“Buffer” is one of the many words of electronic and computer engineering with multiple meanings. It can refer to separate physical memory dedicated to temporary data storage, or it can be a software-defined memory allocation for in-process data. Buffer can also refer to a hardware driver for a low impedance load such as a relay or LED.
However, in classic analog signal chain circuits, it refers to a component that resides between two stages to ensure that the signal source is compatible with the characteristics of its load stage. An RF buffer amplifier usually does not perform traditional signal “processing” tasks, such as filtering or rejection of noise. Instead, its usual role is to ensure an adaptation between source and load by isolating the output of the source from the characteristics and variations of the load input.
This may require the buffer to transform the high impedance output of the source, so that it is “comfortable” in driving the low impedance of the load. The right buffer amplifier can make the difference between achieving “good enough” and “really good” system performance.
TI’s very wide bandwidth buffer amplifier
To meet these requirements, Texas Instruments’ BUF802 buffer amplifier offers wide signal bandwidth of up to 3.1 GHz at 1 V pp input (and up to 2 GHz at 2 V pp) with a fast response speed fast (7000 V / µs) and fast settling time (0.7 ns to 1%) (Fig. 1).
Claimed to be the widest high input impedance, bandwidth buffer amplifier of its type on the market, target applications include high-performance and precision test and measurement projects, including oscilloscopes, active probes, and data acquisition systems at broadband connection.
TI further argues that the bandwidth offered by the BUF802 was previously only reachable via application specific integrated circuits (ASICs). ASICs, however, inherently required a significant commitment in terms of design time, complexity, risk, and cost due to the need for dozens of discrete components such as FETs, bipolar transistors, and protection diodes.
The input voltage noise is only 2.3 nV / √Hz, while the input impedance, another critical parameter in these applications, is 50 GΩ in parallel with 2.4 pF. Operating from a ± 4.5 to ± 6.5 V supply, the JFET BUF802 input can easily drive the ubiquitous 50 Ω load. It also offers a user-adjustable quiescent current for power / performance trade-off and an integrated input / output terminal with quick overdrive recovery, which is another concern in these applications.
The BUF802 has been designed to operate in two modes – Buffer (BF) Mode and Composite Loop (CL) – using the parallel main path and the auxiliary path (Fig. 2).
In applications where extreme DC accuracy is not as critical with offsets below 100mV or where the input is AC coupled, the BUF802 can be used as a standalone input buffer where only its main path is used . In this mode, the BUF802 uses the JFET, the output driver and the bipolar transistors in the main path to reproduce the signal applied to the IN input to its OUT output.
Even more precision
If greater accuracy than that offered by the BUF802 alone is required, designers can configure the BUF802 in CL mode in combination with a separate precision amplifier. In this mode, the BUF802 uses the auxiliary path and the main path to control the output voltage and simultaneously provide high DC accuracy and 3 GHz bandwidth with a maximum offset drift of 1 μV / ° C.
In this scenario, the composite circuit splits the applied signal into low and high frequency components and transmits them to different circuits with adequate transfer function. The low and high frequency components of the signal are then recombined within the BUF802 and reproduced on the OUT pin. (No, it is not the topology of the Doherty amplifier, as the BUF802 mode is preset by the user and not determined by the amplitude of the signal.)
In addition to the comprehensive 37-page BUF802 datasheet with specifications, tables, graphs, considerable and much needed application information, and demo PCB layout details, TI also has an informative blog post “Simplify Analog Front Ends with Hi Buffers -Z “.
Additionally, the company offers the BUF802RGTEVM (EVM) evaluation module which is very useful for buffer amplifiers like this one where layout is critical. (Fig. 3). This EVM can be configured for composite loop mode with a precision amplifier or just the BUF802 device alone. It features single or split supplies and SMA connector footprints on all analog inputs and outputs. The EVM layout is optimized to reduce parasitic coupling and provide the best signal fidelity across the entire frequency spectrum.
The BUF802 is available in a 3x3mm, 16-pin VQFN (very thin lead-free) package and is priced at $ 1.80 in 1,000-unit quantities. The BUF802RGTEVM evaluation module costs $ 25.