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)
thanks

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!

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

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

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

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