3 Why use a Fully Differential Amplifier?

A fully differential amplifier (FDA) provides an active gain component while providing the same benefits as a balun, potentially reducing the overall BOM count and the board space for the RF signal chain. However, an FDA does require a power supply and adds noise to the passing signal, and may cause the phase imbalance to become more pronounced.

Figure 3-1 shows an FDA configured in three different typologies, a differential-to-differential, a single-ended-to-differential, and a differential-to-single-ended setup, allowing engineers to accommodate multiple RF configuration needs with a single device.

In Figure 3-2, an FDA is configured as a single-ended-to-differential amplifier and can be used to replace a single-ended gain amplifier and a passive balun to interface with the ADC as shown in Figure 2-1.

Inherently, amplifiers and gain blocks are single-ended and will provide poor phase imbalance and higher even-order distortion. For example, analog inputs of ADCs are typically a differential interface, the two inputs are ideally required to be equal in amplitude and 180-degrees out of phase. However, maintaining this balanced signal can be quite challenging. As these two input signals slip away from ideal 180-degrees, this produces a phase shift, which generates a common-mode signal, causing even-order distortion (HD2) to become more pronounced.

Specifically, for wideband applications, the TRF1208 (a single-channel, 10-MHz to 11-GHz, 3-dB bandwidth, single-ended to differential RF amplifier in a small 4-mm² QFN package) provides superior gain and phase imbalance of ±0.3 dB and ±3 degrees, which is competitive with the best passive wideband baluns on the market. In Figure 3-1, the TRF1208 combined with the ADC12DJ5200RF provides a 2^nd order harmonic distortion lower than –60 dBFS up to 8 GHz. With a discrete wideband low-noise amplifier (LNA) and passive balun solution, the amplifier can be the limiting factor for linearity and ultimately cause unwanted higher HD2 performance as seen in Figure 3-3. The TRF1208 provides up to –30 dBFS better in HD2 performance than a competitor LNA and balun solution.

Figure 3-3 Second Order Harmonic Distortion (HD2) Performance of TRF1208 (FDA) and ADC12DJ5200RF (ADC) vs a Wideband LNA and Balun Solution

Matching a wideband ADC to a single-ended source can be a difficult challenge, especially at frequencies above the fundamental Nyquist zone. The input impedance of an ADC is typically much higher than 50 Ω, generally 100 Ω or in the kΩ range. This means that a higher output impedance balun will typically match better to an ADC input than a 1:1 balun. Due to the band-limiting nature of the ADC, it is often necessary to use a balun that is much wider band in a pure 50-Ω system than the required system bandwidth. However, passive baluns can become larger in size, the lower frequency the system goes, causing the single-ended amplifier and balun solution to increase the total system size.

Figure 3-4, shows that a balun and gain block solution that covers a frequency range up to 12 GHz is a significantly larger solution size than an FDA, such as the TRF1208. The TRF1208 is housed in a 4-mm² QFN package with an overall board solution size of 120 mm² versus mechanically built baluns that can reach package sizes greater than 64 mm² and the board space usage can average around 171 mm². With the addition of a gain block to the balun solution, the board space can approach 200 mm² of PCB area.

Figure 3-4 Solution Size Comparison Of Surface-Mount Broadband Balun and TRF1208