ADA4856-3

Data Sheet

Rev. B | Page 14 of 20

USING THE ADA4856-3 IN GAINS EQUAL TO +3,

+4, AND +5

Depending on certain applications, it might be useful to have a

fixed gain amplifier that can provide various gains. The advantage

of having a fixed gain amplifier is the ease of layout, the reduced

number of components needed, and the matching of the gain

and feedback resistors.

Gain of +3 Configuration

Figure 38 shows the ADA4856-3 used as an amplifier with a

fixed gain of +3. No external resistors are required, just a simple

trace connecting certain inputs and outputs. Connect VIN to U1,

which is set to a gain of +2, and U2, which is set to unity. U3 then

takes the output of U1 and gains it up by +2 and subtracts the

output of U2 to produce VOUT. As shown in Figure 41, the large

signal frequency response for G = +3 is flat out to 65 MHz, with a

bandwidth of 165 MHz, a 2 V p-p output voltage, and a 100 Ω load.

ADA4856-3

PD

16

15

14

13

5

6

7

12

11

10

9

1

2

3

4

0.1µF

0.1µF

0.1µF

10µF

0.1µF

+

10µF

–VS

–VS

+VS

+VS

VOUT

VIN

8

Figure 38. Gain of +3

Gain of +4 Configuration

To get a gain of +4, set one amplifier to a gain of +1 and set the

other two amplifiers to a gain of +2. Figure 39 shows VIN going in

U2 at unity, then U1 takes the output of U2 and gains it by +2, and

then feeds it to U3, which also gains it by +2 to produce VOUT.

ADA4856-3

PD

16

15

14

13

5

6

7

12

11

10

9

1

2

3

4

0.1µF

0.1µF

0.1µF

10µF

0.1µF

+

10µF

–VS

–VS

+VS

+VS

VOUT

VIN

8

Figure 39. Gain of +4

As shown in Figure 41, the large signal frequency response

for G = +4 is also flat out to 65 MHz, and it has a bandwidth

of 180 MHz.

Gain of +5 Configuration

The gain of +5 is very similar to the G = +3 configuration but with

U2 set to a gain of −1, which ends up being added to twice the

output of U1 to generate VOUT with G = +5.

ADA4856-3

PD

16

15

14

13

5

6

7

12

11

10

9

1

2

3

4

0.1µF

0.1µF

0.1µF

10µF

0.1µF

+

10µF

–VS

–VS

+VS

+VS

VOUT

VIN

8

Figure 40. Gain of +5

Figure 41 shows the large signal frequency response of the three

closed-loop gain sets (+3, +4, and +5) with flatness that extends

to 65 MHz and a −3 dB bandwidth of 190 MHz.

–18

–15

–12

–9

–6

–3

0

3

6

9

12

15

1

10

100

1000

FREQUENCY (MHz)

G = +5

G = +4

G = +3

RL = 100Ω

VS = 5V

VOUT = 2V p-p

Figure 41. Large Signal Frequency Response for All Three Gains