Buffer amplifier (unity-gain buffer)

01

Match the load, output current and speed

TI's BUF634A is an open-loop buffer able to drive 250 mA of output current on supplies of ±2.25 V to ±18 V (36 V total), with bandwidth adjustable from 35 MHz to 210 MHz by an external resistor between the V− and BW pins, 3,750 V/µs slew rate, 1.5 mA quiescent current at 35 MHz, internal output current limit and thermal shutdown, in packages with thermal pads, over −40 to +125 °C. TI lists uses including semiconductor testers, headphone, valve, solenoid and line drivers and capacitive-load drivers. [1]

TI's BUF802 is an open-loop unity-gain buffer with a JFET input stage, 3.1 GHz large-signal bandwidth at 1 Vpp, 7,000 V/µs slew rate, 0.7 ns 1% settling, the ability to drive 50 Ω loads, adjustable quiescent current to trade bandwidth and distortion for power, and integrated input and output clamps, on ±4.5 V to ±6.5 V supplies, for oscilloscope front ends, high-frequency data acquisition and active probes. [2]

TI's LMH6321 is a unity-gain buffer that slews at 1,800 V/µs with 110 MHz small-signal bandwidth while driving a 50 Ω load, drives ±300 mA continuously and is stated not to oscillate with large capacitive loads; it runs from 5 V to ±15 V over −40 to +125 °C, comes in an 8-pin SO PowerPAD or 7-pin DDPAK power package, and is listed for line, pin and sonar drivers and motor control. [3]

Renesas describes the HA-5002 as a monolithic current buffer with 1,300 V/µs slew rate, 110 MHz bandwidth, ±200 mA output current and 3 Ω output impedance, and says it replaces the hybrid LH0002. Its attribute table for the HA9P5002-5Z (8-lead SOIC) lists a 10 to 40 V supply range, 8.3 mA supply current per amplifier, 1.5 V output headroom and a 0 to +70 °C temperature range. [4]

Editorial note: the four parts cover different jobs — the BUF634A, LMH6321 and HA-5002 deliver 250 mA, ±300 mA and ±200 mA from supplies up to 36 V, ±15 V and 40 V; the BUF802 buffers a high-impedance source at gigahertz bandwidth into 50 Ω. Start from the load impedance, peak current and signal bandwidth. [1] [3] [4] [2]

Give the supplier the load (resistance, capacitance, cable), peak output current and voltage swing, bandwidth and slew rate, supply rails, gain accuracy and offset needs, whether the buffer sits inside an op-amp loop, input impedance and noise, protection (current limit, thermal shutdown, clamps), package and thermal path, temperature range and quantity.

02

Check gain accuracy, offset, input impedance and noise

For the BUF634A, TI lists open-loop gain of 0.95 to 0.99 V/V into 1 kΩ, 0.93 to 0.95 V/V into 100 Ω and 0.91 to 0.93 V/V into 67 Ω at ±10 V output; input offset of 36 mV (typ.; 65 mV maximum) with 175 µV/°C drift; input impedance of 180 MΩ ∥ 5 pF; output impedance of 5 Ω; voltage noise of 3.4 nV/√Hz; and second- and third-harmonic distortion of −77 dBc at 2 Vpp and 20 kHz. TI says it can be used alone or inside the feedback loop of a precision op amp to combine high precision with high output current and improved capacitive-load drive. [1]

The BUF802 lists 50 GΩ ∥ 2.4 pF input impedance and 2.3 nV/√Hz input voltage noise, and TI describes using it in a composite loop with a precision amplifier where higher precision is required. [2]

Editorial calculation: a gain of 0.95 V/V into 100 Ω is a 5% (about −0.45 dB) shortfall, so a 10 V output needs about 10.5 V at the input when used open-loop — one reason to close the loop around the buffer with a precision op amp. [1]

Renesas lists for the HA-5002 a minimum gain of 0.995, a maximum offset voltage of 5 mV, input bias current of 2,000 nA, input impedance of 3,000 kΩ and PSRR of 64 dB; the page does not state the load for the gain figure. [4]

Editorial calculation: a gain of 0.995 is a 0.5% shortfall, against 5% for the BUF634A's 0.95 V/V into 100 Ω; the HA-5002's 5 mV maximum offset is 13 times lower than the BUF634A's 65 mV maximum (65 ÷ 5). Compare at the same load before relying on either open-loop. [4] [1]

Output protection differs by part: the LMH6321 current limit is set by an external resistor that fixes a reference current, is continuously adjustable from 10 mA to 300 mA with ±5 mA ±5% accuracy, applies to both sourcing and sinking current, and is paired with a thermal-shutdown error flag; TI notes the resistor can be driven from a DAC to change the limit in operation. [3]

Editorial calculation: at a 100 mA limit setting, the stated ±5 mA ±5% tolerance gives a window of about 90 to 110 mA (100 ± 5 ± 5 mA). [3]

03

Gain, distortion and thermal data to ask a buffer supplier for at your load

Ask any supplier for gain, offset and distortion at your load and frequency, and for safe-operating-area or thermal data at your output current. [1] [2]

04

Input stage, bandwidth control, output stage, current limit, clamps and thermal pad

Protection differs part by part: the BUF634A has an internal current limit and thermal shutdown, the LMH6321's limit is set by a resistor from 10 to 300 mA, and the BUF802 adds input and output clamps. [1] [3] [2]

05

Supplies, bandwidth, output current, slew rate, offset, impedance and gain for a buffer amplifier

Rows list the parts in the order BUF634A, LMH6321, BUF802, HA-5002 wherever each publishes a value; ask for gain and offset at your own load, since the HA-5002 page gives no load for its gain figure. [1] [3] [2] [4]

06

Send the load, current and signal data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Load: [Ω; pF; cable]
Output: [peak A; V swing]
Speed: [MHz; V/µs]
Accuracy: [gain; offset; open-loop or in-loop]
Input: [impedance; noise]
Supply/protection: [rails; limits]
Package/thermal: [package; θJA]
Quantity: [units]

See manufacturer records Send an enquiry through CNFX