OPA650 datasheet(13/24 Pages) TI2

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OPERATING SUGGESTIONS

Optimizing Resistor Values

Inverting Amplifier Operation

Bandwidth vs Gain: Noninverting Operation

OPA2652

SBOS125A – JUNE 2000 – REVISED MAY 2006

causes the phase margin to approach 90

° and the

bandwidth to more closely approach the predicted

value of (GBP/NG). At a gain of +5, the 45MHz

bandwidth shown in the Electrical Characteristics is

Because the OPA2652 is a unity gain stable voltage

close to that predicted using this simple formula.

feedback op amp, a wide range of resistor values

may be used for the feedback and gain setting

resistors. The primary limits on these values are set

by dynamic range (noise and distortion) and parasitic

Because

the

OPA2652

general-purpose,

capacitance considerations. For a noninverting unity

wideband voltage feedback op amp, all of the

gain follower application, the feedback connection

familiar op amp application circuits are available to

should be made with a 25

Ω resistor, not a direct

the designer. Inverting operation is one of the more

short. This configuration isolates the inverting input

common

requirements

and

offers

several

capacitance from the output pin and improves the

performance benefits. Figure 29 shows a typical

frequency response flatness. Usually, the feedback

inverting configuration.

resistor value should be between 200

Ω and 1.5kΩ.

In the inverting configuration, three key design

Below

200

Ω, the feedback network presents

consideration must be noted. First, the gain resistor

additional output loading that can degrade the

(RG) becomes part of the signal channel input

harmonic distortion performance of the OPA2652.

impedance. If input impedance matching is desired

Above

1.5k

Ω, the typical parasitic capacitance

(which is beneficial whenever the signal is coupled

(approximately 0.2pF) across the feedback resistor

through a cable, twisted pair, long PCB trace or other

may cause unintentional bandlimiting in the amplifier

transmission line conductor), RG may be set equal to

response.

the required termination value and RF adjusted to

A good rule of thumb is to target the parallel

give the desired gain. This approach is the simplest,

combination of RF and RG (see Figure 28) to be less

and

results

optimum

bandwidth

and

noise

than approximately 300

Ω. The combined impedance

performance. However, at low inverting gains, the

interacts

with

the

inverting

input

resulting feedback resistor value can present a

capacitance, placing an additional pole in the

significant load to the amplifier output. For an

feedback network, and thus a zero in the forward

inverting gain of –1, setting RG to 50Ω for input

response. Assuming a 2pF total parasitic on the

matching eliminates the need for RM but requires a

inverting node, holding RF || RG < 300Ω keeps this

50W feedback resistor. This configuration has the

pole above 250MHz. By itself, this constraint implies

interesting advantage that the noise gain becomes

that the feedback resistor RF can increase to several

equal to 2 for a 50

Ω source impedance—the same

Ω at high gains. This increase is acceptable as long

the

noninverting

circuits

considered

above.

as the pole formed by RF and any parasitic

However, the amplifier output now sees the 50

Ω

capacitance appearing in parallel is kept out of the

feedback resistor in parallel with the external load. In

frequency range of interest.

general, the feedback resistor should be limited to

the 200

Ω to 1.5kΩ range. In this case, it is preferable

to increase both the RF and RG values as shown in

Figure 29, and then achieve the input matching

Voltage

feedback

amps

exhibit

decreasing

impedance with a third resistor (RM) to ground. The

closed-loop

bandwidth

the

signal

gain

total

input

impedance

becomes

the

parallel

increased. In theory, this relationship is described by

combination of RG and RM.

the Gain Bandwidth Product (GBP) shown in the

specifications.

Ideally,

dividing

GBP

the

The second major consideration, touched on in the

noninverting signal gain (also called the Noise Gain,

paragraph,

that

the

signal

source

or NG) predicts the closed-loop bandwidth. In

impedance becomes part of the noise gain equation

practice, this prediction only holds true when the

and influences the bandwidth. For the example in

phase margin approaches 90

°, as it does in high

Figure 29, the RM value combines in parallel with the

gain

configurations.

low

gains

(increased

external 50

Ω source impedance, yielding an effective

feedback factor), most amplifiers exhibit a wider

driving impedance of 50

Ω || 57.6Ω = 26.8Ω. This

bandwidth and lower phase margin. The OPA2652 is

impedance is added in series with RG for calculating

compensated

give

flat

response

the noise gain (NG). The resulting NG is 1.94 for

noninverting

gain

(see

Figure

28).

This

Figure 29 (an ideal source would cause NG = 2.00).

configuration results in a typical gain of +1 bandwidth

The

third

important

consideration

inverting

of 700MHz, far exceeding that predicted by dividing

amplifier

design

setting

the

bias

current

the 200MHz GBP by NG = 1. Increasing the gain

cancellation resistor on the noninverting input (RB). If

this resistor is set equal to the total DC resistance

looking out of the inverting node, the output DC

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