![]() Just as the fundamental input signal is split into two signals 180⁰ out-of-phase, the second harmonic, or 2f 0 is split into two signals that are 360⁰ out-of-phase, which is equivalent to in-phase. 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. 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.” 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. Stated differently, when one amplifier operates at maximum voltage, the other amplifier will operate at minimum voltage, per the dot convention. 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. 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 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. Since the amplifiers themselves are generic, they can be assumed to be ideal, perfectly matched in gain and phase, and noninverting. (Recall that the dot convention specifies that the polarity is the same at all terminals marked with a dot). 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. A turn ratio that differs from unity would also change the primary-to-secondary impedance ratio. Therefore, as shown in Figure 1, the impedance of each half of the center-tapped winding is equal to Z HALF = (N 2)(Z IN)/2 = (N 2)(Z OUT)/2 = (1)(50Ω) = 25Ω. Note that by n OUT we mean all the turns on the center-tapped winding. 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. The baluns shown in Figure 1 are also transformers (hence the term “balun transformer”) which provide isolation in addition to impedance transformation. An example showing a pair of generic devices combined with simple transformer baluns is shown in Figure 1.įigure 1: Generic block diagram of a push-pull amplifier using a 1:1 balun transformer.Ī balun transformer (short for balanced-to-unbalanced) is utilized to connect an unbalanced (single-ended, ground-referenced) signal to a balanced (differential, ☑80⁰) pair of signals, or vice versa, to combine a differential pair into a single-ended signal. The focus of this application note is on the push-pull amplifier itself. More than a century after the invention of the push-pull amplifier, a myriad of different configurations has been developed, some of which require prerequisite knowledge of the operation of complex transformers. Finally, applications of the push-pull amplifier configuration are discussed. Several practical construction methodologies for the main components are reviewed, and an example of a high-power, broadband application is presented. This article describes a simple, modern-day, push-pull amplifier configuration along with its performance advantages. 3,4 Over a hundred years ago, Colpitts recognized that “a certain amount of distortion in the output waves is avoided” 3 by utilizing this configuration.įast-forward over 100 years and transpose kHz with GHz we find the push-pull amplifier configuration everywhere. 1,128,292, for an “electric wave amplifier” on February 16, 1915, which covered the push-pull circuit by connecting two vacuum tubes like De Forest’s audions like a primitive precursor to the transistor. 1 Next, Sir John Ambrose Fleming invented the first vacuum tube in 1904 2 and while Lee De Forest added the grid to Fleming’s “valve” in 1906, calling it the “audion,” it was Fritz Lowenstein in his Appatent application who first discovered that applying a negative bias to the grid of De Forest’s tube turned it into an audio amplifier.Įdwin Henry Colpitts of the Western Electric Company was awarded Patent No. Dean of the Bell Telephone Company of Missouri first described the push-pull-connected telephone transmitter in Patent No. Remarkably, the concept of the push-pull connection spans three centuries.
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