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Wideband communications systems have signals occupying multi-octave frequency ranges. For example, CATV signals occupy the 50 MHz to 1000 MHz range, which is more than four octaves. Such signals, when amplified in conventional amplifiers, can be distorted due to the second-order products generated inside the amplifier. For example, the second harmonic of a 50 MHz signal is 100 MHz, and the second harmonic of 400 MHz is 800 MHz. Both are within the band.

An ideal push-pull amplifier can cancel the internally generated products and preserve the signal quality. Figure 1 shows the schematic of such an amplifier. It consists of two baluns and two identical amplifiers. When a signal of amplitude V1 is applied to the input of the first balun (Balun #1), the output signal from the same balun consists of two out-of-phase signals as follows:

and

Factor k1 represents the loss of the signals due to division and insertion loss. These two equal-amplitude, out-of-phase signals are applied to the input of two identical amplifiers. The amplified signals at the output of the two amplifiers are equal in amplitude but are out of phase as follows:

and

A is the amplification factor of the amplifiers. These signals are applied to the two ends of the output balun (balun #2). The combined signal appears at the output of the balun 2 as:

k2 represents the loss of signal in balun 2, and square root 2 is due to the voltage division.

If baluns are ideal, k1 and k2 are as follows:

and

Hence, the amplitude of the output signal is

Consequently, the gain of a push-pull amplifier is the same as that of an individual amplifier, where the output power is twice that of an individual amplifier. Thus, push-pull amplifiers are frequently used for combining power of individual amplifiers. However, there is more to these amplifiers than combining power. This topology of amplifiers helps cancel even harmonics and intermodulations and improves the even-order intermodulation product.

Mini-Circuits HELA-10 is used in this example, and it consists of a pair of amplifiers. Since they are on the same chip, their gain and phase are well matched. If a balanced signal is applied to the input of the HELA-10, then the output is also balanced. By using a set of baluns (or transformers) at the input and output, shown functionally in Figure 2, a single-ended input is first converted into a balanced signal in T1, amplified in HELA-10, and combined in the transformer T2 to produce a single-ended output.

The measured IP2 of the amplifier is 87 dBm to 96 dBm at the normal current and 90 dBm to 98 dBm for the higher current. Such IP2 can meet the demanding requirements of today's communications systems.

Push-pull amplifiers can be used for combining power and to improve IP2 at the same time. For frequencies below 1 GHz, lumped-element transformers provide excellent balance and can be used as baluns.

Radhakrishna Setty is director of engineering at Mini-Circuits, Brooklyn, N.Y.

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