OP179 Datasheet, PDF (11/16 Pages) Analog Devices – Rail-to-Rail High Output Current Operational Amplifiers

OP179/OP279
A Single Supply Headphone Amplifier
Because of its high speed and large output drive, the OP179/
OP279 makes for an excellent headphone driver, as illustrated
in Figure 34. Its low supply operation and rail-to-rail inputs
and outputs give a maximum signal swing on a single +5 V
supply. To ensure maximum signal swing available to drive the
headphone, the amplifier inputs are biased to V+/2, which is in
this case 2.5 V. The 100 kΩ resistor to the positive supply is
equally split into two 50 kΩ with their common point bypassed
by 10 µF to prevent power supply noise from contaminating the
audio signal.
+V + 5V
50k⍀
LEFT
INPUT
50k⍀
10␮F
10␮F
100k⍀
+V + 5V
1/2
OP279
16⍀ ^220␮F
50k⍀
LEFT
HEADPHONE
+V
50k⍀
50k⍀
RIGHT
INPUT
10␮F
10␮F
100k⍀
1/2
OP279
16⍀ ^220␮F
50k⍀
RIGHT
HEADPHONE
Figure 34. A Single Supply, Stereo Headphone Driver
The audio signal is then ac-coupled to each input through a
10 µF capacitor. A large value is needed to ensure that the
20 Hz audio information is not blocked. If the input already has
the proper dc bias, the ac coupling and biasing resistors are not
required. A 220 µF capacitor is used at the output to couple the
amplifier to the headphone. This value is much larger than that
used for the input because of the low impedance of the head-
phones, which can range from 32 Ω to 600 Ω. An additional
16 Ω resistor is used in series with the output capacitor to pro-
tect the op amp’s output stage by limiting capacitor discharge
current. When driving a 48 Ω load, the circuit exhibits less than
0.02% THD+N at low output drive levels (not shown). The
OP179/OP279’s high current output stage can drive this heavy
load to 4 V p-p and maintain less than 1% THD+N.
Active Filters
Several active filter topologies are useful with the OP179/OP279.
Among these are two popular architectures, the familiar Sallen-
Key (SK) voltage controlled voltage source (VCVS) and the
multiple feedback (MFB) topologies. These filter types can be
arranged for high pass (HP), low pass (LP), and bandpass (BP)
filters. The SK filter type uses the op amp as a fixed gain voltage
follower at unity or a higher gain, while the MFB structure uses
it as an inverting stage. Discussed here are simplified, 2-pole
forms of these filters, highly useful as system building blocks.
UNITY-GAIN, SALLEN-KEY (VCVS) FILTERS
High Pass Configurations
In Figure 35a is the HP form of a unity-gain 2-pole SK filter
using an OP179/OP279 section. For this filter and its LP coun-
terpart, the gain in the passband is inherently unity, and the
signal phase is noninverting due to the follower hookup. For
simplicity and practicality, capacitors C1-C2 are set equal, and
resistors R2-R1 are adjusted to a ratio “N,” which provides the
filter damping “α” as per the design expressions. A HP design
is begun with selection of standard capacitor values for C1 and
C2 and a calculation of N; then R1 and R2 are calculated as per
the figure expressions.
In these examples, α (or 1/Q) is set equal to √2, providing a
Butterworth (maximally flat) response characteristic. The filter
corner frequency is normalized to 1 kHz, with resistor values
shown in both rounded and (exact) form. Various other 2-pole
response shapes are also possible with appropriate selection of
α. For a given response type (α), frequency can be easily scaled,
using proportional R or C values.
C1
0.01␮F
IN
C2
0.01␮F
R2
22k⍀
(22.508k⍀)
R1
11k⍀
(11.254k⍀)
IN
R2
11k⍀
(11.254k⍀)
C2
0.01␮F
R1
11k⍀
(11.254k⍀)
+V_{S U1A}
3 ₈ OP279
1
2
4
–V_S
R = R2
0.1␮F
Z_f (HIGH PASS)
OUT
GIVEN: ALPHA, F
SET C1 = C2 = C
ALPHA = 2/(N^0.5) = 1/Q
N = 4/(ALPHA)^2 = R2/R1
R1 = 1/(2*PI*F*C* (N^0.5))
R2 = N*R1
1kHz BW SHOWN
a. High Pass
C1
0.02␮F
OUT
U1B
5 ^OP279
7
6
GIVEN: ALPHA, F
SET R1 = R2 = R
ALPHA = 2/(M^0.5) = 1/Q
N = 4/(ALPHA)^2 = C2/C1
R = R1+R2
0.1␮F
PICK C1
C1 = M*C1
R = 1/(2*P1*F*C1* (M^0.5))
1kHz BW SHOWN
Z_f (LOW PASS)
b. Low Pass
Figure 35. 2-Pole Unity-Gain Sallen Key HP/LP Filters
Low Pass Configurations
In the LP SK arrangement of Figure 35b, R and C elements are
interchanged, and the resistors are made equal. Here the C2/C1
ratio “M” is used to set the filter α, as noted. This design is begun
with the choice of a standard capacitor value for C1 and a calcu-
lation of M, which forces a value of “M × C1” for C2. Then, the
value “R” for R1 and R2 is calculated as per the expression.
For highest performance, the passive components used for tun-
ing active filters deserve attention. Resistors should be 1%, low
TC, metal film types of the RN55 or RN60 style, or similar.
REV. F
–11–