about high gain amplifier

[RFAmplifier]

How to ensure that high gain Amplifier(>90dB) would not self-excitation?
can you suggest a good way for me?

the last device is MOTO MRF9060,but once self-excitated, the MRF9060 is burnt.
if self-excitation is inescapable ,then,how to protect the MRF9060?

 

[vfone]

Give more information about your amplifier. MRF9060 has only 17dB of gain. This transistor even is not the best in terms of ruggedness (VSWR max 10:1) is pretty good in terms of stability.
Check your load and the Idq to do not exceed the limits.

 

[flatulent]

One method commonly used in the past is to have the gain of a system split between the sections that operate at different frequencies. A HF superheterodyne receiver is an example.

Putting the amplifier chain in a waveguide small enough in cross section for the amplifier gain to be below one at the frequency where the waveguide passes signals was commonly done in the early RADAR IF strips. This only guards against external coupling from output to input.

There is the necessity to not have coupling through the power supply. The filtering has to be effective at the amplifier frequency.

 

[RFAmplifier]

all effects is good before leave factory.
but MRF9060 is burnt in engineering.

we estimate the cause of self-excitation is coupling from output antenna to input antenna(repeater), in other words, spacial isolation is not enough.but the distance
between the two antennas is unchangeable,therefore i think, protect MRF9060 is all i can do.not guard against self-excitaton?right?

somebody suggested that i should use modules(not transistor),if there is self-excitation,the module will not be burnt.

thanks for concern.

 

[RFAmplifier]

can anyone help me?
i'm anxious

 

[VSWR]

A block diagram of your setup would be helpful.

Is the output frequency from the amplifier module same as the input frequency? Or does the self oscillation starts due to another frequency is present at the input of the amplifier?

If the later is the case, can't you simply suppress this frequency using a high-Q notch filter?

 

[RFAmplifier]

I have already used a dielectirc filter at the input of amplifier.
but the self-excitation is inescapable at the amplifier frequency.right?

 

[RFAmplifier]

after all,the gains is a little hign.
about the block diagram,i don't know how to describe,it's used in spacial repeater.
is it a technical difficulty?

 

[flyhigh]

Hi,

one way to protect the amplifier until you find the reason for burning it is to limit its current. Then you are sure that it is not burned out by self oscillation. The current limit should be just above the current it is normally taking from drain (collector) circuit.

I have some experience with self burning of MW transistors even with this protection set on, it was in the case of poor source groundings (poor vias). Poor grounding can potentially have at least two effect, causing amplifier to oscillate and preventing the housing to act as a heat sink, so you can check that as well.

flyhigh