Photoelectric Galvanometer Amplifier, Balanced

a DC amplifier used to measure very small currents or voltages. Such amplifiers consist of a magnetoelectric or electrostatic mirror galvanometer, which converts the current or voltage being measured into the deflection of a light beam, and a photoelectric transducer, which converts small deviations (usually fractions of a degree) of the galvanometer mirror into a current or voltage.

Figure 1. Schematic of a balanced photoelectric galvanometer amplifier for measuring voltage: (U_x) voltage being measured, (MG) mirror galvanometer, (L) light source, (O) objective lens, (P₁) and (P₂) photoresistors, (E_aux) auxiliary voltage source, (l_g) current in the galvanometer circuit, (l_b) current in the load circuit, (R₁) and (R₂) resistors, (R_b) balance resistor, (R₁) load, (U_b) balance voltage, (M) milliammeter

Figure 1 shows a balanced photoelectric galvanometer amplifier for voltage measurements. The voltage U_x generates a current I_g in the galvanometer circuit, and the galvanometer mirror is deflected. The luminous flux is deflected by the mirror onto the photoresistors of the transducer connected in a bridge circuit and is redistributed in such a way that the current in one of the photoresistors increases and the current in the other decreases. As a result, a current difference I_b arises in the load circuit and increases until U_i is compensated (balanced) by a drop in voltage in the balance resistor U_b = I_b · R_b. The value U_x is determined from I_b · I_g and I_b vary correspondingly with a change in U_x (by ∆_g and ∆I_b, respectively). The ratio k = ∆I_b/∆I_g is called the coefficient of amplification and ranges from 10³ to 10⁸, depending on the design of the instrument.

Instruments of this type may be used to measure voltages from 10^–6 to 1 volt and currents from 10^–9 to 5 × 10^–2 ampere.