Voltage Modulator Circuit Design

[Politecnico]

Dear All

Hi,
I am designing a Voltage Modulator to be employed in Envelope Tracking PA.



According to the circuit above, we have the detected envelope of the input RF signal as Vin to the voltage modulator. First, it goes through LINEAR AMPLIFIER stage and then HYSTERESIS COMPARATOR to drive / drive not the SWITCHING stage.

1) what is the least required GAIN and OUTPUT CURRENT of the Linear Amplifier Stage? Besides, Assuming input RF signal according to WiMAX or WCDMA standards, what is the envelope voltage range?
(right now, I have designed the linear stage with GAIN=55 dB. Is that enough or we need more?)

2) At the Comparator, What voltages does it compare? one side is the output of the Linear Amplifier, and what is on the other side?

3) Is TSMC 0.16um with bias of 3.3v suitable / sufficient to design the whole circuit?

Please correct me if any of my understandings about Voltage Modulator is wrong.
Since I am beginner in designing low frequency analog circuits, any comment is highly appreciated.

 

[dick_freebird]

I think a hysteretic comparator could be kind of a nuisance
for RF apps since you can't know its frequency of operation.
That tends to bother the folks responsible for the spectral
mask compliance.

The comparator appears to be sensing PA load current and
tries to hold op amp current to a fixed maximum while the
op amp asserts the desired voltage. Your op amp output
current and the Rsense go together.

A 0.16um transistor is not going to stand off 3.3V, there
must be an I/O device. Your standoff voltage will have
to be the difference between max VDD and min VCC
(minimum power setting) on the high side switch, and
max VDD to GND on the low side switch, while min VDD
is set by the PA's max output power level and its load
impedance. Max-min is whatever supply tolerance you
(or your "customer") choose to support.

This seems to be all about the load - SiGe PA and its
output network, including antenna loading - so you
want to look more at that before trying to pick a
device technology.