Is really necessary the bias resistor for monolithic RF amplifier (like minicircuit)?

[borgomauro]

I cannot figure out why in the datasheet of minicircuit amplifier (like ERA3, ERA5, etc) there is a "required" resistor.
Suppose I have a power supply of 12 V, and a switching for other function to obtain 5 V.
If a monolithic works at 5V, I think the best choice is to apply a voltage coming for example from the switching instead having dissipated power to have a drop from 12V to 5 with a resistor.
Is there something I'm Missing ???
Best regards,
Mauro

 

[betwixt]

The output and supply share the same pin. The resistor is necessary to allow the power source and signal to be separated. It is also used to set the desired current in the amplifier and to provide the correct load impedance.
Brian.

 

[borgomauro]

The output and supply share the same pin. The resistor is necessary to allow the power source and signal to be separated. It is also used to set the desired current in the amplifier and to provide the correct load impedance.
Brian.

Dear Brian, the resistor is not for signal and power source separation (you need an inductor to have an high impedance at high frequency). I found the reason is that the resistor is part of the biasing network, essentially for thermal feedback. In any case, thanks for your answer.

 

[volker_muehlhaus]

Dear Brian, the resistor is not for signal and power source separation (you need an inductor to have an high impedance at high frequency).

The resistor will provide some limited decoupling from the RF signal path. That is the reason why the data sheets calls the inductor "optional": the resistor already provides some decoupling. If you can accept some output power loss in that resistive feed, you can get away without the inductor.

 

[ucah]

All the devices you are mentioning require a current source for biasing. If you use a voltage source you will simply burn the device. The resistor is there to give a rudimentary current source out of your voltage source. Better results can be obtained with a proper current source, like a current mirror. Do not forget the inductor which serves to isolate the amp from the current source at rf frequencies.

 

[biff44]

I agree with the last post. These devices are really designed to work with a fixed internal voltage, like 4 volts, and allow some variation in operating current (more current needed to achieve the full rated output RF power, but can be throttled back if in current if you do not need the full output power). They are designed that way so that there are a minimum number of pins on the package. You will have to either use a resistor, or an active quasi current-source bias.

other amplifiers are available that allow you to vary the gate/base biases independent of the drain/collector bias.