LMH6502 Datasheet, PDF(14/19 Page) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB Variable Gain Amplifier

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LMH6502 Datasheet, PDF (14/19 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB Variable Gain Amplifier

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Typical Performance Characteristics Unless otherwise specified: VS = Â±5V, 25ËC, VG = VGMAX, VCM

= 0V, RF = 1kâ¦, RG = 174â¦, both inputs terminated in 50â¦, RL = 100â¦, Typical values, results referred to device

output. (Continued)

Feedthrough from VG

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Application Information

THEORY OF OPERATION

A simplified schematic is shown in Figure 1. +VIN and âVIN

are buffered with closed loop voltage followers inducing a

signal current in Rg proportional to (+VIN) - (âVIN), the dif-

ferential input voltage. This current controls a current source

which supplies two well-matched transistor, Q1 and Q2.

The current flowing through Q2 is converted to the final

output voltage using RF and the output amplifier, U1. By

changing the fraction of the signal current "I" which flows

through Q2, the gain is changed. This is done by changing

the voltage applied differentially to the bases of Q1 and Q2.

For example, with VG = 0V, Q1 conducts heavily and Q2 is

off. With none of "I" flowing through RF, the LMH6502âs input

to output gain is strongly attenuated. With VG = +2V, Q1 is off

and the entire signal current flows through Q2 to RF produc-

ing maximum gain. With VG set to 1V, the bases of Q1 and

Q2 are set to approximately the same voltage, Q1 and Q2

have the same collector currents - equal to one half of the

signal current "I", thus the gain is approximately one half the

maximum gain.

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FIGURE 1. LMH6502 Block Diagram

CHOOSING RF & RG

Maximum input amplitude and maximum gain are the two

key specifications that determine component values in a

LMH6502 application.

The output stage op amp is a current-feedback type amplifier

optimized for RF = 1kâ¦. RG can then be computed as:

(1)

To determine whether the maximum input amplitude will

overdrive the LMH6502, compute:

VDMAX = (RG + 3.0â¦) x 1.70mA

(2)

the maximum differential input voltage for linear operation. If

the maximum input amplitude exceeds the above VDMAX

limit, then LMH6502 should either be moved to a location in

the signal chain where input amplitudes are reduced, or the

LMH6502 gain AVMAX should be reduced or the values for

RG and RF should be increased. The overall system perfor-

mance impact is different based on the choice made. If the

input amplitude is reduced, re-compute the impact on signal-

to-noise ratio. If AVMAX is reduced, post LMH6502 amplifier

gain, should be increased, or another gain stage added to

make up for reduced system gain. To increase RG and RF,

compute the lowest acceptable value for RG:

RG > 590 x VDMAX - 3â¦

(3)

Operating with RG larger than this value insures linear op-

eration of the input buffers.

RF may be computed from selected RG and AVMAX: RF

should be > = 1kâ¦ for overall best performance, however RF

< 1kâ¦ can be implemented if necessary using a loop gain

reducing resistor to ground on the inverting summing node of

the output amplifier (see application note QA-13 for details).

ADJUSTING OFFSET

Offset can be broken into two parts; an input-referred term

and an output-referred term. The input-referred offset shows

up as a variation in output voltage as VG is changed. This

can be trimmed using the circuit in Figure 2 by placing a low

frequency square wave (VLOW = 0V, VHIGH = 2V into VG with

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