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OPAMPSOT23EVAL

700MHz, SOT-23, Low Distortion Current Feedback Operational Amplifier

Renesas Electronics 

HFA1150

Application Information

Relevant Application Notes

The following Application Notes pertain to the HFA1150:

• 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

• AN9891-Operating the HFA1150 from 5V Single

Supply

These publications may be obtained from Intersil’s web site

(http://www.intersil.com).

Performance Differences Between Packages

The HFA1150 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 HFA1150 in various gains. Although the

bandwidth dependency on ACL isn’t as severe as that of a

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

relationship between bandwidth and RF. All current feedback

amplifiers require a feedback resistor, even for unity gain

applications, and the RF, in conjunction with the internal

compensation capacitor, sets the dominant pole of the

frequency response. Thus, the amplifier’s bandwidth is

inversely proportional to RF. The HFA1150 is optimized for a

RF = 576/499 (SOIC/SOT-23), at a gain of +2. Decreasing

RF decreases stability, resulting in excessive peaking and

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

RF can be decreased in a trade-off of stability for bandwidth.

The table below lists recommended RF values for various

gains, and the expected bandwidth.

OPTIMUM FEEDBACK RESISTOR

RF ()

BANDWIDTH (MHz)

ACL

SOIC/SOT-23

SOIC/SOT-23

-1

422/464

650/540

+1

383, (+RS = 226)/

600/500

549, (+RS = 100)

+2

576/499

700/540

+5

348/422

480/400

+10

178/348

5V Single Supply Operation

380/300

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 AN9891 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 avoided by

placing a resistor (RS) in series with the output prior to the

capacitance.

Figure 1 details starting points for the selection of this resistor.

The points on the curve indicate the RS and CL 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.

RS and CL form a low pass network at the output, thus limiting

system bandwidth well below the amplifier bandwidth of

700MHz/540MHz (SOIC/SOT-23, AV = +2). By decreasing RS

as CL increases (as illustrated by the curves), the maximum

bandwidth is obtained without sacrificing stability. In spite of

this, bandwidth still decreases as the load capacitance

increases. For example, at AV = +2, RS = 20, CL = 22pF, the

SOIC bandwidth is 410MHz, but the bandwidth drops to

110MHz at AV = +2, RS = 5, CL = 390pF.

FN4836 Rev 1.00

June 2004

Page 4 of 11

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