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參數資料
| 型號： | LM1837N |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 2 CHANNEL, AUDIO PREAMPLIFIER, PDIP18 |
| 封裝： | SOP-18 |
| 文件頁數： | 3/14頁 |
| 文件大小： | 450K |
| 代理商： | LM1837N |
Application Hints (Continued)
The turn-on delay is set by R1 and C2 delay can be approx-
imated by
Delay time t e R1C2 Ln
25
VODC
JR3R2J (13)
EXAMPLE
If we desire a tape preamp with 100 mV output signal from a
tape head with a nominal output of 05 mV at 1 kHz for
standard ferric cassette tape the external components are
determined as follows The value of R2 is arbitrarily set to
10 kX
R1 e R2 e 10k
This minimizes errors due to the output amplifier bias cur-
rents
C1 e
b
480 c 10b3
10 kX
b
100 mV
05 mV
(
e
2400 pF
x00022 mF
Use 00022 mF and determine
R4 e
1
2qC1(1326)
e
546 kX
x549 kX 1%
R3 e
1
2qC1(5196)
e
139 MX
x14 MX 1%
To bias the output amplifier output voltage at 6V (half sup-
ply)
R5 e
25(14 MX)
6 b 25
e
1MX
The maximum variation in the output is found using equation
(11)
D
VOUT e g19V
The low frequency response and turn-on delay determine
the value of C2 For R1 e 10k and C2 e 10 mF the low
frequency 3 dB point is 16 Hz and the turn-on delay is 04
seconds from equation (12)
The complete circuit is shown in
Figure 2 A circuit with 5%
components and biased for a minimum supply of 10V is
shown in
Figure 1 If additional gain is needed R1 and R2
can be reduced without changing the frequency response of
the circuit
DIODE SWITCHING
The LM1837 has a diode in series with each output for
source switching applications The outputs of several func-
tional blocks can be diode OR-connected as shown in
Fig-
ure 9
By removing the power supply from the FM demodulator its
output diode will be cut off by the LM1837 output DC volt-
age R6 is used to bias ON the diode of the LM1837 when
power is applied to it When the output is taken from pin 1 or
pin 18 the THD will be higher because of the current modu-
lation in the diode
TLH7902 – 18
FIGURE 9
CROSSTALK AND CHANNEL SEPARATION
When two signal sources share a common reference point
which is separated from ground by a resistance there will
always be some amount of interchannel crosstalk (the re-
ciprocal of channel separation) induced The coupling meth-
od of
Figure 1 is examined to determine whether the in-
duced crosstalk is acceptably low
Figure 10 is the equivalent AC circuit for the connection
scheme of
Figure 1 RB is the Thevenin resistance of the
common bias point RIN is the preamplifier input resistance
ZS is the impedance of the playback head and VS7 VS8
VS11 and VS12 are the open-circuit output voltages of the
sources If we set VS8 VS11 and VS12 equal to zero we
can define crosstalk for this circuit as V12V7 where V7
and V12 are the AC signal voltages appearing at the two
preamplifier inputs assuming RB m RIN 3
The crosstalk can be shown to be
V12
V7
e
RB
RB a ZS a RIN 3
Since ZS is dependent on the measurement frequency and
the particular head used we choose the worst-case condi-
tion and set ZS e 0 The minimum value of RIN is 150 kX
and RB j 100X This yields a crosstalk figure of
V12
V7
e
100
50100
eb
54 dB
This is 14 dB better than the minimum guaranteed channel
separation so the connection method of
Figure 1 will pro-
vide acceptable crosstalk levels
Reference 1
CCIRARM A
Practical Noise Measurement
Method by Ray Dolby David Robinson and Kenneth Gun-
dry AES Preprint No 1353 (F-3)
11
Obsolete
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