The type of amplifier which exhibits crossover distortion in its output is:
Amplifier Distortion: Understanding Crossover Distortion in Amplifiers
An amplifier is an electronic device that increases the power of a signal. Different types of amplifiers, categorized as Class A, Class B, Class AB, and Class C, operate with varying conduction angles, efficiencies, and distortion characteristics. The specific type of distortion known as crossover distortion is a significant issue in the output of certain amplifier classes.
Class-B Amplifier Operation
A Class-B amplifier is designed to improve efficiency compared to Class-A amplifiers. It typically uses a push-pull configuration, where two transistors are used, with each transistor conducting for approximately half of the input signal cycle (180 degrees). One transistor amplifies the positive half of the signal, and the other amplifies the negative half. This operation mode, where each device is active for only half of the input waveform, leads to higher efficiency but also introduces a specific type of distortion.
Crossover Distortion Explained
Crossover distortion is a type of distortion that occurs in the output of push-pull amplifiers, most notably in Class-B amplifiers. Here's why it happens:
- When the input signal crosses the zero-voltage point (moving from positive to negative, or negative to positive), there is a brief period where neither of the push-pull transistors is fully turned on.
- Transistors require a certain minimum voltage (for example, approximately \(\text{0.7 V}\) for silicon BJT transistors) across their base-emitter junction to begin conducting.
- As the input signal approaches zero, the voltage across the base-emitter junction of the "on" transistor drops below this threshold, causing it to turn off. Simultaneously, the input signal is not yet high enough in the opposite polarity to turn on the other transistor.
- This results in a "dead zone" or a period of zero output voltage around the signal's zero-crossing point.
- The output waveform appears "flattened" or "clipped" at the zero-crossing points, leading to a noticeable distortion, especially at low signal levels. This distortion is called crossover distortion because it occurs when the signal crosses from one half-cycle to the other.
Comparison of Amplifier Classes
To better understand why Class-B amplifiers are particularly prone to crossover distortion, let's compare them with other common amplifier classes:
| Amplifier Class | Conduction Angle | Efficiency (Theoretical Max) | Distortion Characteristics |
|---|---|---|---|
| Class A | \(\text{360}^\circ\) (Full cycle) | \(\text{25-50%}\) | Very low distortion, but high power consumption (always on). |
| Class B | \(\text{180}^\circ\) (Half cycle) | \(\approx \text{78.5%}\) | High crossover distortion at zero crossings. |
| Class AB | \(\text{180}^\circ\) to \(\text{360}^\circ\) (Slightly more than half) | \(\approx \text{50-70%}\) | Reduced crossover distortion compared to Class B, good compromise. |
| Class C | Less than \(\text{180}^\circ\) | \(\approx \text{90%}\) | Very high distortion, suitable only for RF tuned circuits where distortion can be filtered out. |
From the comparison, it is clear that while Class-B amplifiers offer significantly better efficiency than Class-A, this comes at the cost of introducing noticeable crossover distortion in their output. Class-AB amplifiers were developed to mitigate this issue by biasing the transistors to conduct slightly more than 180 degrees, effectively reducing the "dead zone."
Important Questions from Power Amplifier
_______ is usually used in RF power amplifier and in amateur radio.
The collector current of a class C amplifier is _______.
A single stage amplifier employing one active device is powered by a 9 V battery which has a current drain of 20 mA. If load voltage is 3 V at 12 mA, then determine η.
- A tuned Class-C amplifier has a power supply voltage of 12 V. What is the ideal peak-to-peak output voltage?
In push-pull amplifier there occurs cancellation of:
