A balun transformer (short for balanced-to-unbalanced) is utilized to connect an unbalanced (single-ended, ground-referenced) signal to a balanced (differential, ±180⁰) pair of signals, or vice versa, to combine a differential pair into a single-ended signal. The baluns shown in Figure 1 are also transformers (hence the term “balun transformer”) which provide isolation in addition to impedance transformation. In this example, we have chosen to illustrate perhaps the simplest of impedance transformations by setting the turns ratio n_OUT/n_IN = N = 1.  Note that by n_OUT we mean all the turns on the center-tapped winding. Therefore, as shown in Figure 1, the impedance of each half of the center-tapped winding is equal to Z_HALF = (N²)(Z_IN)/2 = (N²)(Z_OUT)/2 = (1)(50Ω) = 25Ω.  A turn ratio that differs from unity would also change the primary-to-secondary impedance ratio. 

Push-Pull Amplifier Operation

The balun transformer with a center-tapped secondary in Figure 1 is a fundamental element of the push-pull amplifier configuration, and the dot convention is shown to make the figure comprehensive. (Recall that the dot convention specifies that the polarity is the same at all terminals marked with a dot). Since the amplifiers themselves are generic, they can be assumed to be ideal, perfectly matched in gain and phase, and noninverting. The push-pull amplifier works by taking the unbalanced (single-ended) primary signal, splitting it into a balanced pair of two signals that are 180⁰ out-of-phase, then driving two amplifying devices, one at an effective phase shift of 0⁰ and the other at -180⁰.

The input balun transformer is then mirrored at the output of the amplifier pair such that the balanced or center-tapped winding may be considered the primary, and the single-ended winding the secondary. The amplifier output signals are applied to the center-tapped primary, and since the devices are perfectly out-of-phase when one amplifier is driving current, the opposing amplifier is sinking current. Stated differently, when one amplifier operates at maximum voltage, the other amplifier will operate at minimum voltage, per the dot convention. This pushing and pulling action across the center-tapped secondary (hence the name, “push-pull amplifier”) results in the devices combining to yield the sum of their powers, or twice the power of each amplifier at the single-ended winding of the output balun transformer. Typically, the device with the highest output voltage (appearing to source current) can be viewed as “pushing” and the one with the lowest output voltage (appearing to sink current) as “pulling.” 

In addition to the inherent power combination that occurs in the push-pull amplifier, one of its primary advantages is the cancellation of even-order harmonics. Just as the fundamental input signal is split into two signals 180⁰ out-of-phase, the second harmonic, or 2f₀ is split into two signals that are 360⁰ out-of-phase, which is equivalent to in-phase. Imagine the center-tapped primary of the output balun transformer being excited by two signals of identical phase at a frequency of 2f₀. In an ideal case, there would be no current flow in the winding, no transformer flux, and no second harmonic output.  Essentially, any even-order harmonic signal will result in purely common mode excitation of the balun transformer. 

Receiver LNAs and broadband power amplifiers that span greater than an octave of bandwidth benefit from this harmonic cancellation. That’s why this fundamental and longstanding amplifier configuration is so ubiquitous, having found its way into everything from low-power LNAs to ultra-high-power transmitters.

Balun Transformer Variants in Push-Pull Amplifier Designs

While the example in Figure 1 shows a generic push-pull amplifier configuration, real-world application circuits vary widely. The input and output baluns in particular offer the designer many options depending on the requirements and desired performance parameters for a given system. In the 127 years since Dean first described the push-pull configuration, and the 107 years since Colpitts invented the push-pull amplifier, volumes have been written about how to match pairs of amplifier devices and how to construct the various combining mechanisms. The full breadth of design techniques and topologies goes beyond the scope of this article, but let’s discuss a few examples by way of introduction.

One of the most compelling developments in RF component design of the 20^th century was the invention of the Guanella balun (see Figure 2).⁵ Gustav Guanella, a Swiss Radio Engineer, discovered that a balun transformer could be constructed from coaxial transmission line. Another article on the Mini-Circuits blog discussing RF transformer types explains, “This 1:1 type of balun transformer creates a high choking reactance on the outer conductor of the coaxial cable, effectively reducing common mode signals while allowing the internal currents of the coaxial transmission line to pass unimpeded.”⁷ Guanella filed for his patent on April 5, 1945, and many physical balun transformers designed today make use of this early work.