Transistor Amplifiers - Class A, AB, B, & C Circuits

Summary

This video provides a clear and engaging overview of the main classes of transistor amplifiers—Class A, B, AB, and C—explaining their circuit designs, operating principles, efficiencies, and typical applications. It highlights how Class A offers low distortion but poor efficiency, while Class B improves efficiency at the cost of crossover distortion. Class AB balances both by reducing distortion, and Class C achieves extremely high efficiency with significant signal distortion, making it suitable for tuned RF circuits. The video also explores practical enhancements like transformer coupling and explains key concepts such as conduction angles, biasing, and resonant frequency.

Takeaways

• 🔹 Class A amplifiers use a single NPN transistor and conduct for the entire 360° of the input cycle, providing very low distortion but low efficiency (maximum 25%).
• 🔹 Coupling capacitors in amplifiers (C1 and C2) block DC while passing AC signals, allowing proper signal transfer without affecting biasing.
• 🔹 Adding a bypass capacitor across the emitter resistor in a Class A amplifier increases AC voltage gain by allowing the signal to bypass the resistor.
• 🔹 Transformer-coupled Class A amplifiers replace Rc with a transformer, storing and releasing energy via magnetic fields to improve maximum theoretical efficiency to 50%.
• 🔹 Class B amplifiers use two complementary transistors (NPN and PNP), each conducting for 180° of the input cycle, achieving higher efficiency (78.5%) but introducing crossover distortion.
• 🔹 Crossover distortion in Class B amplifiers occurs because both transistors remain off until the input exceeds ±0.7 V, creating a small gap in output signal at zero crossing.
• 🔹 Class AB amplifiers reduce crossover distortion by using diode biasing, where two silicon diodes replace a resistor to provide the correct voltage drop for smoother transistor conduction.
• 🔹 Class C amplifiers are tuned amplifiers with LC circuits, conducting for less than 180° of the input cycle, achieving very high efficiency (up to 99%) but with significant distortion.
• 🔹 The resonant frequency of a Class C tuned amplifier is determined by the formula f_r = 1 / (2π√(LC)), controlling oscillation and energy transfer in the circuit.
• 🔹 Voltage gain in amplifiers is the ratio of output voltage to input voltage, and overall amplifier efficiency is the ratio of AC load power to DC power supplied, expressed as a percentage.
• 🔹 Class A amplifiers are ideal for small-signal amplification due to low distortion, whereas Class B and Class AB are more suitable for higher efficiency power amplification, and Class C is used where high efficiency at high frequencies is critical despite distortion.

Q & A

What is the primary function of a transistor amplifier?

The primary function of a transistor amplifier is to increase the power level of an AC input signal by transferring power from the DC power supply to the input signal.

What distinguishes a Class A amplifier from other amplifier types?

A Class A amplifier conducts for the entire 360° of the input cycle, has very low distortion, but has low efficiency, typically around 25% for resistive loads and up to 50% for transformer-coupled designs.

How does a transformer improve the efficiency of a Class A amplifier?

The transformer stores energy in its magnetic field when the collector current increases and releases it back to the circuit when the current decreases. This can make the output voltage higher than the supply voltage at times, effectively increasing the amplifier's efficiency up to a theoretical maximum of 50%.

What is the main disadvantage of a Class B amplifier?

The main disadvantage of a Class B amplifier is crossover distortion, which occurs because each transistor requires approximately 0.7 volts to turn on, leaving a small gap at the zero crossing of the input signal where no output is produced.

How does a Class AB amplifier reduce crossover distortion?

A Class AB amplifier reduces crossover distortion by biasing the two transistors slightly on using two silicon diodes. The combined voltage drop of the diodes matches the base-emitter voltage drops of the transistors, ensuring a smoother transition between the transistors during zero crossings.

What is the conduction angle of a Class C amplifier and why is it significant?

A Class C amplifier conducts for less than 180° of the input cycle, which makes it highly efficient (up to 99%) but introduces significant distortion. It is typically used in RF applications where a tuned circuit can recover the desired waveform.

Why are coupling capacitors used in transistor amplifier circuits?

Coupling capacitors are used to block DC voltage while allowing the AC input signal to pass through, preventing DC biasing voltages from affecting the next stage of the circuit.

What is the relationship between collector current (IC), base current (IB), and beta (β) in a transistor?

The collector current IC is equal to the base current IB multiplied by beta (β), where β is the current gain of the transistor: IC = β × IB.

How is the resonant frequency of a Class C tuned amplifier determined?

The resonant frequency of a Class C tuned amplifier is determined by the LC circuit using the formula: f = 1 / (2π √(L × C)), where L is the inductance and C is the capacitance of the tuned circuit.

What is the theoretical maximum efficiency of Class A, Class B, Class AB, and Class C amplifiers?

The theoretical maximum efficiencies are approximately: Class A – 25% (resistive load) or 50% (transformer-coupled), Class B – 78.5%, Class AB – between Class A and B depending on biasing, and Class C – up to 99%.

Why is a Class A amplifier typically used for small-signal applications rather than power amplification?

Because Class A amplifiers have low efficiency and dissipate a lot of power as heat, they are better suited for small-signal amplification where low distortion is more important than efficiency.

What causes crossover distortion in Class B amplifiers, and how does Class AB solve it?

Crossover distortion occurs because both transistors in a Class B amplifier are off when the input signal is between -0.7V and +0.7V. Class AB solves this by slightly biasing both transistors on so that they begin conducting before the signal reaches zero, smoothing the transition.