Tripath Technology, Inc. - Technical Information
TC2001
OAOUT1
22
V5
CI
RI
RF
INV1 23
+
-
+
INPUT1
AGND
BIASCAP
V5
CI
RI
+
-
28
27
INV2
+
RF
INPUT2
AGND
OAOUT2
Figure 2: TC2001 Input Stage
Input Capacitor Selection
CIN can be calculated once a value for RIN has been determined. CIN and RIN determine the input
low-frequency pole. Typically this pole is set at 10Hz. CIN is calculated according to:
CIN = 1 / (2π x FP x RIN)
where: RIN = Input resistor value in ohms
FP = Input low frequency pole (typically 10Hz)
Modulator Feedback Design
The modulator converts the signal from the input stage to the high-voltage output signal. The
optimum gain of the modulator is determined from the maximum allowable feedback level for the
modulator and maximum supply voltages for the power stage. Depending on the maximum
supply voltage, the feedback ratio will need to be adjusted to maximize performance. The values
of RFBA, RFBB and RFBC (see explanation below) define the gain of the modulator. Once these
values are chosen, based on the maximum supply voltage, the gain of the modulator will be fixed
even with as the supply voltage fluctuates due to current draw.
For the best signal-to-noise ratio and lowest distortion, the maximum modulator feedback voltage
should be approximately 4Vpp. The modulator feedback resistor RFBC should be adjusted so that
the modulator feedback voltage is approximately 4Vpp. This will keep the gain of the modulator
as low as possible and still allow headroom so that the feedback signal does not clip the
modulator feedback stage. Increasing the value of RFBC will increase the modulator gain.
Sometimes increasing the value of RFBC may be necessary to achieve full power for the amplifier
since the input stage for the TC2001 will clip at approximately 4Vpp. This will ensure that the
input stage doesn’t clip before the output stage.
Figure 3 shows how the feedback from the output of the amplifier is returned to the input of the
modulator. The input to the modulator (FBKOUT1/FBKGND1 for channel 1) can be viewed as
inputs to an inverting differential amplifier. RFBA and RFBB bias the feedback signal to
approximately 2.5V and RFBC scales the large OUT1/OUT2 signal to down to 4Vpp.
19
TK2150 – Rev. 1.0/12.02
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