Push-Pull Amplifier-Increasing The Power Of An Input Signal

Amplifiers have traditionally been vital in increasing a system’s sensitivity or output voltage. However, the ever-increasing communication distances fuelled the need for more robust, more minor, and efficient amplifiers. A prime candidate for such a role is the push-pull amplifier. 

Push-pull amplifiers are better than the previously popular single-stage amplifiers. This is possible since they experience less crossover distortion, have higher efficiencies, and dissipate less heat. 

This article will look at the various classes of push-pull amplifiers, their properties, and their applications.

What Is A Push-Pull Amplifier?

A push-pull amplifier employs two complementary anti-phased transistors to drive current through a load in either direction. 

Here, one transistor supplies positive power to the output load while the other drives the negative current to the ground. 

The output stage consists of two identical amplifiers that push and pull input current to and from the load. 

Many people prefer a push-pull over single-ended amplifiers for excellent performance and less distortion. 

Types Of Push-Pull Amplifiers

Fig 1: A Vintage Amplifier Volume Knob

Class A Push-Pull Amplifier

Class A push-pull amplifiers are the most popular power amplifiers due to their low distortion levels and high linearity. They have one switching transistor that’s always around the center of its load line. 

The continuous biasing makes the amplifier always be in an ON state. Thus, it experiences saturation, has quiescent current, and drives current over the 360-degree input phase. 

Note, a Class A amplifier utilizes a pair of transistors, an input transformer, an output transformer, and a biasing resistor circuit. These amplification characteristics make them critical in audio designs that demand high-fidelity amplification. 

Advantages of Class A Push-Pull Amplifier

• It has a low ripple voltage.
• The amplifier has excellent canceling of non-linear distortions
• It doesn’t have crossover distortions
• The amplifier has low harmonic distortion in power amplifications.

Disadvantages

• Bulky and costly
• It has high heat dissipation as it’s always ON.

Class B Amplifier

Fig 2: Sound Mixer With A Microphone

Class B amplifier’s circuit diagrams are very similar to class A amplifiers. However, it lacks the biasing resistors present in Class A amplifiers. The positive and negative halves of the input signal are directed to different parts of the push-pull amplifier circuit. 

The output to this amplifier is continuously switched ON and OFF. Therefore, each identical transistor amplifies just a half of the input. Resultantly, you obtain a “push-pull” transistor behavior whereby one is OFF when the other is in an ON state. 

Class B amplifiers have higher efficiency and lack the quiescent current present in class A amplifiers. This is because they don’t have a DC bias. 

They also have a better power gain than class A or AB amplifiers.

Advantages

• They have high power efficiency.
• The amplifiers have sound power amplification
• Their simpler circuitry is thus cheaper than class A amplifiers.

Disadvantages

• The amplifier has higher crossover distortion than class A amplifiers
• It requires a well-regulated power supply

Class AB Amplifier

A class AB amplifier electronic circuit is very similar in construction and operation to class A amplifiers. 

The only variation between the two is the rating of the biasing resistors R1 and R2. Class AB amplifiers are biased at the base terminal. In this case, the cut-in or forward bias voltage is approximately 0.7 volts. 

Therefore, the OFF time for both transistors is significantly less, reducing the crossover distortion. However, the crossover distortion in the output waveform is still greater than class A but less than class B. 

Finally, class-AB amplifiers have less efficiency and experience losses in their power output even when there isn’t any input.

Advantages

• The amplifier has good linear characteristics.
• It has a simple design
• It has a very high sound quality

Disadvantages

• It’s less efficient than class AB amplifiers
• It generates lots of heat

Push-pull Amplifier Applications

Fig 3: An Audio Control Panel

Below are the push-pull amp applications.

• They’re critical in low-cost applications as they’re cheaper
• They’re common in digital switching designs
• The amplifiers are best in radio-frequency (RF) systems
• They’re critical in applications that demand less power dissipation
• They’re common in long-distance communications that need minimal distortion

How To Create A Push-Pull Amplifier

Circuit Diagram

Materials Needed

• A 6-0-6 Transformer 
• A one BC557 Transistor
• A one 2N2222 Transistor
• A 1k Resistor
• A load

How To Connect The Amplifier Circuitry

The actual push-pull amplifier circuit is the class B amplifier’s circuit. Therefore, we need a 6 Volt RMS, center-tapped transformer as our input source to connect the above materials. 

You can then join the transformer with the bases of transistors Q1 and Q2 through a one kilo-ohm resistor. Connect the collector terminal of the transistor Q1 to a 12v DC source while grounding the transistor Q2 collector terminal. 

Proceed to connect the two transistors’ emitter terminals. You can then tap a single connection to the center of a center-tapped transformer at the output. Alternatively, you can use an LED as your load. 

Which Push-Pull Amplifier Has The Highest Efficiency?

Fig 5: Old Analogue Audio-Frequency Power Amplifiers

At a theoretical value of 78.5%, the Class B amplifier has the highest efficiency. Therefore, a maximum of 78.5% of the input signal turns to sound while the rest dissipates as heat.  

Conclusion

The Q-point of an amplifier determines its classification. Therefore, for class A amplifiers, your circuitry’s Q-point position should be at the midpoint of the load line. 

Lowering the load line shifts the amplifier down, and so we get classes B and AB down the line. Having that information is crucial in your study on amplifiers.

With the information you’ve on push-pull amplifiers from this article, you’re in a better position to do your project. 

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