THS4271D datasheet(21/50 Pages) TI1

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THS4271

249

Ω

130

Ω

100 pF

0.1

µF

6.8

µF

−VS

−5 V

Ω Source

100 pF

0.1

µF

6.8

µF

+VS

5 V

0.1

µF

249

Ω

61.9

Ω

499

Ω

THS4271

THS4275

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SLOS397F – JULY 2002 – REVISED OCTOBER 2009

WIDEBAND, INVERTING GAIN OPERATION

must be taken when dealing with low inverting gains,

as the resulting feedback resistor value can present a

Since

the

THS4271

and

THS4275

are

significant load to the amplifier output. For an

general-purpose,

wideband

voltage-feedback

inverting gain of 2, setting Rg to 49.9 Ω for input

amplifiers,

several

familiar

operational

amplifier

matching eliminates the need for RM but requires a

applications circuits are available to the designer.

100-

Ω feedback resistor. This has an advantage of

Figure 76 shows a typical inverting configuration

the noise gain becoming equal to 2 for a 50-

Ω source

where the input and output impedances and noise

impedance—the same as the noninverting circuit in

gain from Figure 75 are retained in an inverting circuit

Figure 75. However, the amplifier output now sees

configuration. Inverting operation is one of the more

the 100-

Ω feedback resistor in parallel with the

common

requirements

and

offers

several

external load. To eliminate this excessive loading, it is

performance benefits. The inverting configuration

preferable to increase both Rg and Rf, values, as

shows improved slew rates and distortion due to the

shown in Figure 76, and then achieve the input

pseudo-static voltage maintained on the inverting

matching impedance with a third resistor (RM) to

input.

ground. The total input impedance becomes the

parallel combination of Rg and RM.

The next major consideration is that the signal source

impedance becomes part of the noise gain equation

and hence influences the bandwidth. For example,

the RM value combines in parallel with the external

50-

Ω

source

impedance

(at

high

frequencies),

yielding an effective source impedance of 50

Ω ||

61.9

Ω = 27.7 Ω. This impedance is then added in

series with Rg for calculating the noise gain. The

result is 1.9 for Figure 76, as opposed to the 1.8 if RM

is eliminated. The bandwidth is lower for the gain of

–2 circuit, Figure 76 (NG=+1.9), than for the gain of

+2 circuit in Figure 75.

The last major consideration in inverting amplifier

design is setting the bias current cancellation resistor

on the noninverting input. If the resistance is set

equal to the total dc resistance looking out of the

Figure 76. Wideband, Inverting Gain

inverting terminal, the output dc error, due to the input

Configuration

bias currents, is reduced to (input offset current)

multiplied by Rf in Figure 76, the dc source

impedance looking out of the inverting terminal is 249

In the inverting configuration, some key design

Ω || (249Ω + 27.7 Ω) = 130 Ω. To reduce the

considerations must be noted. One is that the gain

additional high-frequency noise introduced by the

resistor (Rg) becomes part of the signal channel input

resistor at the noninverting input, and power-supply

impedance. If the input impedance matching is

feedback, RT is bypassed with a capacitor to ground.

desired (which is beneficial whenever the signal is

coupled through a cable, twisted pair, long PCB

trace, or other transmission line conductors), Rg may

be set equal to the required termination value and Rf

adjusted to give the desired gain. However, care

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THS4271D Datasheet(PDF) 21 Page - Texas Instruments

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