Measuring amplifier, a device for amplifying electrical signals (current, voltage) and also for converting voltage into current and the other way round. According to the form of their frequency response, a distinction is drawn between: selective measuring amplifiers, intended to amplify harmonic signals of a definite frequency; wideband alternating-current amplifiers; and direct-current amplifiers, which can amplify signals of arbitrary form. Every measuring amplifier contains an element which, controlled by the signal being amplified, regulates the flow of energy from an external source to the output of the measuring amplifier, and this is what produces the amplification. As a rule, measuring amplifiers are built in several stages, the output signal of the first controlled element being used to control the second element and so on.
According to the kind of input controlled element, a distinction is drawn between electronic (mainly semiconductor), magnetic, dielectric, photogalvanometric and other measuring amplifiers. Electronic measuring amplifiers are the most widespread. In vacuum-tube measuring amplifiers the controlled element is the vacuum tube, and in semiconductor ones the semiconductor triode. In magnetic measuring amplifiers the current flowing in the control winding changes the magnetic permeability of a ferromagnetic core and thereby changes the inductive reactance of the second winding, and with it the current flowing through that winding from the power supply. In dielectric measuring amplifiers the control voltage changes the capacitance of a capacitor, which makes it possible to control the current flowing through the capacitor from the power supply. In photogalvanometric measuring amplifiers the control current flowing through the galvanometer coil deflects the moving system, with the small mirror fixed to it, in proportion. The illumination of the photoresistors, and with it their resistance, changes as a result, and this changes the current in the circuit connected to the power supply.
A problem common to all measuring amplifiers is achieving a high stability of the gain (conversion factor). The most radical remedy is strong negative feedback. The gain of modern measuring amplifiers reaches 106 and more, the input resistance 1016 Ω, the intrinsic error as a percentage of the measuring range from 0.01% to a few per cent at large gains, and the frequency range up to several tens of MHz. The use of measuring amplifiers makes it possible to measure signals down to 10-17 A and 10-9 V.
According to the kind of input controlled element, a distinction is drawn between electronic (mainly semiconductor), magnetic, dielectric, photogalvanometric and other measuring amplifiers. Electronic measuring amplifiers are the most widespread. In vacuum-tube measuring amplifiers the controlled element is the vacuum tube, and in semiconductor ones the semiconductor triode. In magnetic measuring amplifiers the current flowing in the control winding changes the magnetic permeability of a ferromagnetic core and thereby changes the inductive reactance of the second winding, and with it the current flowing through that winding from the power supply. In dielectric measuring amplifiers the control voltage changes the capacitance of a capacitor, which makes it possible to control the current flowing through the capacitor from the power supply. In photogalvanometric measuring amplifiers the control current flowing through the galvanometer coil deflects the moving system, with the small mirror fixed to it, in proportion. The illumination of the photoresistors, and with it their resistance, changes as a result, and this changes the current in the circuit connected to the power supply.
A problem common to all measuring amplifiers is achieving a high stability of the gain (conversion factor). The most radical remedy is strong negative feedback. The gain of modern measuring amplifiers reaches 106 and more, the input resistance 1016 Ω, the intrinsic error as a percentage of the measuring range from 0.01% to a few per cent at large gains, and the frequency range up to several tens of MHz. The use of measuring amplifiers makes it possible to measure signals down to 10-17 A and 10-9 V.