Electrometer tetrodes lend themselves to use in a balanced bridge-type circuit, notably that due to Penick, in which the output meter indications are made sensibly independent of supply voltage over a limited range of the latter. Producing a working circuit has, however, been largely a matter of trial and error. By contrast, the new circuit is amenable to a straightforward analytical design procedure, and can provide the basis of a very simple high input-impedance null detector having an excellent performance and a wide range of applications.