Buffer Amplifier Design
by Dennis L. Feucht
Innovatia Laboratories
The x1 voltage amplifier, or "buffer," is a standard building block of analog design. This article presents the design of a discrete, low-parts-count, high-performance, matched-transistor buffer, and covers some of the finer points in optimizing its design.
In an era when a buffer is most easily implemented with an op amp, why use a discrete circuit? When high precision and minimal space are not worth the extra cost, a dual-JFET amplifier costing 50¢ in parts can deliver multiple hundreds of megahertz of bandwidth with an offset error of 10 mV, or less, and an offset drift of 10 mV/°C or less. By applying ingenuity good performance can be attained with discrete-component circuits, which will still be available when the op amp is obsolete. The buffer can become part of your design library.
Buffer Amplifier Circuit
The design goal of the x1 voltage amplifier is that of the ideal voltage amplifier: infinite input impedance, zero output impedance, and linearity. To achieve high input impedance, a JFET instead of a BJT is used, as shown in the buffer circuit below.
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