OP275_15 Datasheet, PDF (9/12 Pages) Analog Devices – Dual Bipolar/JFET, Audio Operational Amplifier

OP275
16V–20V
–+
0.1F
V+
DUT
V–
0.1F
+–
16V–20V
5V
R_L
1k
D1
1N4148
+15V
1k
D3
2N4416 _{1/2 OP260AJ}
OUTPUT
D4 ^{(TO SCOPE)}
D2
R_F
2k
R_G
222
2N2222A
10k
10k
750
1F
–
IC2
+
15k
–15V
SCHOTTKY DIODES D1–D4 ARE
HEWLETT-PACKARD HP5082-2835
IC1 IS 1/2 OP260AJ
IC2 IS PMI OP41EJ
Figure 11. OP275’s Settling Time Test Fixture
+15V
0.1F
10F
+
C_FB
R_FB
2
–
8
1/2
1
V_IN
3 ^OP275
+
4 0.1F
10F
+
R_L
2k
V_OUT
–
R_S
C_S
C_IN
+
V_OUT
Figure 14. Compensating the Feedback Pole
–15V
Figure 12. Unity Gain Follower
+15V
0.1F
10F
+
10pF
V_IN
4.99k
2.49k
2_– 8
1/2
1
3
OP275
+
4
10F
0.1F
+
4.99k
2k
V_OUT
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
–
–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_FBC_FB, the effect of the feedback
pole is completely removed.
R_S*
OP275
V_IN
+
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
REV. C
–9–