OP275GSZ datasheet(9/12 Pages) AD

9 / 12 page

OP275

–8–

OP275

–9–

+15V

0.1F

VIN

VOUT

2k

–15V

10F

0.1F

1/2

OP275

10F

–

Figure 12. Unity Gain Follower

0.1F

+15V

10F

VIN

VOUT

2k

–15V

10F

0.1F

10pF

4.99k

2.49k

4.99k

1/2

OP275

–

Figure 13. Unity Gain Inverter

In inverting and noninverting applications, the feedback resis-

tance forms a pole with the source resistance and capacitance

(RS and CS) and the OP275’s input capacitance (CIN), as shown

in Figure 14. With RS and RF in the kilohm range, this pole

can create excess phase shift and even oscillation. A small

capacitor, CFB, in parallel and RFB eliminates this problem.

By setting RS (CS + CIN) = RFBCFB, the effect of the feedback

pole is completely removed.

16V–20V

0.1F

V+

5V

1k

+15V

2N4416

1k

OUTPUT

(TO SCOPE)

1F

10k

IC2

2k

750

2N2222A

15k

–15V

1N4148

DUT

1/2 OP260AJ

16V–20V

0.1F

10k

–

SCHOTTKY DIODES D1–D4 ARE

HEWLETT-PACKARD HP5082-2835

IC1 IS 1/2 OP260AJ

IC2 IS PMI OP41EJ

V–

222

–

Figure 11. OP275’s SettlingTimeTest Fixture

RFB

CIN

CFB

VOUT

–

Figure 14. Compensating the Feedback Pole

Attention to Source Impedances Minimizes Distortion

Since the OP275 is a very low distortion amplifier, careful atten-

tion should be given to source impedances seen by both inputs.

As with many FET-type amplifiers, the p-channel JFETs in the

OP275’s input stage exhibit a gate-to-source capacitance that var-

ies with the applied input voltage. In an inverting configuration,

the inverting input is held at a virtual ground and, as such, does

not vary with input voltage.Thus, since the gate-to-source voltage

is constant, there is no distortion due to input capacitance modu-

lation. In noninverting applications, however, the gate-to-source

voltage is not constant.The resulting capacitance modulation

can cause distortion above 1 kHz if the input impedance is

greater than 2 k and unbalanced.

OP275

VIN

VOUT

RS*

RS = RG//RF IF RG//RF > 2k

FOR MINIMUM DISTORTION

–

Figure 15. Balanced Input Impedance to Minimize

Distortion in Noninverting Amplifier Circuits

Figure 15 shows some guidelines for maximizing the distortion

performance of the OP275 in noninverting applications.The best

way to prevent unwanted distortion is to ensure that the parallel

combination of the feedback and gain setting resistors (RF and

RG) is less than 2 k. Keeping the values of these resistors small

has the added benefits of reducing the thermal noise of the circuit

REV. C

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