T E C H N I C A L I N F O R M A T I O N
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. 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.
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.
1/2 TA3020
V5
RFBA
RFBA
RFBC
RFBC
Processing
&
FBKOUT1
FBKGND1
OUT1
OUT 1 GROUND
Modulation
RFBB
AGND
RFBB
Figure 3: Modulator Feedback
TA3020, Rev 2.1, 01.01
19
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
