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57-65 GHz differential and transformer-coupled power and variable-gain amplifiers using a commercial 90 nm digital CMOS process are presented. On-chip transformers combine bias, stability and input/interstage matching networks to enable compact designs. Balanced transmission lines with artificial dielectric strips provide substrate shielding and increase the effective dielectric constant up to 54 for further size reduction. Consequently, the designed three-stage power amplifier occupies only an area of only 0.15 mm2. Under a 1.2 V supply, it consumes 70 mA and obtains small-signal gains exceeding 15 dB, saturated output power over 12 dBm and associated peak power-added efficiency (PAE) over 14% across the band. The variable-gain amplifier, based on the same principle, achieved a peak gain of 25 dB with 8 dB of gain variation.