For example: Agilent E4440A or Anritsu 68369
Stanford Research Systems Amplifiers
RF Amplifiers
Stanford Research Systems FS710
FS710 10 MHz, AGC Distribution Amplifier
Manufacturer: Stanford Research Systems
Stanford Research Systems SR530
SR530 Dual Phase Lock-In Amplifier
Manufacturer: Stanford Research Systems
Stanford Research Systems SR550
SR550 Low Noise Lock-In Preamplifier
Manufacturer: Stanford Research Systems
Stanford Research Systems SR552
SR552 Low Noise Bipolar Input Preamplifier
Manufacturer: Stanford Research Systems