AD831

REV. B

–9–

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 22), the gain

setting expression is

R1

+ R2

R2









Equation 2

R2

R1

IF

OUTPUT

IFN

VP

IFP

AP

GND

VN

RFP

RFN

VN

VP

LON

LOP

VP

GND

BIAS

VN

AD831

Top View

50

Ω

50

Ω

6

7

8

4

5

9

10

11

12

13

AN

VFB

COM

OUT

14

15

16

17

18

1

2

3

19

20

Figure 22. Output Amplifier Feedback Connections for

Increasing Gain

Driving Filters

The output amplifier can be used for driving reverse-terminated

loads. When driving an IF bandpass filter (BPF), for example,

proper attention must be paid to providing the optimal source

and load terminations so as to achieve the specified filter re-

sponse. The AD831’s wideband highly linear output amplifier

affords an opportunity to increase the RF-to-IF gain to compen-

sate for a filter’s insertion and termination losses.

Figure 23 indicates how the output amplifier’s low impedance

(voltage source) output can drive a doubly-terminated bandpass

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 sig-

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

R2

51.1

Ω

R1

110

Ω

IF

OUTPUT

IFN

VP

IFP

AP

GND

VN

RFP

RFN

VN

VP

LON

LOP

VP

GND

BIAS

VN

AD831

Top View

50

Ω

50

Ω

6

7

8

4

5

9

10

11

12

13

AN

VFB

COM

OUT

14

15

16

17

18

1

2

3

19

20

BPF

Figure 23. Connections for Driving a Doubly-Terminated

Bandpass Filter

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

with concomitant reduction in the bandwidth of this amplifier

(Figure 24). Note also that the Johnson noise of these gain-set-

ting resistors, as well as that of the BPF terminating resistors, is

ultimately 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

12

10

0

10

1000

100

8

6

4

2

G = 1

G = 2

G = 4

Figure 24. Output Amplifier 1 dB Compression Point for

Gains of 1, 2, and 4 (Gains of 0 dB, 6 dB, and 12 dB,

Respectively)