Variable gain amplifier (VGA)

01

Choose the gain-control type and range

TI's LMH6401 is a wideband, digitally controlled VGA (DVGA) for DC-to-RF, IF and high-speed time-domain applications. Gain is set over an SPI interface across a 32 dB range from −6 dB to 26 dB in 1 dB steps, with a power-down mode, a differential 100 Ω input and a differential output with common-mode control to match an ADC input; TI positions it as an ADC driver for DC- or AC-coupled systems needing automatic gain control. [1]

TI's VCA824 is a DC-coupled, wideband, linear-in-V/V, continuously variable voltage-controlled gain amplifier. A high-impedance gain-control input varies gain down by 40 dB from a maximum set by external gain and feedback resistors, intended between 2 V/V and 40 V/V; the internal architecture combines two input buffers, a multiplier core and a current-feedback output stage, so no external buffering is needed. [2]

TI's LMH6881 is a programmable differential amplifier that TI presents as able to replace both fixed-gain fully differential amplifiers and VGAs: it needs no external gain-setting components and covers 6 dB to 26 dB in 0.25 dB steps, with a 100 Ω input that also accepts 50 Ω single-ended sources, DC or AC coupling, and gain set by SPI serial commands or by parallel pins — the parallel pins allow it to be soldered down at a fixed gain with no control circuit. A dual version is offered as the LMH6882. [3]

The F1240 product page describes a dual-channel IF digital VGA with 20 dB (typ.) maximum gain, a 31 dB gain-control range set by 6-bit control in 0.50 dB steps over parallel or serial control, settling in under 15 ns, 200 Ω differential matched input and output that need no termination resistor, and individual power-down per channel. [4]

Editorial note: a digital-step VGA gives repeatable gain settings from a controller; a voltage-controlled VGA gives continuous adjustment suited to analog AGC loops. State which your control loop needs and the gain step or control-voltage range. Editorial calculation: the LMH6881's 20 dB range in 0.25 dB steps gives 81 gain settings, against 33 settings for the LMH6401's 32 dB range in 1 dB steps and 63 settings for the F1240's 31 dB range in 0.50 dB steps (within the 64 codes of its 6-bit control word). [1] [3] [2] [4]

Give the supplier the frequency range, gain range and step size or control-voltage range, gain accuracy, noise figure, output IP3 and harmonic distortion at your output level, input and output impedance, single-ended or differential operation, ADC common-mode requirement, supply rails and power budget, temperature range, package and quantity.

02

Check bandwidth, noise figure, distortion and power

The LMH6401 lists 4.5 GHz −3 dB bandwidth at 26 dB gain, 8 dB noise figure at 1 GHz (100 Ω source), output IP3 of 43 dBm at 200 MHz, 33 dBm at 1 GHz and 27 dBm at 2 GHz, and harmonic distortion at maximum gain for 2 Vpp differential output of HD2 −73/HD3 −80 dBc at 200 MHz, −63/−63 dBc at 1 GHz and −58/−54 dBc at 2 GHz, with 82 ps rise and fall times. It runs from 5.0 V at 69 mA (345 mW), single or split supply, from −40 to +85 °C in a 3 × 3 mm 16-pin UQFN. [1]

The VCA824 lists 710 MHz small-signal bandwidth at G = 2 V/V, 320 MHz bandwidth for 4 Vpp at G = 10 V/V, 0.1 dB gain flatness to 135 MHz, 2,500 V/µs slew rate, gain accuracy of ±0.3 dB at 20 dB, and ±90 mA output current, for differential line receivers, equalisers, variable attenuators and voltage-tunable filters. [2]

The LMH6881 lists 2,400 MHz small-signal bandwidth, OIP3 of 44 dBm and HD3 of −100 dBc at 100 MHz, a 9.7 dB noise figure and power-down capability, in a 4.00 × 4.00 mm 24-pin WQFN, for oscilloscope front ends, spectrum-analyzer gain blocks, differential ADC and cable drivers, IF/RF and baseband gain blocks and medical imaging. [3]

For the F1240, the maker lists a 10 MHz to 500 MHz frequency range, a noise figure of about 4.0 dB that degrades by less than 2 dB at 10 dB reduced gain, output IP3 of +47 dBm (typ.) and 79 mA supply current per channel in a 5 mm × 5 mm 32-pin package. [4] The page says its FlatNoise design keeps output IP3 and noise figure flat as gain is backed off, to hold system SNR in an IF-sampling receiver.

Editorial calculation: the LMH6401's 32 dB range in 1 dB steps gives 33 gain settings; at 5.0 V × 69 mA the dissipation is 345 mW, matching the datasheet figure. [1]

03

Gain-accuracy, noise and distortion curves to request from a VGA maker

Ask any supplier for gain-accuracy, noise-figure and distortion curves at your frequency and output level, and for the control interface timing. [1] [2]

04

Attenuator or multiplier core, fixed-gain stage, control interface, output and common mode

The rows cover both architectures in the sources: the LMH6401's 1 dB attenuator steps ahead of a 26 dB fixed amplifier, and the VCA824's multiplier core; state which one your part uses.

05

Gain range, bandwidth, noise figure, impedance, power and ESD across four VGAs

Where a row gives more than one value, they follow the citation order LMH6401, LMH6881, F1240, VCA824; the 100 Ω input value covers both TI digital parts.

06

Send the frequency, gain and signal-level data together

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

Band: [MHz–GHz]
Gain: [range; step or control voltage]
Noise/linearity: [NF; OIP3; HD at level]
Interface: [SPI; analog]
IO: [single-ended or differential; impedance]
ADC: [common-mode voltage; full scale]
Power/package: [rails; mW; package]
Quantity: [units]

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