4

Application Information

Relevant Application Notes

The following Application Notes pertain to the HFA1155:

• AN9787-An Intuitive Approach to Understanding

Current Feedback Amplifiers

• AN9420-Current Feedback Amplifier Theory and

Applications

• AN9663-Converting from Voltage Feedback to Current

Feedback Amplifiers

• AN9897-Operating the HFA1155 from 5V Single

Supply

These publications may be obtained from Intersil’s web site

(www.intersil.com) or via our AnswerFax system.

Performance Differences Between Packages

The HFA1155 is a high frequency current feedback amplifier.

As such, it is sensitive to parasitic capacitances which

influence the amplifier’s operation. The different parasitic

capacitances of the SOIC and SOT-23 packages yield

performance differences (notably bandwidth and bandwidth

related parameters) between the two devices - see Electrical

Specification tables for details.

Because of these performance differences, designers

should evaluate and breadboard with the same package

style to be used in production.

Note that some “Typical Performance Curves” have separate

graphs for each package type. Graphs not labeled with a

specific package type are applicable to both packages.

Optimum Feedback Resistor

The enclosed frequency response graphs detail the

performance of the HFA1155 in various gains. Although the

voltage feedback amplifier, there is an appreciable decrease

in bandwidth at higher gains. This decrease can be minimized

by taking advantage of the current feedback amplifier’s unique

amplifiers require a feedback resistor, even for unity gain

compensation capacitor, sets the dominant pole of the

frequency response. Thus, the amplifier’s bandwidth is

overshoot (Note: Capacitive feedback causes the same

problems due to the feedback impedance decrease at higher

frequencies). At higher gains the amplifier is more stable, so

gains, and the expected bandwidth.

5V Single Supply Operation

This amplifier operates at single supply voltages down to

4.5V. The dramatic supply current reduction at this operating

condition (refer also to Figure 25) makes this op amp an

even better choice for low power 5V systems. Refer to

Application Note AN9897 for further information.

Driving Capacitive Loads

Capacitive loads, such as an A/D input, or an improperly

terminated transmission line will degrade the amplifier’s

phase margin resulting in frequency response peaking and

possible oscillations. In most cases, the oscillation can be

prior to the capacitance.

Figure 1 details starting points for the selection of this

combinations for the optimum bandwidth, stability, and

settling time, but experimental fine tuning is recommended.

Picking a point above or to the right of the curve yields an

overdamped response, while points below or left of the curve

indicate areas of underdamped performance.

limiting system bandwidth well below the amplifier bandwidth

maximum bandwidth is obtained without sacrificing stability.

In spite of this, bandwidth still decreases as the load

OPTIMUM FEEDBACK RESISTOR

SOIC/SOT-23

BANDWIDTH (MHz)

SOIC/SOT-23

-1

576/576

370/360

+1

370/365

+2

715/604

380/355

+5

402/475

300/300

+10

182/182

230/250

HFA1155