5

Unity Gain Considerations

Unity gain selection is accomplished by floating the -Input of

the HFA1212. Anything that tends to short the -Input to

GND, such as stray capacitance at high frequencies, will

cause the amplifier gain to increase toward a gain of +2. The

result is excessive high frequency peaking, and possible

instability. Even the minimal amount of capacitance

associated with attaching the -Input lead to the PCB results

in approximately 6dB of gain peaking. At a minimum this

requires due care to ensure the minimum capacitance at the

-Input connection.

Table 1 lists five alternate methods for configuring the

HFA1212 as a unity gain buffer, and the corresponding

performance. The implementations vary in complexity and

involve performance trade-offs. The easiest approach to

implement is simply shorting the two input pins together, and

applying the input signal to this common node. The amplifier

bandwidth decreases from 430MHz to 280MHz, but

excellent gain flatness is the benefit. A drawback to this

approach is that the amplifier input noise voltage and input

offset voltage terms see a gain of +2, resulting in higher

noise and output offset voltages. Alternately, a 100pF

capacitor between the inputs shorts them only at high

frequencies, which prevents the increased output offset

voltage but delivers less gain flatness.

Another straightforward approach is to add a 620W resistor

in series with the amplifier’s positive input. This resistor and

the HFA1212 input capacitance form a low pass filter which

rolls off the signal bandwidth before gain peaking occurs.

This configuration was employed to obtain the data sheet

AC and transient parameters for a gain of +1.

Pulse Overshoot

The HFA1212 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 overshoot on the negative portion

of the output waveform (see Figure 6, Figure 9, and Figure

12). This overshoot isn’t present for small bipolar signals

(see Figure 4, Figure 7, and Figure 10) or large positive

signals (see Figure 5, Figure 8 and Figure 11).

PC Board Layout

This amplifier’s frequency response depends greatly on the

care taken in designing the PC board (PCB). 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 (10

µF) tantalum in parallel with a small value

(0.1

µF) 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.

An example of a good high frequency layout is the

Evaluation Board shown in Figure 3.

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

in the curves), the maximum bandwidth is obtained without

sacrificing stability. In spite of this, bandwidth decreases as

the load capacitance increases.

TABLE 1. UNITY GAIN PERFORMANCE FOR VARIOUS

IMPLEMENTATIONS

APPROACH

PEAKING

(dB)

BW

(MHz)

±0.1dB GAIN

FLATNESS (MHz)

Remove -IN Pin

4.5

430

21

0

220

27

Remove -IN Pin

0.5

215

15

Short +IN to -IN (e.g.,

Pins 2 and 3)

0.6

280

70

100pF Capacitor

Between +IN and -IN

0.7

290

40

HFA1212