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Difference between Line Driver Open Collector Push Pull transistor Configurations

This page compares Line Driver vs Open Collector vs Push Pull transistor configurations and mentions difference between line driver, open collector transistor and push pull transistor with respect to their advantages and disadvantages.

Line driver circuit

Line driver circuit

The figure-1 depicts line driver sourcing output circuit. In "ON" condition, the circuit provides "Vcc" at the output. In "OFF" condition, the circuit output floats. Hence it requires sinking input for its proper operation.

Advantages:
➨It can be used to directly drive the LEDs.
➨It is simple circuit to implement.

Disadvantages:
➨The circuit can provide only one output state i.e. HIGH (Vcc).
➨The circuit output floats in "OFF" state.

Open collector transistor

Open collector transistor

The figure-2 depicts open collector transistor sinking output configuration. In "ON" condition, open collector configuration provides path to the GND (Ground). In "OFF" condition, open collector output will float.

Advantages:
➨The outputs can be tied directly together. This results into logical ANDing of the output. Hence AND gate circuit can be made by connection of its outputs.
➨Standard TTL gates with totem pole output circuit can provide 0.4mA for "HIGH" current output and 1.6 mA for "LOW" current. Open collector gates offer higher current ratings.
➨Open collector configuration can provide various voltages for "HIGH" output. This is very useful to interface different logic families having different voltage/current requirements.

Disadvantages:
➨The circuit can provide only one output state i.e. Ground or zero voltage. It can not provide high (i.e. Vcc) output as required for some applications.
➨It uses pull up resistor in KOhm range and hence it offers slow switching speed due to relatively long time constant.

Push pull transistor configuration

Push pull transistor

The figure-3 depicts push pull transistor configuration. It combines both line driver circuit and open collector transistor circuits. This circuit provides path to the GND in "OFF" state. It provides "Vcc" supply during the "ON" state.

This circuit is commonly used in microcontroller and digital outputs. When output is logic high, the transistor sources current from Vcc and pushes it out into the output terminal. This stage is called "push" stage. In "Pull" stage, transistor pulls the output to the ground.

Advantages:
➨It can provide two outputs i.e. High ('Vcc') and Low ('GND' or '-Vcc' if configured).
➨It is used as push pull amplifier in which upper transistor pushes the signal to high during positive half cycle where as the bottom transistor pulls the signal low during the negative half cycle of the input AC waveform.
➨It offers higher efficiency which is 78.5% for class B push pull.
➨No power is drawn from DC supply under no signal condition.
➨It eliminates even order harmonics in AC output signal.
➨There are no DC components in the output signal.

Disadvantages:
➨Cross over distortion
➨Requires biasing current
➨It requires two identical transistors.
➨Q point stabilization


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