Tait TP9100 - Page 63

TP9100 Service Manual Circuit Descriptions 63
© Tait Electronics Limited May 2005
Audio Signal
for Speaker
The audio signal for the speaker is generated by IC204 (VOUTAUXP ball).
This signal is post-volume-control decoupled by C238 and fed to the audio
power amplifier.
Audio Power
Amplifier
The signal from C238 is fed to the audio power amplifier IC200. IC200 has
22dB of gain and a differential output configuration. Gain is configured by
the resistors R260 and R259. When operational, the output bias voltage for
IC200 is approximately half the radio supply voltage. When not operational,
the output becomes high impedance.
Control of Audio
Power Amplifier
Power up, power down, and muting of IC200 is controlled by one signal
from the digital section,
DIG AUD PA EN. The network consisting of Q200,
R254 and R256 converts the digital signal to the single three-level analog
signal required by IC200. A “pop-free” audio PA on/off control is activated
by a fast transition on the
MODE pin of IC 200.
Control of Internal
Speaker Audio
The differential signal to the internal speaker can be disconnected by dual-
MOSFET IC207, under control of the digital signal
DIG INT SPK INH.
The external speaker audio signal is permanently connected to the output
of IC200.
Microphone Signals There are two microphone source signals:
■ ACC MIC AUD from accessory connector
■ internal speaker as a microphone
The biasing for electret microphones is provided by a filtered 3.0V supply
via R226. The components R209 and C202 provide the supply filtering.
The microphone inputs to IC204 (
VINAUXP, VINAUXN, VINNORP and VINNORN
balls) are differential. The negative inputs are decoupled to the filtered 3.0V
supply by C215 and C216. The positive input of
ACC MIC AUD is biased to
approximately 1.5V by R229 and R232. AC coupling and DC input
protection is provided by C213.
The microphone signal from the internal speaker is converted from
differential to single/ended by IC202 and then fed to
VINNORP ball of IC204
as the internal microphone signal.
Accessory Audio
Input
The accessory audio input signal ACC AUD TAP IN is DC-coupled to the ADC
input of IC205. R241 combined with internal clamping diodes in IC205
provide DC protection for the ADC input. IC205 provides the input biasing
of approximately 1.5 V.