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HFA1135EVAL

360MHz, Low Power, Video Operational Amplifier with Output Limiting

Intersil 

HFA1135

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, and the

expected bandwidth, for various closed loop gains.

TABLE 1. OPTIMUM FEEDBACK RESISTOR

GAIN

(AV)

-1

+1

+2

+5

+10

RF (Ω)

330

1.5k

250

330

180

250

BANDWIDTH

(MHz)

290

660

360

315

200

90

Non-inverting Input Source Impedance

For best operation, the DC source impedance seen by the

non-inverting input should be ≥50Ω. This is especially

important in inverting gain configurations where the non-

inverting input would normally be connected directly to GND.

Pulse Undershoot and Asymmetrical Slew Rates

The HFA1135 utilizes a quasi-complementary output stage

to achieve high output current while minimizing quiescent

supply current. In this approach, a composite device

replaces the traditional PNP pulldown transistor. The

composite device switches modes after crossing 0V,

resulting in added distortion for signals swinging below

ground, and an increased undershoot on the negative

portion of the output waveform (see Figures 9, 13, and 17).

This undershoot isn’t present for small bipolar signals, or

large positive signals. Another artifact of the composite

device is asymmetrical slew rates for output signals with a

negative voltage component. The slew rate degrades as the

output signal crosses through 0V (see Figures 9, 13, and

17), resulting in a slower overall negative slew rate. Positive

only signals have symmetrical slew rates as illustrated in the

large signal positive pulse response graphs (see Figures 7,

11, and 15).

PC Board Layout

This amplifier’s frequency response depends greatly on the

care taken in designing the PC board. The use of low

inductance components such as chip resistors and chip

capacitors is strongly recommended, while a solid

ground plane is a must!

Attention should be given to decoupling the power supplies.

A large value (10µF) tantalum in parallel with a small value

(0.1µF) chip capacitor works well in most cases.

Terminated microstrip signal lines are recommended at the

input and output of the device. Capacitance directly on the

output must be minimized, or isolated as discussed in the

next section.

Care must also be taken to minimize the capacitance to

ground at the amplifier’s inverting input (-IN), as this

capacitance causes gain peaking, pulse overshoot, and if

large enough, instability. To reduce this capacitance, the

designer should remove the ground plane under traces

connected to -IN, and keep connections to -IN as short as

possible.

An example of a good high frequency layout is the

Evaluation Board shown in Figure 2.

Driving Capacitive Loads

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

terminated transmission line 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 660MHz (AV = +1). 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 = +1, RS = 50Ω, CL = 20pF, the overall

bandwidth is 170MHz, but the bandwidth drops to 45MHz at

AV = +1, RS = 10Ω, CL = 330pF.

50

45

40

35

30

25

20

15 AV = +1

10

5

0

0 40 80

AV = +1

AV = +2, RF = 250Ω

120 160 200 240 280 320 360 400

LOAD CAPACITANCE (pF)

FIGURE 1. RECOMMENDED SERIES RESISTOR vs LOAD

CAPACITANCE

5

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