Help: PA/LNA connect together

[hmsheng]

lna in series with pa

Hi,

Generally, in RF transceiver, PA/LNA connect to antenna through a T/R switch. For time division system, such as Bluetooth, how if we connect PA and LNA together? That is, in TX mode tune on PA and turn off LNA, in RX mode turn on LNA and tune off PA. Most of such products and papers are designed with T/R switch. Why?

If we connect PA/LNA together and PA output power is 3dBm to 50 Ohm antenna, can the large PA output swing destroy LNA input device (NMOS, 0.18um)?

Regards,
HM

 

[betwixt]

Yes, the reason for the T/R switch is to provide a first line of defense against the PA output damaging the receiver first stage. If TX and RX frequencies are significantly different it is possible to use filtering techniques but when they are the same or very close you have no option but the use a switch to 'disconnect' the antenna path not being used.

Brian.

 

[hmsheng]

The differential LNA is commen source cascode type with a degeneration inductor between the input NMOS source terminal and ground. The input NMOS is 0.18um 1.8V device. If the PA output port and the LNA input port connect to Antenna together and PA output power of 3dBm. In TX mode LNA is disabled. But the input NMOS gate will be stressed by the PA output signal. Because of the AC coupling and biased to ground the LNA input port swing is about -1.8V~1.8V. Can this signal destroy the LNA input device or cause the NMOS breakdown?

Thx

 

[dick_freebird]

3dBm is not much output power but you would need to
account for worst-case VSWR effects as well, possibly
doubling the voltage peak value. And many radios do
more than +3dBm output.

Circulators are another approach, but bulky / heavy.
They are "always-on", though, and may be the only
option for a full-duplex link.