How to protect output of power amp against short circuit and mismatch in antenna?

[7rots51]

How to protect output of power amp against short circuit and mismatch in antenna?

please give some circuits if possible for learning purposes.

 

[vfone]

Re: PA output protection

Place an isolator between PA output and antenna, or a coupler and a detector which DC output will shut-down the RF input.

 

[7rots51]

Re: PA output protection

If I use bidirectional coupler,what signal should be used for protection? forward or reverse?

please place more comment and details here.

 

[vfone]

Re: PA output protection

Have to use the reverse port.
The detected voltage is proportional with VSWR due to the load mismatch. This voltage will drive a circuit to reduce the RF input power or the gain of the power amplifier (usually of the driver).

 

[biff44]

PA output protection

I agree, a simple circuit will use a diode and directional coupler to detect the reverse power, and the diode output voltage will exceed a threshold in a comparator if the reflected power gets too big.

A fancier circuit will use two diodes and sense both output power AND reverse power, and compute a crude return loss estimate. That way you can work on the ratio, with would be better if your transmitter has to work with different user controllable output powers.

 

[7rots51]

Re: PA output protection

my output power is fixed.
Can I use RF detector /controllers like as LTC 4400 instead of simple diode?
or diode is enough?
what type of diodes I should use?

 

[vfone]

Re: PA output protection

Just a low power Schottky diode will be fine for this application.

 

[quangt]

Re: PA output protection

Hi All,

I don't think there is anything that can do to protect the PA from high VSWR apart from using the isolator at the output of the PA.

Using the directional coupler and diodes as mentioned in above doesn't help at all. Because the diodes isn't not fast enough. Not just the diodes but also other circuitry that used to control the PA output power.

There may be a way to protect the PA by re-shaping the output waveforms of the PA so that it is not peaking or excess the maximum break down voltage under the high VSWR conditions.

 

[biff44]

PA output protection

Not fast enough? You can probably crowbar the supply voltage in a millisecond or so. Since most devices blow up due to thermal overload caused by the reflected power being absorbed, a millisecond is pretty quick.

 

[vfone]

Re: PA output protection

There are diodes with 200 pico-seconds response time.
Directional coupler and detector approach is used since the RF power transistor was invented, until now.

 

[7rots51]

Re: PA output protection

vfone:

There are diodes with 200 pico-seconds response time

please introduce some part numbers.

 

[vfone]

Re: PA output protection

**broken link removed**

 

[quangt]

Re: PA output protection

Even though you may use the fast diodes of 200psec to detect and rectify the signal. But besides that there are also other circuits (like buffers or comparators) in the control loop to stop or foldback the PA output power under the high VSWR conditions and those circuits also have some time delays.

Added after 14 minutes:

I have look at the spec of the diode that have 200pms. the maximum input power rating is only 200mW ;-). If I have to spend some money to buy this diode just for the VSWR protection then I would probably invest my money in buying the isolator.

 

[vfone]

Re: PA output protection

1. This was only an example to see that in this world exist very fast diodes (crazy to be to use a Planar-Doped Barrier Diode in a PA protection)
2. The supporting power of the diode doesn’t matter for this application, because anyway there is a power divider/coupler in front of it, which will reduce the RF power to obtain the proportional DC voltage.
3. Not always an isolator is suitable for high-power protection, especially when the design is looking to shut-down totally the output power if the antenna mismatch become permanent.