Amplifier high impedance

Thermal offset voltage Thermal offset current Low-level amplifier High-impedance amplifier Charge amplifier Integrating amplifier Wideband amplifier Video amplifier Precision amplifier... 

Because of the very large resistance of the glass membrane in a conventional pH electrode, an input amplifier of high impedance (usually 10 —10 Q) is required to avoid errors in the pH (or mV) readings. Most pH meters have field-effect transistor amplifiers that typically exhibit bias currents of only a pico-ampere (10 ampere), which, for an electrode resistance of 100 MQ, results in an emf error of only 0.1 mV (0.002 pH unit). 

The sensitivity of instruments using low resistance circuits is determined primarily by the sensitivity of the galvanometer (Figure 4.5). Electrode systems that have a high resistance, e.g. glass electrodes, require a high impedance voltmeter, which converts the potential generated into current which can be amplified and measured. Such instruments are commonly known as pH meters but may be used for many potentiometric measurements other than pH. 

Because of the very large resistance of ihe glass membrane in a conventional pH electrode, an input amplifier of high impedance (usually If)1-- tnIJ Dl is required lo avoid errors in the pH (or mV) readings. 

If a resistance is placed in the feedback loop (Fig. 6.6b), the bias current ib will also create a difference between Ej and E0 by an amount ibRr. Even very inexpensive (< 1) OAs can have bias currents of less than 10 9 A, which means that the value of Rr will have to exceed 1 MO to create a 1-mV error. Amplifiers with bias currents of less than 0.1 pA (10 13 A) are available. Using the same criterion, Rr may then reach 1000 MQ, a value well beyond any resistance commonly encountered in dynamic electroanalytical techniques. Such amplifiers are, however, eminently useful for constructing pH meters and pH stats and measuring potentials in electrophysiology, where very small high-impedance electrodes are often used. 

It is now necessary to describe the determination of conductivity in ionic solids. The circuit shown in Figure 8.9 is only a schematic representation. This type of measurement in high-impedance samples of ionic solids causes experimental problems that are avoided with the help of the so-called guard ring circuit, which prevents leakage currents affecting the measurement [16]. Besides, these circuits use an amplifier to deal with the high resistance of the sample. 

The photons and metastable helium atoms enter the reaction zone where they meet the eluent from the capillary column. The solute molecules are ionized and the electrons produced are collected at the lower electrode and measured by an appropriate high impedance amplifier. The distance between the collecting electrodes is about 1.5 mm. The helium must be 99.9995 pure. The base current ranges from... 

The principle of detection is as follows. Hydrogen is mixed with the column eluent and burned at a small jet. Surrounding the flame is a cylindrical electrode and a relatively high voltage is applied between the jet and the electrode to collect the ions that are formed in the flame. The resulting current is amplified by a high-impedance amplifier and the output fed to a data acquisition system or a potentiometric recorder. 

Since the phototube displays an inherent high impedance, it is readily adaptable for coupling with the input of a vacuum tube amplifier as shown in figure 40. The 6F5 tube in the illustration is biased to cut-off (wili not conduct) until light falling upon the photocathode of the 919 rube begins to alter this... 

The basic measurement technique for intensity-modulated photovoltage spectroscopy (IMVS) is the same as for IMPS. In principle, IMVS measurements can be made for any constant current condition, but in practice it is usual to make measurements under conditions where the net current is zero. In the case of a photoelectrochemical solar cell, this corresponds to the open-circuit condition, and a high impedance voltage amplifier is used to ensure that a negligible current is drawn from the illuminated device. The output of the voltage amplifier is fed to the FRA, and the remainder of the set up is the same as for IMPS (cf. Fig. 12.26). 

Every electronic component has a characteristic impedance. When a signal is transmitted from a unit with a high-impedance output to a low-impedance input, there is going to be a loss in the signal unless an impedance-matching device is used to couple the two units. Manufacturers of preamplifiers and amplifiers quote the impedance of the input and output for their products. Coaxial cables have an impedance between 90 and 100 ft. 

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