The usual current technology for low-noise amplification is High Electron-Mobility Transistors (HEMTs) (also known as HFETs, Heterostructure Field-Effect Transistors). These are widely used at frequencies up to 50 GHz (e.g. Lai et al. (1994)) and have been demonstrated up to 140 GHz (Wang et al. 1995). A typical amplifier might have a noise temperature of 0.5 K/GHz, 20 dB of gain and 30-50% bandwidth. The relatively high operating temperature (10-15 K) compared to bolometers (0.05-0.3 K) means that the cryogenic technology required is much simpler.
An alternative to HEMTs is superconductor-insulator-superconductor (SIS) mixers. These are often used for (non-CMB) heterodyne observations at frequencies 100--500 GHz, and provide gain and down-conversion in a single package. It is now possible to have SIS receivers with at 200 GHz with a bandwidth esentially limited only by the IF amplifier. SIS mixers have been used for CMB astronomy (Meinhold & Lubin (1991)) but bolometers have generally been superior for total-power measurements due to their large bandwidths. SIS mixers may become important when interferometric CMB measurements are pushed to higher frequencies.