OPA690 datasheet(32/45 Pages) TI

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BANDWIDTH VS GAIN:

1/4

OPA4830

Source

750W

374W

1.5kW

57.6W

+5V

1.5kW

150W

0.1 F

6.8 F

0.1 F

INVERTING AMPLIFIER OPERATION

OPA4830

SBOS350A – DECEMBER 2006 – REVISED MAY 2008.................................................................................................................................................... www.ti.com

benefits. It also allows the input to be biased at VS/2

NONINVERTING OPERATION

without any headroom issues. The output voltage can

be independently moved to be within the output

Voltage-feedback

amps

exhibit

decreasing

voltage range with coupling capacitors, or bias

closed-loop

bandwidth

the

signal

gain

adjustment resistors.

increased. In theory, this relationship is described by

the gain bandwidth product (GBP) shown in the

Electrical Characteristics. Ideally, dividing GBP by the

noninverting signal gain (also called the noise gain, or

NG) predicts the closed-loop bandwidth. In practice,

this calculation only holds true when the phase

margin approaches 90

°, as it does in high-gain

configurations. At low gains (increased feedback

factors), most amplifiers exhibit a more complex

response with lower phase margin. The OPA4830 is

compensated to give a slightly peaked response in a

noninverting gain of 2V/V (see Figure 74). This

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

bandwidth of 110MHz, far exceeding that predicted

by dividing the 110MHz GBP by 2V/V. Increasing the

gain causes the phase margin to approach 90

° and

the bandwidth to more closely approach the predicted

value of (GBP/NG). At a gain of +10V/V, the 11MHz

bandwidth illustrated in the Electrical Characteristics

Figure 88. AC-Coupled, G = –2V/V Example Circuit

agrees with that predicted using the simple formula

and the typical GBP of 110MHz.

In the inverting configuration, three key design

Frequency response in a gain of +2V/V may be

considerations must be noted. The first consideration

modified to achieve exceptional flatness simply by

is that the gain resistor (RG) becomes part of the

increasing the noise gain to 3V/V. One way to do this,

signal channel input impedance. If input impedance

without affecting the +2V/V signal gain, is to add a

matching is desired (which is beneficial whenever the

2.55k

Ω resistor across the two inputs (see Figure 78).

signal is coupled through a cable, twisted pair, long

A similar technique may be used to reduce peaking in

PCB trace, or other transmission line conductor), RG

unity-gain

(voltage

follower)

applications.

For

may be set equal to the required termination value

example, by using a 750

Ω feedback resistor along

and RF adjusted to give the desired gain. This

with a 750

Ω resistor across the two op amp inputs,

approach is the simplest and results in optimum

the voltage follower response is similar to the gain of

bandwidth and noise performance.

+2V/V response of Figure 73. Further reducing the

value of the resistor across the op amp inputs further

However,

low

inverting

gains,

the

resulting

dampens

the

frequency

response

because

feedback resistor value can present a significant load

increased noise gain. The OPA4830 exhibits minimal

to the amplifier output. For an inverting gain of 2,

bandwidth reduction going to single-supply (+5V)

setting RG to 50Ω for input matching eliminates the

operation as compared with ±5V. This minimal

need for RM but requires a 100Ω feedback resistor.

reduction is because the internal bias control circuitry

This configuration has the interesting advantage of

retains nearly constant quiescent current as the total

the noise gain becoming equal to 2 for a 50

Ω source

supply voltage between the supply pins changes.

impedance—the same as the noninverting circuits

considered above. The amplifier output now sees the

100

Ω feedback resistor in parallel with the external

load. In general, the feedback resistor should be

All of the familiar op amp application circuits are

limited to the 200

Ω to 1.5kΩ range. In this case, it is

available with the OPA4830 to the designer. See

preferable to increase both the RF and RG values, as

Figure 88 for a typical inverting configuration where

shown in Figure 88, and then achieve the input

the I/O impedances and signal gain from Figure 72

matching impedance with a third resistor (RM) to

are retained in an inverting circuit configuration.

ground. The total input impedance becomes the

Inverting operation is one of the more common

parallel combination of RG and RM.

requirements

and

offers

several

performance

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