ST AN1282 APPLICATION NOTE
PREAMPLIFIER AND SPEAKER DRIVER USING TS925
The TS925 is an input/output rail to rail quad B iCMOS 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 t hese features, the following
technical note highlights m any of the advantages
of the device in a global audio application.
APPLICATION CIRCUIT
Figure 1 shows two operators (A1, A4) use d in a
preamplifier configuration, and the two others in a
push-pull configuration driving a headset.
The phantom ground i s used as a c ommon ref erence l evel (V
CC/2
).
AN1282
APPLICATION NOTE
by F. MARICOURT
Preamplifier : the operators A1 and A4 are wired
with a non inverting gain of respectively :
❑ A1# (R4/(R3+R17))
❑ A4# R6/R5
With the following values chosen :
❑ R4=22kΩ - R3 =50Ω - R17=1.2kΩ
❑ R6=47kΩ - R5 =1.2 kΩ,
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 power supp ly is delivered from two L R6 batter-ies (2x1.5V nominal).
Figure 1 : Electrical Schematic
The mean rectified peak level of the output signal
envelope is used to control the preamplifier gain.
June 2000
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AN1282 - APPLICATION NOTE
Figure 2 : Frequency Response of the Global
Preamplifier Chain
Figure 3 : Volta ge Noise De nsity versus
Frequency at Preamplifier Output
maximum o utput swing and a hi gh fidelity for reproducing sound and music.
Figure 4 shows the available signal swing at the
headset outputs : two other rail to rail competit or
parts are employe d in the sa me circuit for compari-40 Sson (note the much reduced clipping level and crossover distortion)
Figure 4 : Maximum Voltage Swing at
Headphone Outputs (R
) 32Ω)
L
Figure 5 : THD+Noise versus Frequency
(headphone outputs)
Headphone amp lifier : the operators A2 and A3
are organized in a push-pull configuration with a
gain of 5. The stereo inputs can be connected to a
CD-player and the TS925 drives directly the
head-phone speakers.This configuration shows
the abil-ity of the circuit to drive 32Ω load with a
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