OP249

–11–

REV. C

The OP249 was carefully designed to provide symmetrically

matched slew characteristics in both the negative and positive

directions, even when driving a large output load.

An amplifier’s slewing limitation determines the maximum

frequency at which a sinusoidal output can be obtained without

significant distortion. It is, however, important to note that the

nonsymmetric stewing typical of previously available JFET

amplifiers adds a higher series of harmonic energy content to

the resulting response—and an additional dc output compo-

nent. Examples of potential problems of nonsymmetric

slewing behavior could be in audio amplifier applications,

where a natural low distortion sound quality is desired, and in

servo or signal processing systems where a net dc offset cannot

be tolerated. The linear and symmetric stewing feature of the

OP249 makes it an ideal choice for applications that will exceed

the full-power bandwidth range of the amplifier.

As with most JFET-input amplifiers, the output of the OP249

may undergo phase inversion if either input exceeds the speci-

fied input voltage range. Phase inversion will not damage the

amplifier, nor will it cause an internal latch-up condition.

Supply decoupling should be used to overcome inductance and

resistance associated with supply lines to the amplifier. A 0.1

µF

and a 10

µF capacitor should be placed between each supply

pin and ground.

OPEN-LOOP GAIN LINEARITY

The OP249 has both an extremely high open-loop gain of

1 kV/mV minimum and constant gain linearity. This feature of

the OP249 enhances its dc precision, and provides superb accu-

racy in high closed-loop gain applications. Figure 43 illustrates

the typical open-loop gain linearity—high gain accuracy is as-

sured, even when driving a 600

Ω load.

OFFSET VOLTAGE ADJUSTMENT

The inherent low offset voltage of the OP249 will make offset

adjustments unnecessary in most applications. However, where

a lower offset error is required, balancing can be performed with

simple external circuitry, as illustrated in Figures 44 and 45.

VERTICAL 50 V/DIV

INPUT VARIATION

HORIZONTAL 5V/DIV

OUTPUT CHARGE

Figure 43. Open-Loop Gain Linearity. Variation in Open-

Loop Gain Results in Errors in High Closed-Loop Gain

1/2

OP249

–V

+V

R3

R1

200k

R5

50k

R2

31

R4

R2

R1

Figure 44. Offset Adjust for Inverting Amplifier

Configuration

1/2

OP249

–V

+V

R1

200k

R3

50k

R2

33

R5

R2

R1

R4

1 +

R5

R4

IF R2 << R4

GAIN =

= 1 +

R5

R4 + R2

Figure 45. Offset Adjust for Noninverting Amplifier

Configuration

In Figure 44, the offset adjustment is made by supplying a small

voltage at the noninverting input of the amplifier. Resistors R1

and R2 attenuates the pot voltage, providing a

±2.5 mV (with

±15 V) adjustment range, referred to the input. Figure 45

illustrates offset adjust for the noninverting amplifier configura-

tion, also providing a

±2.5 mV adjustment range. As indicated

in the equations in Figure 45, if R4 is not much greater than R2,

there will be a resulting closed-loop gain error that must be

accounted for.