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Ultra Low Bias Current Varactor Bridge Operational Amplifiers
Models 310, 311
FEATURES -14 Guaranteed Low Bias: 10 A max Low Voltage Drift: 10V/C max (310K, 311K) Versatility: Noninverting, model 31IJ/K Inverting, model 310J/K 14 High Input Impedance: 10 (311J, 311K) APPLICATIONS Femto-ammeter Electrometer Long-Time Integrator Small-Rate Differentiator Flame Current Detector Phototube Amplifier Log Compressor pH Meter
GENERAL DESCRIPTION The models 310 and 311 are operational amplifiers with extremely low input bias currents and high input impedances. As such, they are applicable to a large number of electrometer amplifier applications which have traditionally been fulfilled (not without difficulty) by vacuum electrometer tubes. These specialized requirements are characterized by extremely high source impedances or where infinitesimal currents must be measured or amplified. Because of varactor bridge inputs, the all solid state 310 and 311 amplifiers are ideally suited to this type of requirement. Voltage drifts are small, noise is minimized, and the cost is low. In principles the operation of varactor bridge amplifiers is similar to that of vibrating reed electrometers (parametric), but also includes the inherent advantages of solid state design. VOLTAGE SOURCE OR CURRENT SOURCE? Model 310 - The model 310 is designed such that the high quality signal input is the inverting input terminal, and is most appropriate for measurements of current signals. This type of signal source is common to gas chromatograph flame detectors, photomultiplier tubes, radiation detectors, etc. The inverting model 310 is also useful for logarithmic compression over an extremely wide dynamic range and in the construction of very-long-time-constant integrators or differentiators. Input signals from picoamps to milliamps may be accommodated with femtoamp current resolution.
Model 311 - The311 is similar to the 310 but the high quality input is the positive or non-inverting input. It is primarily intended for measurement of voltages from very high source impedances. Such sources are found in the glass electrodes of pH cells and other scientific measurement apparatus. Another such source is charge stored on the plates of a capacitor, as is found in long-time track-andhold applications. Table 1. Comparison of Electrometer Types
TYPE VARACTOR BRIDGE VIBRATING CAPACITOR MOSFET ELECTROMETER TUBES
I-Stability V-Stability Bandwidth Overload Protection CMR Microphonics Warm-up Size Price
good excellent narrow easy excellent fair fast small low
excellent excellent narrow easy excellent fair fast large high
good fair wide difficult fair good fast small medium
good poor wide poor poor slow large medium
The chart above shows the relative advantages of various electrometer types. The best performance possible is given by the vibrating reed electrometer. it should be remembered, however, that this is a large, expensive laboratory instrument not generally suitable for instrument construction.
Information furnished by Intronics is believed to be accurate and reliable. However, no responsibility is assumed by Intronics for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intronics.
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