Buffer Amplifier Links Page
Mark Connelly, WA1ION - South Yarmouth, MA, USA - rev. 20130403
Buffer amplifiers convert a high impedance input, such as from a short whip antenna or from a high-Q tuned L-C tank circuit, to a low impedance output suitable for driving 50 or 75 ohm coaxial cable to a receiver's input. These amplifiers provide considerable current gain. They may or may not also provide voltage gain, depending on the configuration. Buffer circuits are important building blocks in both receiving and automatic testing applications.
The TMB-1 Tri-Mode Buffer Amplifier Box article contains a discussion of a generic buffer amplifier circuit. A pin-connection table for several integrated circuits is included.
BUF-A is an early design incorporating the LH0033CG made by National Semiconductor and other vendors. This IC was prized for its low noise floor and linear response, but it is generally considered obsolete since the mid 1990's. The BUF-A card was used in several of my designs including the MWT-3 Regenerative Tuner.
BUF-B is a high-gain circuit similar to that in the MFJ-1024 active whip and in several articles published in the ham radio press over the years. The input is a dual-gate MOSFET and the output is a 2N5109 bipolar transistor. Feedback is used to reduce distortion. My DXP-1 Phasing Unit article uses a BUF-B card.
BUF-C1-25 uses the possibly-obsolete National LH0002CH and a step-up transformer (when desired) for an optional lower-impedance input. BUF-C1-25 is used in the DL-2 Phasing Unit design.
BUF-C2 has a JFET front end followed by a National LH0002CH buffer chip.
BUF-D is a high-gain design using a JFET input and a VMOS (VN10KM) output stage. It is used in the RL-1 Remotely-Tuned Loop and the RW-1 Remotely-Tuned Whip.
BUF-E and BUF-E1 use the National Semiconductors LM6221N or LM6321N. These chips have good low-noise performance. BUF-E1 is a reduced card size version of BUF-E and, therefore, is now the preferred version. These buffer cards are used in several of my projects.
BUF-F uses an inexpensive Harris-Intersil HA3-5002-5 chip. If a field of many active whips is needed, as in a computer-controlled phased array, this may be the design you want.
BUF-G is an adaptation of Lyle Koehler's front end card that uses a JFET input (MPF102) and bipolar output (2N2907). This circuit has a lot of gain for its low current consumption. It is not particularly well suited to the presence of large signals, so it is best used in high-Q tuned applications rather than in broadband ones.
BUF-H uses a widely-available TI - Burr Brown BUF634P chip.
BBVA-A is suitable for medium impedance inputs (300 ohms to 3K typical). It uses a VN10KM VMOS FET that has more input capacitance (but greater power dissipation) than a conventional JFET. This circuit is inexpensive and provides a lot of gain with an acceptably low noise floor. Current consumption is on the high side, about 100 mA.
PSA-A is a buffer amplifier with shiftable phase by means of an external potentiometer. It is used in the DXP-2 and Superphaser-2 phasing units.
One-Chip Active Whips, an article from 1991, describes the benefits of using integrated circuit buffer amplifiers in active whip antenna designs.
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