Line Driver vs. Open Collector vs. Push-Pull Transistor Configurations

This article compares line driver, open collector, and push-pull transistor configurations, highlighting the differences between them and their respective advantages and disadvantages.

Line Driver Circuit

Figure 1: Line driver sourcing output circuit

The line driver circuit, as depicted in Figure 1, acts as a sourcing output. When “ON,” it provides Vcc at the output. When “OFF,” the output floats, requiring a sinking input for proper operation.

Advantages:

• Can directly drive LEDs.
• Simple circuit to implement.

Disadvantages:

• Provides only one output state (HIGH/Vcc).
• Output floats in the “OFF” state.

Open Collector Transistor

Figure 2: Open collector transistor sinking output configuration

Figure 2 illustrates an open collector transistor configuration. In the “ON” state, it provides a path to ground (GND). When “OFF,” the output floats.

Advantages:

• Outputs can be tied directly together, creating a logical AND. This allows for the creation of AND gate circuits by connecting outputs.
• Standard TTL gates with totem pole outputs can provide 0.4mA for “HIGH” and 1.6mA for “LOW” current. Open collector gates offer higher current ratings.
• Offers various voltage levels for “HIGH” output, useful for interfacing logic families with different voltage/current requirements.

Disadvantages:

• Provides only one output state: Ground (zero voltage). Cannot provide a high (Vcc) output when required.
• Uses a pull-up resistor (in kOhm range), resulting in slower switching speeds due to a relatively long time constant.

Push Pull Transistor Configuration

Figure 3: Push pull transistor configuration

The push-pull transistor configuration (Figure 3) combines elements of both line driver and open collector circuits. It provides a path to ground in the “OFF” state and Vcc during the “ON” state. This configuration is commonly found in microcontrollers and digital outputs.

When the output is logic high, the transistor sources current from Vcc, pushing it to the output terminal (the “push” stage). In the “pull” stage, the transistor pulls the output to ground.

Advantages:

• Provides two outputs: High (Vcc) and Low (GND or -Vcc, depending on configuration).
• Used as a push-pull amplifier, where the upper transistor pushes the signal high during the positive half-cycle, and the bottom transistor pulls the signal low during the negative half-cycle of the input AC waveform.
• Offers higher efficiency (78.5% for class B push-pull).
• No power is drawn from the DC supply when there is no signal.
• Eliminates even-order harmonics in the AC output signal.
• No DC components in the output signal.

Disadvantages:

• Crossover distortion can occur.
• Requires biasing current.
• Requires two identical transistors.
• Q-point stabilization is needed.