AD5539 Ver la hoja de datos (PDF) - Analog Devices

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Fabricante

AD5539

Operational Amplifier Ultrahigh Frequency

Analog Devices 

AD5539

Figure 10. Full Power Response

Figure 11. Deviation from Ideal Gain

vs. Closed-Loop Voltage Gain

Figure 12. AD5539 Circuit

FUNCTIONAL DESCRIPTION

The AD5539 is a two-stage, very high frequency amplifier.

Darlington input transistors Q1, Q4–Q2, Q3 form the first

stage—a differential gain amplifier with a voltage gain of ap-

proximately 50. The second stage, Q5, is a single-ended ampli-

fier whose input is derived from one phase of the differential

amplifier output; the other phase of the differential output is

then summed with the output of Q5. The all NPN design of the

AD5539 is configured such that the emitter of Q5 is returned,

via a small resistor to ground; this eliminates the need for sepa-

rate level shifting circuitry.

The output stage, consisting of transistors Q9 and Q10, is a

Darlington voltage follower with a resistive pull-down. The bias

section, consisting of transistors Q6, Q7 and Q8, provides a

stable emitter current for the input section, compensating for

temperature and power supply variations.

SOME GENERAL PRINCIPLES OF HIGH FREQUENCY

CIRCUIT DESIGN

In designing practical circuits with the AD5539, the user must

remember that whenever very high frequencies are involved,

some special precautions are in order. All real-world applica-

tions circuits must be built using proper RF techniques: the use

of short interconnect leads, adequate shielding, groundplanes,

and very low profile IC sockets. In addition, very careful bypass-

ing of power supply leads is a must.

Low-impedance transmission line is frequently used to carry sig-

nals at RF frequencies: 50 Ω line for telecommunications pur-

poses and 75 Ω for video applications. The AD5539 offers a

relatively low output impedance; therefore, some consideration

must be given to impedance matching. A common matching

technique involves simply placing a resistor in series with the

amplifier output that is equal to the characteristic impedance of

the transmission line. This provides a good match (although at a

loss of 6 dB), adequate for many applications.

All of the circuits here were built and tested in a 50 Ω system.

Care should be taken in adapting these circuits for each particu-

lar use. Any system which has been properly matched and ter-

minated in its characteristic impedance should have the same

small signal frequency response as those shown in this

data sheet.

–6–

REV. B

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