Class AB Output Stage: Other Design Techniques

Summary

This transcript explains a variation of the Class A/B amplifier's output stage, focusing on the addition of emitter follower input stages and resistors for improved input resistance and thermal stability. It addresses the prevention of crossover distortion, thermal runaway, and compensates for transistor mismatches. Additionally, it explores using compound devices, such as Darlington pairs, to increase current gain, replacing standard output transistors in Class A/B stages. It highlights the challenges in biasing these compound devices, emphasizing the need for a higher base-emitter voltage compared to traditional transistors.

Takeaways

• 🔌 The class A/B output stage can be modified by adding emitter follower input stages to increase input resistance and reduce loading effects.
• 📡 Emitter follower input stages help to bias the output transistors and prevent crossover distortion even with small input signals.
• 🔍 Resistors R3 and R4 are added for two main reasons: to correct for transistor mismatches and to provide thermal stability.
• 🔥 The configuration is prone to thermal runaway due to a positive feedback loop, which can be mitigated by the stabilizing effect of R3 and R4.
• 🔬 An increase in current through R3 causes a voltage rise across it, which can decrease the collector current and prevent thermal runaway.
• 🏗️ Key advantages of the circuit include increased input resistance, compensation for mismatches, and most importantly, thermal stability.
• 🔄 Another design technique involves using compound devices like Darlington pairs to increase the current gain of the output stage.
• 🔄 Darlington pairs consist of two transistors (NPN and NPN or PNP and NPN) connected in such a way that their beta values multiply, increasing the overall current gain.
• 🔄 For the PMP device in a class A/B output stage, a compound PNP transistor is often used instead of a PNP Darlington pair due to higher beta values.
• 🔌 In a class A/B output stage using compound devices, the biasing voltage (Vcc) needs to be higher to ensure the compound devices are turned on.

Q & A

What is the purpose of adding emitter follower input stages to the output transistors?

The emitter follower input stages are added to increase the input resistance of the output stage, helping with loading effects from prior stages and assisting in biasing the output transistors.

How does the circuit avoid crossover distortion?

The circuit avoids crossover distortion by ensuring the transistors are always on, even when the input signal is very small. This is achieved through the VBE rise and fall across the input and output transistors.

What is the role of resistors R3 and R4 in the circuit?

Resistors R3 and R4 provide thermal stability and help compensate for transistor mismatches. They prevent thermal runaway by limiting the positive feedback loop that could lead to increased junction temperature and power dissipation.

How do resistors R3 and R4 help prevent thermal runaway?

R3 and R4 increase the voltage drop across them as the current increases, which reduces the VBE of the output transistors, thus decreasing the collector current and stabilizing the circuit to avoid thermal runaway.

What is the advantage of using a Darlington pair in the output stage?

A Darlington pair increases the current gain of the circuit. The beta of the Darlington pair is approximately the product of the individual betas of the two transistors, effectively boosting the overall current gain.

Why is a compound PNP transistor used instead of a PNP Darlington pair?

A compound PNP transistor is preferred because the beta of a PNP transistor is typically much lower than an NPN transistor. Using a compound PNP configuration, which includes an NPN transistor, helps achieve higher current gain through beta multiplication.

What is the main difference in biasing a Darlington pair compared to a regular transistor?

Biasing a Darlington pair requires a higher voltage, typically around 2 volts, because two VBE junctions need to be turned on. In contrast, a regular transistor requires only around 0.7 volts for each output device.

Why does increasing the input resistance of the output stage help with loading effects?

Increasing the input resistance minimizes the loading on previous stages, allowing the signal to pass through with less distortion and avoiding unwanted signal loss.

What happens to the circuit when the output transistor sources maximum current?

When the output transistor sources maximum current, the power dissipation increases, which can raise the junction temperature. Without thermal stability mechanisms like R3 and R4, this could lead to thermal runaway.

How does the VBE change across the transistors affect the input signal?

The VBE rise and fall across the input and output transistors ensure that the input signal is transferred to the output without distortion, maintaining a smooth transition and keeping the transistors on even at small signal levels.