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HFA1205IB

Dual, 425MHz, Low Power, Video Operational Amplifier

Intersil 

HFA1205

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 HFA1205 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 7, 11, 15 and

19). This undershoot isn’t present for small bipolar signals,

or large positive signals (see Figures 5, 6, 9, 10, 13, 14, 17

and 18). 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 7, 11, 15, and 19),

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 5,

9, 13 and 17).

PC Board Layout

The frequency response of this amplifier depends greatly on

the amount of 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). The larger this

capacitance, the worse the gain peaking, resulting in pulse

overshoot and possible instability. Reduce this capacitance

by removing the ground plane under traces connected to -

IN, and keep connections to -IN as short as possible.

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 425MHz (for AV = +2). By decreasing RS as

CL increases (as illustrated in 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 = 50Ω, CL = 22pF,

the overall bandwidth is limited to 230MHz, and bandwidth

drops to 80MHz at AV = +2, RS = 7Ω, CL = 390pF.

50

AV = +2

40

30

20

10

0

0

50 100 150 200 250 300 350 400

LOAD CAPACITANCE (pF)

FIGURE 1. RECOMMENDED SERIES OUTPUT RESISTOR vs

LOAD CAPACITANCE

Evaluation Board

The performance of the HFA1205 may be evaluated using

the HA5023 Evaluation Board.

The performance of the HFA1205IB (SOIC) may be

evaluated using the HA5023 Evaluation Board and a SOIC

to DIP adaptor like the Aries Electronics Part Number

08-350000-10.

The schematic for amplifier 1 and the board layout are

shown in Figure 2 and Figure 3. Resistors RF, RG and RS

may require a change to the appropriate value (see

“Optimum Feedback Resistor” section) for the gain being

evaluated.

To order evaluation boards (Part Number HA5023EVAL),

please contact your local sales office.

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