AN1282 Datasheet, PDF (1/2 Pages) STMicroelectronics – PREAMPLIFIER AND SPEAKER DRIVER USING TS925

AN1282
APPLICATION NOTE
PREAMPLIFIER AND SPEAKER DRIVER USING TS925
The TS925 is an input/output rail to rail quad BiC-
MOS operational amplifier. It is able to operate
with low supply voltages (2.7V) and to drive low
output loads such as 32Ω.
As an illustration of these features, the following
technical note highlights many of the advantages
of the device in a global audio application.
APPLICATION CIRCUIT
Figure 1 shows two operators (A1, A4) used in a
preamplifier configuration, and the two others in a
push-pull configuration driving a headset.
The phantom ground is used as a common refer-
ence level (V_CC/2).
The power supply is delivered from two LR6 bat-
ter-ies (2x1.5V nominal).
by F. MARICOURT
Preamplifier : the operators A1 and A4 are wired
with a non inverting gain of respectively :
u A1# (R4/(R3+R17))
u A4# R6/R5
With the following values chosen :
u R4=22kΩ - R3=50Ω - R17=1.2kΩ
u R6=47kΩ - R5=1.2kΩ,
the gain of the preamplifier chain is thus 58dB.
Alternatively, the gain of A1 can be adjusted by
choosing a JFET transistor Q1 instead of R17.
This JFET voltage controlled resistor arrangement
forms an automatic level control (ALC) circuit,
use-ful in many MIC preamplifier applications.
The mean rectified peak level of the output signal
envelope is used to control the preamplifier gain.
Figure 1 : Electrical Schematic
June 2000
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