DESIGN-IN TOOLS

MACROMODELS

DEMONSTRATION FIXTURES

OPERATING RECOMMENDATIONS

OPTIMIZING RESISTOR VALUES

BANDWIDTH vs GAIN: NONINVERTING

OPA2889

SBOS373B – JUNE 2007 – REVISED AUGUST 2008 ....................................................................................................................................................... www.ti.com

Computer simulation of circuit performance using

SPICE

is

often

useful

when

analyzing

the

performance of analog circuits and systems. This

Two printed circuit boards (PCBs) are available to

principle is particularly true for video and RF amplifier

assist in the initial evaluation of circuit performance

circuits where parasitic capacitance and inductance

using the OPA2889 in its two package options. Both

can have a major effect on circuit performance. A

of these are offered free of charge as unpopulated

SPICE model for the OPA2889 is available through

PCBs, delivered with a user’s guide. The summary

the Texas Instruments web page (www.ti.com). This

information for these fixtures is shown in Table 1.

model does a good job of predicting small-signal ac

and transient performance under a wide variety of

Table 1. Demonstration Fixtures by Package

operating conditions. It does not do as well in

LITERATURE

predicting

the

harmonic

distortion

or

dG/dP

PRODUCT

PACKAGE

ORDERING NUMBER

NUMBER

characteristics. This model does not attempt to

OPA2889ID

SO-8

DEM-OPA-SO-2A

SBOU003A

distinguish between the package types in their

OPA2889IDGS

MSOP-10

DEM-OPA-MSOP-2B

SBOU040

small-signal ac performance.

The demonstration fixtures can be requested at the

Texas Instruments web site (www.ti.com) through the

OPA2889 product folder.

OPERATION

Because

the

OPA2889

is

a

unity-gain

stable,

voltage-feedback op amp, a wide range of resistor

Voltage-feedback

op

amps

exhibit

decreasing

values may be used for the feedback and gain setting

closed-loop bandwidth as the signal gain increases.

resistors. The primary limits on these values are set

In theory, this relationship is described by the Gain

by dynamic range (noise and distortion) and parasitic

Bandwidth Product (GBP) shown in the Electrical

capacitance

considerations.

For

a

noninverting

Characteristics.

Ideally,

dividing

GBP

by

the

unity-gain

follower

application,

the

feedback

noninverting signal gain (also called the Noise Gain,

connection should be made with a direct short.

or

NG)

predicts

the

closed-loop

bandwidth.

In

Usually, the feedback resistor value should be

practice, this principle only holds true when the phase

between 200

Ω and 1.5kΩ. Below 200Ω, the feedback

margin approaches 90°, as it does in high gain

network presents additional output loading which can

configurations. At low gains (increased feedback

degrade the harmonic distortion performance of the

factors), most amplifiers exhibit a more complex

OPA2889.

Above

1.5k

Ω,

the

typical

parasitic

response with lower phase margin. The OPA2889 is

capacitance

(approximately

0.2pF)

across

the

compensated to give a slightly peaked response in a

feedback

resistor

can

cause

unintentional

noninverting gain of 2V/V (see Figure 50). This

band-limiting in the amplifier response.

compensation results in a typical gain of +2V/V

bandwidth of 60MHz, far exceeding that predicted by

A good rule of thumb is to target the parallel

dividing the 75MHz GBP by 2. Increasing the gain

combination of RF and RG (see Figure 50) to be less

causes the phase margin to approach 90° and the

than approximately 400

Ω. The combined impedance

bandwidth to more closely approach the predicted

RF || RG interacts with the inverting input capacitance,

value of (GBP/NG). At a gain of +10, the 8MHz

placing an additional pole in the feedback network

bandwidth shown in the Electrical Characteristics

and thus, a zero in the forward response. Assuming a

agrees closely with that predicted using the simple

2pF total parasitic on the inverting node, holding RF ||

formula and the typical GBP of 75MHz.

RG < 400Ω keeps this pole above 160MHz. By itself,

this constraint implies that the feedback resistor RF

The frequency response in a gain of +2V/V may be

can increase to several k

Ω at high gains. This

modified to achieve exceptional flatness simply by

increase in resistor size is acceptable as long as the

increasing the noise gain to 2.5V/V. One way to

pole formed by RF and any parasitic capacitance

modify the response without affecting the +2V/V

appearing in parallel is kept out of the frequency

signal gain, is to add a 750

Ω resistor across the two

range of interest.

inputs, as shown in the circuit of Figure 50. A similar

technique may be used to reduce peaking in

unity-gain

(voltage

follower)

applications.

For

example, by using a 750

Ω feedback resistor along

22

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Product Folder Link(s): OPA2889