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THS4500

THS4501

SLOS350D − APRIL 2002 − REVISED JANUARY 2004

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CHOOSING THE PROPER VALUE FOR THE

FEEDBACK AND GAIN RESISTORS

The selection of feedback and gain resistors impacts

circuit performance in a number of ways. The values in this

section provide the optimum high frequency performance

(lowest distortion, flat frequency response). Since the

THS4500 family of amplifiers is developed with a voltage

feedback architecture, the choice of resistor values does

not have a dominant effect on bandwidth, unlike a current

feedback amplifier. However, resistor choices do have

second-order effects. For optimal performance, the

following feedback resistor values are recommended. In

higher gain configurations (gain greater than two), the

feedback resistor values have much less effect on the high

frequency performance. Example feedback and gain

resistor values are given in the section on basic design

considerations (Table 3).

Amplifier loading, noise, and the flatness of the frequency

response are three design parameters that should be

considered when selecting feedback resistors. Larger

resistor values contribute more noise and can induce

peaking in the ac response in low gain configurations, and

smaller resistor values can load the amplifier more heavily,

resulting in a reduction in distortion performance. In

addition, feedback resistor values, coupled with gain

requirements, determine the value of the gain resistors,

directly impacting the input impedance of the entire circuit.

While there are no strict rules about resistor selection,

these trends can provide qualitative design guidance.

APPLICATION CIRCUITS USING FULLY

DIFFERENTIAL AMPLIFIERS

Fully differential amplifiers provide designers with a great

deal of flexibility in a wide variety of applications. This

section provides an overview of some common circuit

configurations and gives some design guidelines.

Designing the interface to an ADC, driving lines

differentially, and filtering with fully differential amplifiers

are a few of the circuits that are covered.

BASIC DESIGN CONSIDERATIONS

The circuits in Figures 100 through 104 are used to

highlight basic design considerations for fully differential

amplifier circuit designs.

Table 3. Resistor Values for Balanced Operation

in Various Gain Configurations

Gain

R2 & R4 (

Ω)

R1 (

Ω)

R3 (

Ω)

RT (Ω)

392

412

383

54.9

499

523

487

53.6

392

215

187

60.4

1.3k

665

634

52.3

1.3k

274

249

56.2

3.32k

681

649

52.3

1.3k

147

118

64.9

6.81k

698

681

52.3

NOTE: Values in the table above assume a 50

Ω source impedance.

VOCM

−

Vout+

Vout−

Figure 100

Equations for calculating fully differential amplifier resistor

values in order to obtain balanced operation in the

presence of a 50-

Ω source impedance are given in

equations 6 through 9.

T +

–

1–

2(1

)K)

+ R2

+ R4

+ R1 * R

s || RT

(6)

1 +

) R2 β2 +

) R

T || RS

) R

T || RS ) R4

+ 2

1–

1 ) β2

T ) RS

+ 2

1–

1 ) β2

(7)

(8)

(9)

For more detailed information about balance in fully

differential amplifiers, see Fully Differential Amplifiers,

referenced at the end of this data sheet.

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