AD818

REV. B

–10–

–IN

+IN

NULL 1

NULL 8

OUTPUT

Figure 34. AD818 Simplified Schematic

THEORY OF OPERATION

The AD818 is a low cost, video operational amplifier designed

to excel in high performance, high output current video

applications.

The AD818 (Figure 34) consists of a degenerated NPN differ-

ential pair driving matched PNPs in a folded-cascode gain stage.

The output buffer stage employs emitter followers in a class AB

amplifier which delivers the necessary current to the load, while

maintaining low levels of distortion.

The AD818 will drive terminated cables and capacitive loads of

10 pF or less. As the closed-loop gain is increased, the AD818

will drive heavier cap loads without oscillating.

INPUT CONSIDERATIONS

circuits where the input to the AD818 will be subjected to tran-

sient of continuous overload voltages exceeding the

±6 V maxi-

mum differential limit. This resistor provides protection for the

input transistors by limiting their maximum base current.

For high performance circuits, it is recommended that a “bal-

ancing” resistor be used to reduce the offset errors caused by

bias current flowing through the input and feedback resistors.

terminal. The offset voltage error will then be reduced by more

than an order of magnitude.

GROUNDING AND BYPASSING

When designing high frequency circuits, some special precau-

tions are in order. Circuits must be built with short interconnect

leads. When wiring components, care should be taken to pro-

vide a low resistance, low inductance path to ground. Sockets

should be avoided, since their increased interlead capacitance

can degrade circuit bandwidth.

Feedback resistors should be of low enough value (

≤1 kΩ) to

assure that the time constant formed with the inherent stray

capacitance at the amplifier’s summing junction will not limit

performance. This parasitic capacitance, along with the parallel

may result in peaking. A small capacitance (1–5 pF) may be

used in parallel with the feedback resistor to neutralize this

effect.

Power supply leads should be bypassed to ground as close as

possible to the amplifier pins. Ceramic disc capacitors of

0.1

µF are recommended.

10k

1

7

6

8

4

3

2

AD818

Figure 35. Offset Null Configuration

OFFSET NULLING

The input offset voltage of the AD818 is inherently very low.

However, if additional nulling is required, the circuit shown in

Figure 35 can be used. The null range of the AD818 in this

configuration is

±10 mV.

SINGLE SUPPLY OPERATION

Another exciting feature of the AD818 is its ability to perform

well in a single supply configuration. The AD818 is ideally

suited for applications that require low power dissipation and

high output current.

Referring to Figure 36, careful consideration should be given to

the proper selection of component values. The choices for

this particular circuit are: R1 + R3 R2 combine with C1 to

form a low frequency corner of approximately 10 kHz. C4 was

inserted in series with R4 to maintain amplifier stability at high

frequency.

Combining R3 with C2 forms a low pass filter with a corner

frequency of approximately 500 Hz. This is needed to maintain

the input divider. The values for R2 and C2 were chosen to

demonstrate the AD818’s exceptional output drive capability. In

this configuration, the output is centered around 2.5 V. In order

to eliminate the static dc current associated with this level, C3

R2

3.3k

R1

3.3k

R3

100

C2

3.3 F

C1

0.01 F

C4

0.001 F

R4

1k

3

2

AD818

7

3.3 F

0.01 F

4

SELECT C1, R1, R2

FOR DESIRED LOW

FREQUENCY CORNER.

6

C3

0.1 F

150

1k

Figure 36. Single Supply Amplifier Configuration