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Help me with building a balanced amplifier (UHF)

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molloy

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Balanced amplifier

I have to design a balanced amplifier (UHF). I'd like to simulate even the 2nd harmonic to understand its behaviour in the circuit but i dont know well how to do. The second harmonic could cause a saturation of the amplifier and so it is very important to know its influence. The problem is that i just have an equivalent circuit of the output of the transistor (and it doesnt exist a non linear model of the transistor)
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Borber

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The second harmonic of what? Is it componnent of input signal or...
If your amplifier is not overdriven to nonlinear region (-1dB compression) you will have no problem with second harmonic.
Your question is not quite understendable!
 

nandopg

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Need more details...

Dear molloy,

Let us know what kind of balanced structure you are using in your project. Basically there are two ways to get a balanced amplifier: a push-pull connection and a connection using power combiner/splitter.
The push-pull connection has the advantage to cancel the even harmonics (including the second) and because of that, in some case, is the preferable connection.

If you are using power combiner/splitter to get the balanced structure you are right to be worried about the second harmonic specially if you are pushing the active devices into class AB to get a better efficiency.

To try to provide you some help it would be necessary to know some details about your project as:
1- How are you connecting the active device.
2- What is the active device you are using
3- How much efficiency are you looking for
4- Design targets as frequency response, output power, driver level, etc...

NandoPG
 

molloy

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You are right, the both of you. Well:
I choose a configuration with a balun, just to reduce the second harmonic.
The amplifier works in AB mode (and so i need a 2nd harmonic characterization)
The output power is very high, lets say about 54-55dBm
The band is UHF
emmm i cant say nothing about the device unfortunately, because of my job. Anyway i hope you could help me the same


thanks
 

nandopg

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Some points...

From you last post:

1- Push-pull configuration in RF PAs is preferable when you have transistors in twin package and in class B operation. In this case you have a strong decreasing in all even harmonics. By the way true push-pull operation is only defined in Class B operation.

2- As you go into class AB the use of baluns as a summing device shows any advantage and an disadvantage comes up: you don't have isolation between the transistor. This means, if the PA for any reason get unbalanced the likelihood to blow the transistors is pretty high.

3- For Class AB and Class A the transistors should be connected through a power combiner/splitter in order to have insensitivity respect to possible unbalancing between the two halves of the PA. This is even true at the power level you mentioned.

4- As a rule of thumb for UHF Class AB devices connected through a power combiner/splitter the second harmonic is about -40dBc. You can use this figure to determine the order of the harmonic filter.

Concluding: For the reasons presented you should not use baluns as summing device for a Class AB PA.

NandoPG
 

molloy

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The ldmos i use is the type with two devices in a case. I will use two of these in a balanced configurations (and so with a 90° hybrid), but im forced to use a push-pull configuration with a balun because of the output impedance of the LDMOS (about 3-3.5ohm) that would be very difficult to match.
Just to be clear i include a data-sheet of a similar LDMOS

thk

stefano
 

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