A unique output is seen in the LM111/211/311 and LM119/219/319 families. The output transistor exposes both the emitter and the collector creating a floating output.

Figure 2-2 LM311 Block Diagram

A problem arises when the comparator inputs require a split power supply to accept bipolar input signals. For most open collector output comparators, this results in an output Low swing that is equal to the negative power supply pin (V- or V_EE). If the comparator is using a split supply, such as +12V and -12V, the output LOW is -12V and NOT ground (0V). Allowing for a negative supply allows the comparator to directly accept bipolar input signals without level shifting, attenuation or clamping, thus improving accuracy. Level shifting is possible at the output by using a resistor- divider string to achieve an above ground swing, but this is not the best way.

The exposed output transistor pins allows the floating output to be either Common Collector (load to pull-up voltage, or sinking current), or common emitter (with the load to ground, or sourcing current), as shown in the following images. This allows the user to determine the output High and Low voltage levels independent of the V_CC and V_EE voltages.

Figure 2-3 Open Collector Configuration

Figure 2-4 Open Emitter Configuration

As shown in Section 2.1, if a Common Collector logic output is required (the most common use), the Emitter pin is tied to system ground, and the pull-up resistor is connected between the Collector pin and a positive logic supply. The logic output is taken from the Collector pin, as shown in Figure 2-3. This is the same configuration as the Open Collector output, except that the output now swings Low to system ground (0V).

A usage example is where the input section power supplies (V+ and V-) are a split supply of +12V and -12V to accept bipolar input signals, but the output has to drive 3.3V digital logic. This configuration is shown in Figure 2-5 .

Figure 2-5 Bipolar Input Using Open Collector Output Example

The output Emitter is tied to GND, and the pull-up resistor is placed between the Collector pin and the pull-up voltage. The Collector pin becomes the output. If the pull-up voltage is +3.3V, this creates a 0 to 3.3V ground referenced output swing directly compatible with 3.3V digital logic. No level shifting required!

If a high side sourcing or a grounded load is required, the Common Emitter configuration can be used, as shown in Figure 2-6. For a Common Emitter output, the Collector pin is tied to the positive source voltage, and the load is connected between the Emitter pin and negative voltage (usually ground). This is useful when the load must be returned to ground, as shown in Figure 2-6.

Figure 2-6 Bipolar Input With Open Emitter Output Example

Logic output is now taken from the Emitter pin. Note there is an inversion in the logic when using common emitter output. The inputs can be simply reversed to correct the logic. Observe limits on the output stage voltages in the Absolute Maximum table.

Examples of the OC/OE output is the LM111, LM211, LM311, LM119, LM219, LM319 and LM6511.