50029DOC.pdf

(149 KB) Pobierz
Using the LC506 and
LC549 Amplifiers
LC506 / LC549 APPLICATION NOTE
PURPOSE OF THIS APPLICATION NOTE
This application note describes circuits which utilize the
LC549 low distortion push-pull amplifier and its matching
preamplifier LC506. Specific attention has been given to
realizing hearing aid circuits for the medium and high
power ear level use. Methods of implementing tone controls,
preset gain and maximum power output adjustment have
been described. Typical acoustic data is presented for a
variety of configurations.
INTRODUCTION
The general purpose circuit incorporating infinite volume
control range and gain trim independent of volume control
shown in Figure 1 serves to illustrate the functions of the
LC506 and LC549.
The LC506 preamplifier contains a voltage regulator which
effectively removes battery line signals and supplies bias
for external circuitry such as a buffered electret microphone.
Since the LC506 is configured as a opamp with negative
feedback, gain trim is accomplished by controlling the AC
impedance between pin 1 and ground, where minimum
impedance corresponds to maximum gain. To obtain
infinite volume control range, an interstage potentiometer
arrangement is used.
The LC549 output amplifier internally phase splits the
preamp signal and feeds a Class B output transistor pair.
Negative feedback, sensed from the output terminals,
allows the LC549 to operate at a very low quiescent current
while maintaining low distortion. The capacitor across the
output ensures stability, while the decoupling capacitors
connected to pins 1 and 4 maintain bandwidth and remove
unwanted common battery line signals. The LC549 can
drive a wide range of load impedances.
For maximum power output of about 6 mW with a 1.3 V
supply, a transformer-coupled load which reflects 300Ω
across pins 5 to 8 is recommended. In high power hearing
aid applications, this represents a peak undistorted output
in excess of 135 dB SPL. By substituting a 3 kΩ receiver,
the hearing aid output can be reduced to 124 dB SPL with
resultant reduction in peak current demand. For general
purpose applications, the transformer may be replaced by
two 100
Ω
resistors connected between +V
B
and pins 5
and 8. The resulting balanced low impedance output can
be used to feed signals over a long cable with a minimum
of noise pick up. In this application, the terminating load
should be balanced centre-tapped transformer or opamp
connected in a differential input configuration.
As the gain between the LC506 and LC549 is divided
almost equally, noise with minimum volume control setting
is lower than with arrangements where the majority of the
gain is in the output stage. Further, since the LC506 output
and the LC549 input are single ended, simple tone networks
can be incorporated at the interstage point.
+
6.8
+
V
B
= 1.3 VDC
LC506
3
4
6.8
LC549
4
6
5
REGULATOR
V
OUT
0.1
2
V
IN
Source
+
PREAMP
1
6
0.1
0.1
2
N/C
OUTPUT
DRIVER
3
2.2
230
0.068
Load
470
Typ.
30K
Gain
Trim
10K
5
Volume
Control
10K
1
7
8
6.8
+
All resistors in ohms, all capacitors in µF unless otherwise stated
Fig.1 LC506 / LC549 Bread Board Circuit
Document No. 500 - 29 - 2
GENNUM CORPORATION P.O. Box 489, Stn A, Burlington, Ontario, Canada L7R 3Y3
Japan Branch: A-302, Miyamae Village, 2-10-42 Miyamae, Suginami-ku, Tokyo 168, Japan
tel. (905) 632-2996 fax: (905) 632-5946
tel. (03) 3334-7700
fax: (03) 3247-8839
+
4.7
6.8
N/C
MPO
Trim
V
B
= 1.3 VDC
0.1
Mic
+
2
1
3
4
3
6
0.1
0.1
2
4
5
LC506
5
6
LC/LD549
1
7
8
0.047
470
Typ.
2.2
GT
10K
V
C
= 10K
+
6.8
+
6.8
* 1.0µF
C
M
All resistors in ohms, all capacitors in µF unless otherwise stated
* Capacitor C
M
will improve stability margin when MPO trim resistor is incorporated.
Fig.2 LC506 / LC549 Typical Hearing Aid Circuit
HEARING AID CIRCUITS
HIGH POWER HEARING AID AMPLIFIER
Using a receiver with an impedance of nominally 470
Ω
the
circuit of Figure 2 will provide a peak output of approximately
135 dB SPL with V
B
at 1.3 V. This value can be reduced
with a simple MPO trim resistor in series with the receiver
centre-tap, which effectively reduces the available output
collector voltage swing without affecting the gain below
clipping.
A resistor of 4.7
Ω
is placed in series with the battery line
only for the purpose of simulating internal battery resistance.
The acoustic performance of the circuit of Figure 2 is
outlined in Table 1. The frequency response of the circuit
is shown in Figure 3, where the input is 55 dB SPL and
circuit gain is maximum.
For a change in V
B
from 2.0 to 1.1 V, the amplifier performs
well with little change in performance. Below 1.1 V, the
LC506 regulator is no longer able to maintain its output
voltage, and the amplifier shuts off.
TABLE 1
PARAMETER
Acoustic Gain
Electrical Gain
Gain Trim
Gain Dependence on V
B
Acoustic Total Harmonic Distortion
Maximum Acoustic Output
Similarly, stability and low distortion is maintained even
with significant internal battery impedances: for an increase
from 0Ω to 20Ω the total harmonic distortion at 1 kHz with
50 dB SPL input only increases from 0.95 to 1.05% while
the amplifier remains very stable.
0
RELATIVE ACOUSTIC RESPONSE (dB)
-10
-20
-30
-40
-50
100
1K
10K
FREQUENCY (Hz)
Fig.3 LC506 / LC549 Acoustic Response
VALUE
77.2
77.3
31
1.0
0.95
130.8
134.7
UNITS
dB
dB
dB
dB/V
%
dB SPL
dB SPL
dB
dBSPL
CONDITIONS
50 dB SPL input at 1 kHz
50 dBSPL input at 1 kHz
50 dBSPL input at 1 kHz
V
B
varied from 1.1 to 1.5 V
50 dBSPL input at 1 kHz
SSPL 90 HF Average
7% THD at Receiver Peak Output Frequency
Control varied from 1
Ω
to 3.3
Ω
"A" weighted at Maximum Gain
MPO Control Range
Acoustic Input Referred Noise
23
8
2
500 - 29 - 2
TONE CONTROL SCHEMES
The single-ended nature of the interconnection between
the LC506 and the LC549 allows the use of simple tone
networks at the interstage point to implement low frequency
cut. It is also possible to use a low network at the interstage
point to implement low frequency cut. It is also possible to
use a low network at the front end of the aid between the
microphone and the LC506, but some performance
degradation will result. A comparison of these two methods
is as follows:
• Front End Tone Control:
Advantage
- No interaction with volume control
Disadvantage - Does not reduce noise bandwidth of
preamp, but reduces mic noise bandwidth
• Interstage Tone Control:
Advantage
- Reduces both mic and preamp noise
bandwidth
Disadvantage - Breakpoint frequency depends somewhat
on volume control setting
Realizations of these two schemes are shown in Figures 4
and 5. As their effects are very similar, a typical response
plot for both is shown in Figure 6, demonstrating the
+
change in low frequency rolloff achieved with the tone
control at its two extremes.
In Figure 4, a front-end tone control is used. The tone
control (R1) can be directly coupled to the microphone
output as long as the total resistance to ground in approximately
50Ω or more. The DC current drain imposed by such a load
is not high enough to upset the internal microphone bias.
In the minimum low frequency cut position, C1 is shorted
out by R1’s wiper, and C2 along with the microphone
output impedance plus the LC506 input impedance, determine
the low frequency -3 dB breakpoint at around 210 Hz. For
maximum cut, two breakpoints are in effect. The first, due
to C1, becomes the dominant one, and is determined
primarily by R1’s track resistance in parallel with the series
combination of R2 and the microphone output impedance.
However, the combination of C2’s reactance and the LC506
input impedance lowers the effective impedance from R2
to ground yielding a breakpoint frequency of approximately
1.2 kHz. The breakpoint due to C2 now shifts slightly
higher, largely due to C1’s presence.
4.7
V
B
= 1.3 VDC
6.8
C2
0.022
2
1
N/C
C1
0.0068
Mic
R1
50K
R2
18K
3
4
3
6
0.1
5
LC506
5
6
0.1
R
VC
=
10K
2
4
LC/LD549
1
7
8
0.047
470
Typ.
+
Tone
2.2
Control
GT
10K
0.068
+
6.8
+
6.8
All resistors in ohms, all capacitors in µF unless otherwise stated
Fig.4 Front End Tone Control
+
4.7
6.8
C2
0.022
N/C
V
B
= 1.3 VDC
3
2
1
4
3
6
0.1
Mic
+
0.033
LC506
5
6
C1
0.0068
2
5
LC/LD549
4
1
7
8
R
VC
=
10K
R1
50K
R2
18K
0.047
470
Typ.
2.2
GT
10K
C2
0.022
Tone
Control
+
6.8
+
6.8
All resistors in ohms, all capacitors in µF unless otherwise stated
Fig.5 Interstage Tone Control
3
500 - 29 - 2
In Figure 5, the same tone control configuration is used at
the interstage point, but is preceded now by a volume
control which presents a variable source impedance.
Fortunately, The effect of the impedance looking into
RVC’s wiper on C1’s breakpoint is swamped somewhat by
R1 and R2. However, its effect is most noticeable in the
minimum cut case where the source impedance change,
seen by C2, is the most pronounced, but is masked to a
degree by the LC549 input impedance.
In both Figures 4 and 5, the volume control configuration
used is the most desirable, as it yields infinite volume
control range, independent of gain trim settings.
High frequency rolloff is accomplished with the capacitor
shown in dashed lines in both Figures 4 and 5. In each
case, a high frequency -3 dB breakpoint has been set to
approximately 1.2 kHz. Its effect on frequency response is
plotted in Figure 7.
Note that the capacitor value is different in the two circuits,
as the source impedance seen by the capacitor in Figure
4 is the LC506 output impedance, and in Figure 5, the
capacitor sees the microphone output impedance.
The two configurations were compared on the basis if
output noise with a white noise input representing microphone
noise. The gain trim and volume control were set for
maximum gain in each case and the tone control was
varied between maximum and minimum cut.
A variation of the circuit of Figure 5 is shown in Figure 8,
where the tone control and volume control positions are
reversed. The advantage of this configuration is that the
tone control sees relatively small changes in impedance
with volume control changes, as the volume control track
resistance is small relative to the LC549 input impedance.
The disadvantage is a higher component count due to the
extra coupling capacitor.
RELATIVE ACOUSTIC RESPONSE (dB)
0
0
RELATIVE ACOUSTIC RESPONSE (dB)
1K
10K
-10
-10
-20
-20
-30
-30
-40
-40
-50
100
-50
100
1K
10K
FREQUENCY (Hz)
FREQUENCY (Hz)
Fig.6 Tone Control Adjustment
Fig.7 High Frequency Control
TABLE 2
White Noise Input
= 24 dB SPL
Minimum Cut
Maximum Cut
Figure 4 Circuit
Output Noise (dBA)
92.2
90.5
Figure 5 Circuit
Output Noise (dBA)
92.2
87.8
Table 2 shows that the Figure 5 circuit is superior in terms of noise reduction
with tone setting, as expected
4
500 - 29 - 2
+
4.7
6.8
C2
0.022
C2
C1
0.1 0.01 0.068
6
V
B
= 1.3 VDC
N/C
3
2
1
4
3
6
0.1
2
5
Mic
+
0.033
LC506
5
LC/LD549
4
1
7
8
R
VC
=
10K
R1
50K
R2
18K
Tone
Control
0.047
470
Typ.
2.2
GT
10K
+
6.8
+
6.8
All resistors in ohms, all capacitors in µF unless otherwise stated
Fig.8 Alternate Tone Control Method
PERFORMANCE WITH HIGH IMPEDANCE RECEIVERS
The circuit of Figure 2 was used to drive both a miniature
2900
Ω
design and a standard-sized 3690
Ω,
3 terminal
receiver. With the input adjusted to give 105 dB SPL at
1 kHz in each case, the frequency responses were plotted
for each in Figure 9.
As Figure 9 was plotted using a 2 cm
3
coupler and tubing,
caution should be used in interpretation of this graph.
While the 2 cm
3
coupler is readily available and simple to
use, it does not give an accurate representation of hearing
aid response at high frequencies. Other available measurement
techniques may in fact show a greater difference in the
performance of the two receivers.
CONCLUSIONS AND RECOMMENDATIONS
The LC506 / LC549 combination provides the hearing aid
designer with a versatile amplification system capable of
giving stable, low distortion operation in both high and
medium power aids, and over a wide range of battery
voltages. The circuit is stable over a wide range of battery
impedances.
Two methods of incorporating tone control have been
suggested as illustrated in Figures 4 and 5. The main
difference between the two is noise performance, with the
circuit in Figure 5 showing the greatest noise reduction
with maximum low frequency tone cut.
0
RELATIVE ACOUSTIC RESPONSE (dB)
-10
-20
2900Ω
-30
-40
3690Ω
-50
100
1K
10K
FREQUENCY (Hz)
Fig.9 Frequency Response using High Impedance Receivers
Figure 8 illustrates a version of Figure 5 which yields lower
interaction between volume and tone controls, but is higher
in parts count. The volume control scheme suggested in
each case is believed to be the optimum, as it provides
infinite volume control range independent of the gain trim
setting. Despite the slight interaction between volume and
tone controls, it is believed that the Figure 5 circuit is the
most desirable.
Gennum Corporation assumes no responsibility for the use of any circuits described herein and makes no representations that they are free from patent infringement.
© Copyright March 1981 Gennum Corporation .
Revision Date: November 1991.
All rights reserved.
Printed in Canada.
5
500 - 29 - 2

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin