OP275
16V–20V
–
+
+15V
0.1F
V+
R
L
1k
2N4416
D2
1/2 OP260AJ
1k
D3
D4
OUTPUT
(TO SCOPE)
V–
D1
+
–
16V–20V
5V
R
G
222
2N2222A
1N4148
15k
–15V
Figure 11. OP275’s Settling Time Test Fixture
+15V
10F
+
C
FB
R
FB
0.1F
2
V
IN
1/2
1
OP275
3
+
4 0.1F
10F
+
–
8
V
OUT
R
L
2k
–15V
Figure 12. Unity Gain Follower
+15V
10
F
+
0.1
F
10pF
V
IN
4.99k
2
1/2
OP275
3
+
4
4.99k
–
8
1
2k
V
OUT
2.49k
0.1
F
10
F
+
–15V
Figure 13. Unity Gain Inverter
In inverting and noninverting applications, the feedback resis-
tance forms a pole with the source resistance and capacitance
(R
S
and C
S
) and the OP275’s input capacitance (C
IN
), as shown
in Figure 14. With R
S
and R
F
in the kilohm range, this pole
can create excess phase shift and even oscillation. A small
capacitor, C
FB
, in parallel and R
FB
eliminates this problem.
By setting R
S
(C
S
+ C
IN
) = R
FB
C
FB
, the effect of the feedback
pole is completely removed.
REV. C
–9–
–
0.1F
+
R
F
2k
10k
10k
+
–
DUT
1F
–
IC2
+
750
SCHOTTKY DIODES D1–D4 ARE
HEWLETT-PACKARD HP5082-2835
IC1 IS 1/2 OP260AJ
IC2 IS PMI OP41EJ
–
R
S
C
S
C
IN
V
OUT
+
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.
R
G
R
F
–
R
S*
V
IN
+
OP275
V
OUT
*
R
S
= R
G
//R
F
IF R
G
//R
F
> 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 (R
F
and
R
G
) is less than 2 k
. Keeping the values of these resistors small
has the added benefits of reducing the thermal noise of the circuit
AD [ ANALOG DEVICES ]
