one base, and two emitters, and in the circuit it looks like this: In the single-input (inverter) circuit, grounding the input resulted in an output that assumed the "high" (1) state.

In the case of the open-collector output configuration, this "high" state was simply "floating. " Allowing the input to float (or be connected to Vcc) resulted in the output becoming grounded, which is the "low" or 0 state. Thus, a 1 in resulted in a 0 out, and visa-versa. Since this circuit bears so much resemblance to the simple inverter circuit, the only difference being a second input terminal connected in the same way to the base of transistor Q2, we can say that each of the inputs will have the same effect on the output.

Namely, if either of the inputs are grounded, transistor Q2 will be forced into a condition of cutoff, thus turning Q3 off and floating the output (output goes "high"). The following series of illustrations shows this for three input states (00, 01, and 10): In any case where there is a grounded ("low") input, the output is guaranteed to be floating ("high").

Conversely, the only time the output will ever go "low" is if transistor Q3 turns on, which means transistor Q2 must be turned on (saturated), which means neither input can be diverting R1 current away from the base of Q2. The only condition that will satisfy this requirement is when both inputs are "high" (1): In the earlier section on NAND gates, this type of gate was created by taking an AND gate and increasing its complexity by adding an inverter (NOT gate) to the output.

However, when we examine this circuit, we see that the NAND function is actually the simplest, most natural mode of operation for this TTL design. To create an AND function using TTL circuitry, we need to increase the complexity of this circuit by adding an inverter stage to the output, just like we had to add an additional transistor stage to the TTL inverter circuit to turn it into a buffer: Of course, both NAND and AND gate circuits may be designed with totem-pole output stages rather than open-collector.

I am opting to show the open-collector versions for the sake of simplicity.