RF power amplifier design

Hello all,

I have a question on one particular implementation of high power RF/MW amplifier.
Is it possible to achieve large output power by summing the outputs of multiple "low" power amplifiers (i.e. using power combiners, etc). What happens with output voltages/currents if the load is not matched to the output?



My main idea is that I want to make a 40 dBm amplifier, which should be able to work and withstand big output mismatch conditions. But somehow I am confused by the problem of voltages/currents. That is, at 50 ohm load there are ~23 volts rms over the load. If it is disconnected (Rload -> inf) the voltage should rise to approx 46 V). Does this mean that each of the 'low' power amplifiers should be powered with this voltage?

I am not really sure that I managed to describe my problem, but any comment on it would be appreciated!

Cheers,
D

In the real world, high power TV transmitters use 20 X 1kW modules so its a very valid way to proceed. Each module should have its own protection against VSWR and it would be good if it disconnected itself when its faulty. This way a faulty module will cause a slight power reduction and not a total loss . Your maintenance crew turn up once a year and replace every module that is faulty, but the viewer/listener never knows!.
Frank

buffallo said:

Hello all,

I have a question on one particular implementation of high power RF/MW amplifier.
Is it possible to achieve large output power by summing the outputs of multiple "low" power amplifiers (i.e. using power combiners, etc). What happens with output voltages/currents if the load is not matched to the output?

View attachment 123341

My main idea is that I want to make a 40 dBm amplifier, which should be able to work and withstand big output mismatch conditions. But somehow I am confused by the problem of voltages/currents. That is, at 50 ohm load there are ~23 volts rms over the load. If it is disconnected (Rload -> inf) the voltage should rise to approx 46 V). Does this mean that each of the 'low' power amplifiers should be powered with this voltage?

I am not really sure that I managed to describe my problem, but any comment on it would be appreciated!

Cheers,
D

Click to expand...

In general, designing higher-power amplifiers should include the power stage protection from a high VSWR. The heat dissipation as well as voltage or current overshoot should not damage the hardware. RF power amplifiers usually are provided with a VSWR bridge with an output running through a comparator. This device cuts off the RF input to the power stage, or the main DC power, to protect the device. Such circuits can operate over a wide bandwidth.
In microwave RF power amplifiers, ferrite isolators are used which absorb a reflected RF power from a mismatched load. By using such isolator, the final power stage only sees the matched load while reflected power is absorbed in isolator load.

directional couplers could save your PA and circuits from missmatch condition.

pragash said:

directional couplers could save your PA and circuits from missmatch condition.

Click to expand...

To my knowledge, directional coupler isolation or directionality requires a good matching on all ports.
Different coupler designs offer different senstivity to port mismatch.

This all is why isolators and SWR protectors are preferred.