Buffer Amplifiers and Driving Capacitive Loads

Walt Jung and Walt Kester

Buffer Amplifiers

In the early days of high speed circuits, simple emitter followers were often used as high speed buffers. The term buffer was generally accepted to mean a unity-gain, open-loop amplifier. With the availability of matching PNP transistors, a simple emitter follower can be improved, as shown in Figure 6-66A. This complementary circuit offers first-order cancellation of dc offset voltage, and can achieve bandwidths greater than 100 MHz. Typical offset voltages without trimming are usually less than 50 mV, even with unmatched discrete transistors. The HOS-100 hybrid amplifier from Analog Devices represented an early implementation of this circuit. ...

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