Abstract: Two kinds of balance circuits are studied. They can explain various null circuits from a uniform view， such as potentiometer，Wheatstone bridge and operational amplifier. The usage on operational amplifier can overcome the limitation of classic amplification of balance circuit. Especially，it makes the balance circuit reaching equilibrium automotccally and immediately，and can treat the signal form DC to AC，including LF and HF.

Key words: balance circuit, potentiometer, Wheatstone bridge, operational amplifier, duality circuit