AD831APZ datasheet(9/16 Pages) AD

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AD831

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AD831

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For gains other than unity, the amplifier’s output at OUT is

connected via an attenuator network to VFB; this determines

the overall gain. Using resistors R1 and R2 (Figure 6), the gain

setting expression is

dB =

ËÁ

¯˜

log

(2)

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Figure 6. Output Amplifier Feedback Connections

for Increasing Gain

Driving Filters

The output amplifier can be used for driving reverse-terminated

loads. When driving an IF band-pass filter (BPF), for example,

proper attention must be paid to providing the optimal source

and load terminations so as to achieve the specified filter response.

The AD831’s wideband highly linear output amplifier affords an

opportunity to increase the RF to IF gain to compensate for a

filter’s insertion and termination losses.

Figure 7 indicates how the output amplifier’s low impedance

(voltage source) output can drive a doubly terminated band-pass

filter.The typical 10 dB of loss (4 dB of insertion loss and 6 dB

due to the reverse-termination) be made up by the inclusion of a

feedback network that increases the gain of the amplifier by

10 dB (3.162).When constructing a feedback circuit, the signal

path between OUT and VFB should be as short as possible.

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Figure 7. Connections for Driving a DoublyTerminated

Band-Pass Filter

Higher gains can be achieved, using different resistor ratios, but

with concomitant reduction in the bandwidth of this amplifier

(Figure 8). Note also that the Johnson noise of these gain setting

resistors, as well as that of the BPF terminating resistors, is ulti-

mately reflected back to the mixer’s input; thus they should be as

small as possible, consistent with the permissible loading on the

amplifier’s output.

FREQUENCY (MHz)

1000

100

G = 1

G = 2

G = 4

Figure 8. Output Amplifier 1 dB Compression

Point for Gains of 1, 2, and 4 (Gains of 0 dB, 6 dB,

and 12 dB, Respectively)

REV. C

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