Class AB Stage: Biasing Using Diodes

Summary

This script explains the biasing configurations for a Class AB output stage in electronics. It covers the use of diodes to provide the necessary voltage for transistors, preventing issues like thermal runaway. The script highlights how diodes D1 and D2, when placed near output transistors, offer thermal stability through negative feedback. While this setup has advantages like maintaining stable collector current, it also faces limitations, such as needing a minimum bias current and slight instability in the bias voltage when sourcing current. Other configurations offer more stable biasing.

Takeaways

• 🔧 The Class A-B output stage requires a biasing voltage (VBB) to avoid the dead zone, keeping the base-emitter junctions turned on even when the signal is zero.
• 🔋 A common biasing method is the use of two diodes (D1 and D2) in series to provide the VBB voltage needed for the output transistors (QN and QP).
• 💡 Diodes D1 and D2 are typically smaller than the output transistors since they only need to handle base current, unlike the larger output transistors that must supply significant load current.
• 🔄 There is a ratio (n) between the sizes of the output transistors and the diode-connected transistors, which influences the current supplied to the load.
• 🔥 Thermal runaway is a concern in Class A-B amplifiers due to positive feedback, where increased current leads to higher power dissipation and rising temperatures, potentially damaging transistors.
• 🧊 Diodes D1 and D2 provide thermal stability by having a negative temperature coefficient, reducing the voltage across them as the temperature increases, thereby preventing thermal runaway.
• 🌀 The negative feedback loop formed by the diodes helps regulate the current through the output transistors, keeping it relatively constant despite temperature changes.
• 🏗️ In integrated circuits, placing D1 and D2 in close proximity to the output transistors improves thermal stability, while in discrete designs, the diodes can be mounted on the transistor's metal casing.
• ⚖️ One limitation of this configuration is the minimum required bias current (I bias), which must be greater than the maximum load current divided by the beta of transistor QN.
• 📉 Another limitation is that when QN is sourcing current, the current through the diodes decreases, which can slightly reduce the bias voltage and affect stability.

Q & A

What is the purpose of providing a voltage VBB to the bases of the output transistors in a Class AB output stage?

The voltage VBB is provided to the bases of the output transistors to bias the base-emitter junctions so they remain turned on even when the signal is zero, preventing distortion due to the 'dead region' where both transistors would be off.

How are diodes used in the biasing configuration for a Class AB output stage?

Two diodes (D1 and D2) are used in series to provide the necessary bias voltage to the base-emitter junctions of the output transistors. These diodes help stabilize the circuit thermally and ensure continuous operation of the transistors.

What is the function of the current source I_bias in this circuit?

The current source I_bias is responsible for biasing the diodes D1 and D2, ensuring they remain turned on. This biasing keeps the output transistors ready to conduct current as needed.

Why are the output transistors (Q_N and Q_P) typically larger than the biasing diodes?

The output transistors (Q_N and Q_P) are larger because they need to handle large currents to the load, while the diodes only need to carry small currents proportional to the base currents of the output transistors. This difference allows the diodes to be made smaller, saving space and power.

What is thermal runaway and how does this circuit prevent it?

Thermal runaway occurs when increasing current through the transistors causes an increase in power dissipation, leading to further temperature rises, creating a positive feedback loop. This circuit prevents thermal runaway through the use of diodes (D1 and D2) with negative temperature coefficients, which reduce the voltage drop as temperature increases, providing negative feedback and stabilizing the current.

How do the negative temperature coefficients of D1 and D2 contribute to thermal stability?

The negative temperature coefficients of diodes D1 and D2 mean that their voltage drops decrease as temperature rises. This reduces the VBB voltage, which in turn decreases the base-emitter voltages (VBE) of the output transistors, reducing the collector current and counteracting further temperature increases.

What are the limitations of using diodes for biasing in this circuit?

One limitation is that the current through the diodes decreases when the output transistor (Q_N) is sourcing current to the load, causing a slight reduction in the bias voltage (VBB). Another limitation is that the bias current (I_bias) cannot be arbitrarily small, as it must be large enough to supply the base current of the output transistors, limiting how small D1 and D2 can be made.

Why do diodes D1 and D2 need to be placed in close thermal contact with the output transistors?

Placing diodes D1 and D2 in close thermal contact with the output transistors ensures that they experience the same temperature changes as the transistors, allowing them to provide effective thermal feedback to prevent thermal runaway.

How is the minimum bias current for I_bias determined in this circuit?

The minimum bias current for I_bias is determined by the maximum load current divided by the current gain (beta) of the output transistors (Q_N and Q_P). I_bias must be greater than this value to ensure sufficient base current for the output transistors.

What alternatives are there to this diode biasing configuration for a more stable bias voltage?

Other configurations can be used that provide a more stable bias voltage, such as using more complex circuits with additional feedback mechanisms or different types of components that better regulate the bias voltage under varying load conditions.