Push Pull Amplifier

Introduction

A push-pull amplifier consists of an input 0-180-degree power splitter driving two identical devices in antiphase and a 0-180-degree output power combiner adding the output power of the two devices in the amplifier load. This type of splitter and combiner, which are the key elements of the amplifier, are called baluns (BALanced UNbalanced). They transform a balanced system that is symmetrical with respect to ground to an unbalanced system with one side grounded. Note that the microwave push-pull amplifier is two independent devices each amplifying an individual signal of half the total power.

Typical Block Diagram:

Advantages:

1. Four times higher device impedance  (Zin Gate-to-Gate & Zout Drain-to-Drain) in comparison of a single-ended device impedances with the same output power. Thus it is easier to match.
2. Virtual ground, which can be used for more compact and simpler matching structures.
3. Cancellation of even products and harmonics, such as f2 - f1, 2f1, 2f2, f1 + f2, etc.
   

Disadvantages:

1. Poor input and output external match due to the fact that the baluns used for push-pull amplifiers do not eliminate the input and output power reflected by the device.
2. With conventional baluns, isolation between the two sides of the part is theoretically only 6dB; this poor interdevice isolation can cause instability problems.
3. Use of baluns: manually made coaxial baluns are simple to make for lab use but in production they require labor that makes mass production difficult. SMT baluns are available but add cost and tend to occupy more real estate than equivalent quadrature couplers.

 Comparison versus Balanced Configuration

1. Single tone performance should be equivalent. The external elements matching circuits and the splitters and combiners have similar loss. Note that both styles have different advantages in multi-octave amplifiers that do not come into play in this discussion of narrow band (5-10 %) commercial amplifiers. Due to the fact that even products and harmonics are out of the pass band of the matching circuits (internal and external) and the baluns, the cancellation of these products doesn’t exist and consequently for narrow band applications the push-pull configuration is not more efficient.
2. Linearity is the same for amplifiers with less than an octave in bandwidth. Matching circuits (internal and external) are filtering the even products and harmonics for the push-pull configuration and the products and harmonics attenuated or cancelled by the balanced configuration before they reach the output combiner. The output balun and quadrature coupler have limited band and in general cannot cancel these products and harmonics that are out of their pass band.
3. Push-pull configuration has an impedance transformation ratio advantage of two for conventional baluns. This can make design easier, depending on the impedance to be matched.
4. Balanced amplifiers have a significant external match advantage.
5. Balanced amplifiers are more stable due to the good isolation between the two device sides.
6. The virtual ground present for the push-pull configuration can be used to advantage with lumped tuning capacitors between the two sides to make for fast tuning.
7. Both configurations can be tuned using open stubs, which are preferred in production to lumped capacitors for their lower cost (free and no assembly), lower loss, ease to model and their power handling capability.