Audio Circuit. XR-T6420-1 Datasheet
Speakerphone Audio Circuit
The XR-T6420-1 is a monolithic integrated circuit for use
in high performance speakerphone systems. It is designed to
be used with the XR-T6421 Speakerphone Control Circuit.
The XR-T6420-1 contains the audio paths comprising the
following: Two variable gain cells, a microphone amplifier,
a transmitting amplifier, a receive amplifier, and a speaker
FUNCTIONAL BLOCK DIAGRAM
Two Matched Variable GaiA Cells
Internal Microphone Amplifier
Independent Control of Transmitting and Receiving Levels
External Control of Gains and Freq·uency Response
Voltage Controlled Amplifiers
ABSOLUTE MAXIMUM RATINGSI
Power Supply (VCC - VEE)
Derate Above +25°C
Any Input Voltage
aOc to 70°C
VCC - 0.5 V to VEE + 0.5 V
-55°C to +150oC
O°C to 70°C
O°C to 70°C
The speakerphone concept essentially requires that only
one direcction of sound transmission be permitted at any
time. This restraint is brought about by the large gains re-
quired to provide loudspeaker volume and high micro-
phone sensitivity. Owing to the inevitable acoustic
coupling between loudspeaker and microphone, plus
imperfections in the hybrid 2 to 4 wire conversion, it is
necessary to lower the gain in either the transmitting or
receiving path at anyone time to avoid regeneration.
The XR-T6420-1 and XA-T6421 chip set enables the sys-
tem designer to make a highly adaptive, high performance
speakerphone. The XA-T6421 provides for all sensing and
control functions, while the XA-T6420-1 contains all audio
paths needed to switch the gain in either path and provide
interfacing between the system and line.
Test Conditions: TA = 25°C. VCC = +6 V. VEE = -6 V. unless specified otherwise.
MIN TYP MAX UNIT
1.7 2 2.25 V Referenced to VEE
10 kn Typical Input Impedance to GND
Isource (Pin 7)
Isink (Pin 7)
.8 1.3 1.5 V Referenced to VEE
Differential Mode Gain
Common Mode Gain
VOUT (Pin 17)
-4 -1 0
VOUT (Pin 10)
VOUT (Pin 20)
1.2 1.5 1.7
PRINCIPLES OF OPERATION
Power Supply - Normal operation is with two supplies.
VCC is the highest potential and VEE is the lowest. with
the ground pin in between. The circuit can be operated
from a single supply if the ground pin is connected to a low
impedance source of approximately half the supply voltage.
Microphone Amplifier - The microphone amplifier is an
operational amplifier with the noninverting input inter-
nally biased to approximately VEE + 2 Volts. The non-
inverting input impedance is nominally 10.3 Kohms. Gain
and frequency response can be set by external components
using the non inverting configuration for the op amp. The
amplifier has an emitter follower output. therefore. needs
an external pull down resistor to VEE. This resistor is
selected low in value in order to prevent slewing of the
output waveform due to capacitance.
Transmit VCA - This VCA provides a voltage dependent
gain. given in Figure 2. The input. referred to VEE. has a
nominal impedance of 14:8 Kohms. The output is also
referred to VEE and is buffered by an emitter follower.
Transmit Amplifier - This amplifier is a Darlington Com-
mon Emitter Amplifier with a Class A output stage. Pin 8
is approximately 1.1 Volt above VEE. Gain is set by the in-
put and feedback resistor. To increase the output swing.
the output DC level is determined using a resistor from Pin
8 to VEE.
Receive Amplifier - This amplifier has a high impedance
differential input and a fixed gain of one. The inputs must
be referred to a voltage greater than VEE + 1.5 V. The
output is at a fixed DC level with a Class A output stage.
Receive VCA - This VCA provides a voltage dependent
gain. given in Figure 3. The input is referenced to VEE
and has a nominal impedance of 14Kohms. The output
is referred to ground and is buffered by an emitter follower.
Speaker Amplifier - This is an operational amplifier with
the non inverting input referred to ground through a 1.8
Kohm resistor. The gain is externally set using the input
and feedback resistor. The amplifier can be compensated
by a capacitor from the output to Pin 2 or from Pin 2 to
ground. The output is capable of sourcing or sinking 4 mA.
Pin 1 • SPI - Inverting input to speaker amplifier.
Pin 2 • SPC - Speaker amplifier compensation
Pin 3 - spa - Speaker amplifier output.
Pin 4 - VEE - Negative DC supply pin (usually -5 to -10 V).
Pin 5 - GND - Ground pin reference for circuit. Can be
used with ( VCC - VEE )/2 external reference.
Pin. 6 - VCC - Positive DC supply voltage. usually +5 to
Pin 7 - TXO - Output of transmit amplifier.
Pin 8 - TXI - Input of transmit amplifier.
Pin 9 - TXC - Control voltage of transmit VCA. Transfer
function of VCA is:
TVO = 2 (1 +exp (TXD - Vpin 5/VT))-1
whereVT= -KT 2:!26mVat+25°C
Pin 10 - TVO - Output of transmit VCA. Output is an
Pin 11 - TVI - Input of transmit VCA. Input impedance is
nominally 14.9 Kohms.
Pin 12 - MCO - Output of microphone amplifier.
Pin 13 - MC· - Inverting input of microphone amplifier.
Pin 14 - MC+ - Noninverting input of microphone ampli-
fier. Input impedance is nominally 10.3 K ohms.
Pin 15 • RX- - Inverting input to receive amplifier. Input
is high impedance.
Pin 16 - RX+ - Noninverting input to receive amplifier.
Input is high impedance.
Pin 17 • RXO - Output of receive amplifier. Output DC
level is nominally 0 volts.
Pin 18- RVI - Input to receive VCA. Input impedance is
nominally 14 K ohms.
Pin 19 • RXC - Control voltage of receive VCA. Transfer
function of VCA is:
RVO = (1 + exp (( Vpin Q_ RXC)/VT))-1
where VT = ~ 2:! 26 mV at 25°C
Pin 20 • RVO - Output of receive VCA. Output is an