Mid-band Gain
e
R
1
a
150 k
X
150 k
X
(3)
TL/H/7380 – 15
*
For Stability with High Current Loads
FIGURE 13. RIAA Phono Amplifier
Capacitor
C
1
sets
the
corner
frequency
f
2
where
R
1
e
X
C1
.
C
1
e
1
2
q
f
2
R
1
(4)
Capacitor C
2
establishes the corner frequency f
3
where X
C2
equals the impedance of the inverting input. This is normally
150 k
X
. However, in the circuit of
Figure 13 negative feed-
back reduces the impedance at the inverting input as:
Z
e
Z
o
1
a
A
o
b
(5)
Where:
Z
o
e
impedance at node 6 without external feedback
(150 k
X
)
A
o
e
gain without external feedback (50)
b
e
feedback transfer function
b
e
A
o
b
A
A
o
A
A
e
closed loop gain with external feedback.
Therefore
C
2
e
1
2
q
f
3
#
Z
o
1
a
A
o
b
J
e
1
2
q
f
3
#
150k
1
a
50
b
J
(6)
BRIDGE AMPLIFIER
Where more power is desired than can be provided with one
amplifier, two amps may be used in the bridge configuration
shown in
Figure 14 .
TL/H/7380 – 16
*
For Stability with High Current Loads
FIGURE 14. Bridge Configuration
This provides twice the voltage swing across the load for a
given supply, thereby, increasing the power capability by a
factor of four over the single amplifier. However, in most
cases the package dissipation will be the first parameter
limiting power delivered to the load. When this is the case,
the power capability of the bridge will be only twice that of
TL/H/7380 – 17
FIGURE 15A. 8
X
Load
the single amplifier.
Figures 15A and B show output power
versus device package dissipation for both 8 and 16
X
loads
in the bridge configuration. The 3% and 10% harmonic
TL/H/7380 – 18
FIGURE 15B. 16
X
Load
distortion contours double back due to the thermal limiting
of the LM380. Different amounts of heat sinking will change
the point at which the distortion contours bend.
The quiescent output voltage of the LM380 is specified at 9
g
1 volts with an 18 volt supply. Therefore, under the worst
case condition, it is possible to have two volts DC across
the load.
TL/H/7380 – 19
*
For Stability with High Current Loads
FIGURE 16. Quiescent Balance Control
With an 8
X
speaker this 0.25A which may be excessive.
Three alternatives are available; 1) care can be taken to
match the quiescent voltages, 2) a non-polar capacitor may
be placed in series with the load, 3) the offset balance con-
trol of
Figure 16 may be used.
5
