TS925

Using the TS925 as a preamplifier and speaker driver

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3

Using the TS925 as a preamplifier and speaker

driver

The TS925 is an input/output rail-to-rail quad BiCMOS operational amplifier. It is able to operate

with low supply voltages (2.7V) and to drive low output loads such as 32

Ω.

As an illustration of these features, the following technical note highlights many of the

advantages of the device in a global audio application.

3.1

Application circuit

Figure 16 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

The power supply is delivered from two LR6 batteries (2 x 1.5V nominal).

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= 1.2kΩ,

The gain of the preamplifier chain is therefore equal to 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, useful in many microphone preamplifier applications. The mean rectified peak level of

the output signal envelope is used to control the preamplifier gain.