Suggetions for RF power stage amp

Thread Starter

Zeno53

Joined Sep 23, 2020
24
I need to select a power amp to match as the last stage to a previous preamp circuit.
Let's leave out for a moment the laws, regulations, usable frequencies and powers, as currently it is a simulation, this pre-amplification circuit in simulation allows to supply in sinusoidal it will be able to supply from 20mV (+/- 10mVolt) to 2Volt (+/- 1Volt) up to 100MHz then the power amp can be driven with this voltage depending on the stages used. The most important factor is the phase linearity up to 100Mhz and I thought it could be done as there are devices on the market with a bandwidth of several GHz. Without this factor I don't need it. I would prefer to use FETs but all the amps I've tested from J-FETs (very sensitive and ideal) and Mosfets don't have phase linearity. The use of Mosfets is not excluded because even if they use higher input voltages and for this reason I have put as a limit of voltage supplied by the preamplifier up to 1Volt. Furthermore, it could drive low ohmmic loads without any particular problems. Advice on which component to use?
 

Papabravo

Joined Feb 24, 2006
15,518
I need to select a power amp to match as the last stage to a previous preamp circuit.
Let's leave out for a moment the laws, regulations, usable frequencies and powers, as currently it is a simulation, this pre-amplification circuit in simulation allows to supply in sinusoidal it will be able to supply from 20mV (+/- 10mVolt) to 2Volt (+/- 1Volt) up to 100MHz then the power amp can be driven with this voltage depending on the stages used. The most important factor is the phase linearity up to 100Mhz and I thought it could be done as there are devices on the market with a bandwidth of several GHz. Without this factor I don't need it. I would prefer to use FETs but all the amps I've tested from J-FETs (very sensitive and ideal) and Mosfets don't have phase linearity. The use of Mosfets is not excluded because even if they use higher input voltages and for this reason I have put as a limit of voltage supplied by the preamplifier up to 1Volt. Furthermore, it could drive low ohmmic loads without any particular problems. Advice on which component to use?
Most RF power amplifiers operate class C (conduction for less than half a cycle) for efficiency. This compromise pretty much throws linearity and low distortion out the window. I fail to see how you can reasonably achieve your objective. Tell me something I don't know -- perhaps I've missed something. I have a pair of RF Power MOSFETS in my KPA-500 which is good enough for voice communications and FSK-441 meteor scatter. I don't think it would work very well in your application.
 

Thread Starter

Zeno53

Joined Sep 23, 2020
24
What value should I look at on the datasheets indicating the potential linear phase loss? Is it the capacitance? The only one is as mentioned in the other post to use an all-pass filter. If there was a calculator it would be really nice. If it exists. the alternative is to use more power stages divided by band only that the circuit becomes too complicated for my abilities. What do you think about J-fet?
 

DickCappels

Joined Aug 21, 2008
7,176
There are N- and P-channel JFETs. Surely somebody could put together a complimentary pair, but JFETs are not very useful for power, that if you can even find them anymore.

A series of class-B MOSFET stages (or class-A if need be) might get you the X100 voltage gain you crave.
 

Thread Starter

Zeno53

Joined Sep 23, 2020
24
Hi @DickCappels , do you have any specific models in mind that I do a couple of searches? I explain what I want to do because i need some advice if possible, I have to create an amplification stage whose input will be managed by an amplitude of maximum 100mV (but if necessary I can reduce this output value as it is a multistage) but the important thing that it must have the output in phase from 1Hz up to about 100Mhz. I did some tests with some J-Fet and I focused on the low values Ciss aka Input Capacitance and Crss aka Reverse Transfer Capacitance but more or less all of them have a loss of linearity on the phase. These values should be significant as to what is the loss of linearity on the phase. It is fine other like an OpAmp, BJT or transistor which maintains the above characteristics. And I confirm that you don't need a lot of power and class A would be preferable for the kind of sine response it provides. Many thanks!
 

DickCappels

Joined Aug 21, 2008
7,176
The phase shift requirement over such a bandwidth will be a real challenge.

I can't recommend any specific transistors because I haven't done anything like what you want to do.
 

Thread Starter

Zeno53

Joined Sep 23, 2020
24
Up to this point it seems possible. The problem actually seems to go further. The simulation predict a gain of about 1600x. Let's not get too excited because the output, as I said, can reach a few tens of mV. But interesting isn't it? Probably maintains a fairly linear phase even over 100Mhz.



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