The NOT Gate

The single-transistor inverter circuit illustrated earlier is actually too crude to be of practical use as a gate. Real inverter circuits contain more than one transistor to maximize voltage gain (so as to ensure that the final output transistor is either in full cutoff or full saturation), and other components designed to reduce the chance of accidental damage.

Practical Inverter Circuit

Shown here is a schematic diagram for a real inverter circuit, complete with all necessary components for efficient and reliable operation:

This circuit is composed exclusively of resistors, diodes and bipolar transistors. Bear in mind that other circuit designs are capable of performing the NOT gate function, including designs substituting field-effect transistors for bipolar (discussed later in this chapter).

Analyze the Inverter Circuit

Let\\’s analyze this circuit for the condition where the input is “high,” or in a binary “1” state. We can simulate this by showing the input terminal connected to V_cc through a switch:

In this case, diode D₁ will be reverse-biased, and therefore not conduct any current. In fact, the only purpose for having D₁ in the circuit is to prevent transistor damage in the case of a negative voltage being impressed on the input (a voltage that is negative, rather than positive, with respect to ground).

With no voltage between the base and emitter of transistor Q₁, we would expect no current through it, either.

However, as strange as it may seem, transistor Q₁ is not being used as is customary for a transistor. In reality, Q₁ is being used in this circuit as nothing more than a back-to-back pair of diodes. The following schematic shows the real function of Q₁:

The purpose of these diodes is to “steer” current to or away from the base of transistor Q₂, depending on the logic level of the input.

Exactly how these two diodes are able to “steer” current isn\\’t exactly obvious at first inspection, so a short example may be necessary for understanding.

Suppose we had the following diode/resistor circuit, representing the base-emitter junctions of transistors Q₂ and Q₄ as single diodes, stripping away all other portions of the circuit so that we can concentrate on the current “steered” through the two back-to-back diodes:

With the input switch in the “up” position (connected to V_cc), it should be obvious that there will be no current through the left steering diode of Q₁, because there isn\\’t any voltage in the switch-diode-R₁-switch loop to motivate electrons to flow.

However, there will be current through the right steering diode of Q₁, as well as through Q₂\\’s base-emitter diode junction and Q₄\\’s base-emitter diode junction:

This tells us that in the real gate circuit, transistors Q₂ and Q₄ will have base current, which will turn them on to conduct collector current.

The total voltage dropped between the base of Q₁ (the node joining the two back-to-back steering diodes) and ground will be about 2.1 volts, equal to the combined voltage drops of three PN junctions: the right steering diode, Q₂\\’s base-emitter diode, and Q₄\\’s base-emitter diode.

Now, let\\’s move the input switch to the “down” position and see what happens:

If we were to measure current in this circuit, we would find that all of the current goes through the left steering diode of Q₁ and none of it through the right diode. Why is this? It still appears as though there is a complete path for current through Q₄\\’s diode, Q₂\\’s diode, the right diode of the pair, and R₁, so why will there be no current through that path?

Remember that PN junction diodes are very nonlinear devices: they do not even begin to conduct current until the forward voltage applied across them reaches a certain minimum quantity, approximately 0.7 volts for silicon and 0.3 volts for germanium.

And then when they begin to conduct current, they will not drop substantially more than 0.7 volts. When the switch in this circuit is in the “down” position, the left diode of the steering diode pair is fully conducting, and so it drops about 0.7 volts across it and no more.

Recall that with the switch in the “up” position (transistors Q₂ and Q₄ conducting), there was about 2.1 volts dropped between those same two points (Q₁\\’s base and ground), which also happens to be the minimum voltage necessary to forward-bias three series-connected silicon PN junctions into a state of conduction.

The 0.7 volts provided by the left diode\\’s forward voltage drop is simply insufficient to allow any electron flow through the series string of the right diode, Q₂\\’s diode, and the R₃//Q₄ diode parallel subcircuit, and so no electrons flow through that path.

With no current through the bases of either transistor Q₂ or Q₄, neither one will be able to conduct collector current: transistors Q₂ and Q₄ will both be in a state of cutoff.

Consequently, this circuit configuration allows 100 percent switching of Q₂ base current (and therefore control over the rest of the gate circuit, including voltage at the output) by diversion of current through the left steering diode.

Example Gate Circuit

In the case of our example gate circuit, the input is held “high” by the switch (connected to V_cc), making the left steering diode (zero voltage dropped across it). However, the right steering diode is conducting current through the base of Q₂, through resistor R₁:

Logic Level Binary 1

With base current provided, transistor Q₂ will be turned “on.” More specifically, it will be saturated by virtue of the more-than-adequate current allowed by R₁ through the base.

With Q₂ saturated, resistor R₃ will be dropping enough voltage to forward-bias the base-emitter junction of transistor Q₄, thus saturating it as well:

With Q₄ saturated, the output terminal will be almost directly shorted to ground, leaving the output terminal at a voltage (in reference to ground) of almost 0 volts, or a binary “0” (“low”) logic level. Due to the presence of diode D₂, there will not be enough voltage between the base of Q₃ and its emitter to turn it on, so it remains in cutoff.

Logic Level Binary 0

Let\\’s see now what happens if we reverse the input\\’s logic level to a binary “0” by actuating the input switch:

Now there will be current through the left steering diode of Q₁ and no current through the right steering diode. This eliminates current through the base of Q₂, thus turning it off. With Q₂ off, there is no longer a path for Q₄ base current, so Q₄ goes into cutoff as well. Q₃, on the other hand, now has sufficient voltage dropped between its base and ground to forward-bias its base-emitter junction and saturate it, thus raising the output terminal voltage to a “high” state.

In actuality, the output voltage will be somewhere around 4 volts depending on the degree of saturation and any load current, but still high enough to be considered a “high” (1) logic level.

With this, our simulation of the inverter circuit is complete: a “1” in gives a “0” out, and vice versa.

Inverter Circuit High State

The astute observer will note that this inverter circuit\\’s input will assume a “high” state of left floating (not connected to either V_cc or ground).

With the input terminal left unconnected, there will be no current through the left steering diode of Q₁, leaving all of R₁\\’s current to go through Q₂\\’s base, thus saturating Q₂ and driving the circuit output to a “low” state:

The tendency for such a circuit to assume a high input state if left floating is one shared by all gate circuits based on this type of design, known as Transistor-to-Transistor Logic, or TTL.

This characteristic may be taken advantage of in simplifying the design of a gate\\’s output circuitry, knowing that the outputs of gates typically drive the inputs of other gates.

If the input of a TTL gate circuit assumes a high state when floating, then the output of any gate driving a TTL input need only provide a path to ground for a low state and be floating for a high state. This concept may require further elaboration for full understanding, so I will explore it in detail here.

A gate circuit as we have just analyzed has the ability to handle output current in two directions: in and out. Technically, this is known as sourcing and sinking current, respectively.

Sourcing Current

When the gate output is high, there is continuity from the output terminal to V_cc through the top output transistor (Q₃), allowing electrons to flow from ground, through a load, into the gate\\’s output terminal, through the emitter of Q₃, and eventually up to the V_cc power terminal (positive side of the DC power supply):

To simplify this concept, we may show the output of a gate circuit as being a double-throw switch, capable of connecting the output terminal either to V_cc or ground, depending on its state.

For a gate outputting a “high” logic level, the combination of Q₃ saturated and Q₄ cutoff is analogous to a double-throw switch in the “V_cc” position, providing a path for current through a grounded load:

Please note that this two-position switch shown inside the gate symbol is representative of transistors Q₃ and Q₄ alternately connecting the output terminal to V_cc or ground, not of the switch previously shown sending an input signal to the gate!

Conversely, when a gate circuit is outputting a “low” logic level to a load, it is analogous to the double-throw switch being set in the “ground” position.

Sinking Current

Current will then be going the other way if the load resistance connects to V_cc: from ground, through the emitter of Q₄, out the output terminal, through the load resistance, and back to V_cc. In this condition, the gate is said to be sinking current:

The combination of Q₃ and Q₄ working as a “push-pull” transistor pair (otherwise known as a totem pole output) has the ability to either source current (draw in current to V_cc) or sink current (output current from ground) to a load.

However, a standard TTL gate input never needs current to be sourced, only sunk. That is, since a TTL gate input naturally assumes a high state if left floating, any gate output driving a TTL input need only sink current to provide a “0” or “low” input, and need not source current to provide a “1” or a “high” logic level at the input of the receiving gate:

Open-Collector Output

This means we have the option of simplifying the output stage of a gate circuit so as to eliminate Q₃ altogether. The result is known as an open-collector output:

To designate open-collector output circuitry within a standard gate symbol, a special marker is used. Shown here is the symbol for an inverter gate with open-collector output:

Please keep in mind that the “high” default condition of a floating gate input is only true for TTL circuitry, and not necessarily for other types, especially for logic gates constructed of field-effect transistors.

Review

An inverter, or NOT, gate is one that outputs the opposite state as what is input. That is, a “low” input (0) gives a “high” output (1), and vice versa.

Gate circuits constructed of resistors, diodes and bipolar transistors as illustrated in this section are called TTL. TTL is an acronym standing for Transistor-to-Transistor Logic. There are other design methodologies used in gate circuits, some which use field-effect transistors rather than bipolar transistors.

A gate is said to be sourcing current when it provides a path for current between the output terminal and the positive side of the DC power supply (V_cc). In other words, it is connecting the output terminal to the power source ( V).

A gate is said to be sinking current when it provides a path for current between the output terminal and ground. In other words, it is grounding (sinking) the output terminal.

Gate circuits with totem pole output stages are able to both source and sink current. Gate circuits with open-collector output stages are only able to sink current, and not source current. Open-collector gates are practical when used to drive TTL gate inputs because TTL inputs don\\’t require current sourcing.