wideband matching of RF power amp

[maniana]

wideband matching

Hi,
I have an LDMOS that has an output impedance of 15+j1.1 at 330MHz.
I have a big problem to match it in 280-350MHz band using two pi sections.
It tunes very shraply with only 5MHz bandwith.
1. Could anyone suggest a good matching network or suggest some readings with practical examples.

2. if i put a 15ohm port in AWR will the simulation be valid?
how to add a port in AWR that has complex impedance?

 

[kezambo]

wide band match

Hi,
you can use HB1 tone load pull analysis using ads software to find the optimum impedance. then use smitch chart to design the matching network

 

[vfone]

wideband matching network

AWR is fine to find the matching network, with the condition to have the right model of the LDMOS transistor.

In AWR you have in General\Passive\Other various elements that simulate complex impedance, IMPED, LTUNER, etc.

 

[jayce3390]

power amplifier wideband matching

some methods exist to enlarge the operating bandwidth, for exmaple you can add a R+C components between gate and drain. But at the beginning try to find a proper matching network able to match your circuit at your impedance.

But you need to get :
1 . Optimal impedance at 280MHz
2 . Optimal impedance at center frequency
3 . Optimal impedance at 350 MHz

You have to design your matching network using a set of 3 impedances to see if you are able to match the bandwidth.

 

[dream-cast]

wide band matching

it is very hard

 

[maniana]

wideband matching power amplifier



I designed matching in a similar way to the circuit shown in the pic above.
at the output I have 2 pi sections (c10-Z9-c11) build from lumped elements.
Those 2 pi sections transform the optimum output impedance to 50 ohm.
So, if the matching is done by the pi sections, what is the L1 inductor for and how do you pick its value?
I know it provides Vdd, but its value is 55nH in the datasheet of the transistor, so it is not a choke.
I matched the device using 2 pi sections at the input and 2 sections at the output, but the bandwith is only 10MHz at 330MHz and there is plenty of resonances outside the band. in my case L1 is a large value choke.

What is the reason for L1 and how do you calculate it?

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[quangt]

Sometimes L1 can be part of the matching or an RF choke. You need to know what is the output impedance of this device. As a rule of thumb, the value of L1 is usually chosen in the region of at least 10 times of the device output impedance. Large enough block the in band RF signal and small enough to keep the device stable at low frequencies. I hope this will help.

 

[FvM]

Generally, transmission line transformers are well suited to achieve wideband matching where standard LC networks don't work.
See respective RF FET manufacturer application notes, e.g. from Polyfet.

 

[jayce3390]

L1 value :

Move L1 value in S parameter simulation, when high impedance is found at the node in your frequency range you get the L1 value.

Ideally, an open circuit must be synthesized at the node between RF path and bias path.

You are aiming 20% bandwidth around 315 MHz, it's not easy.

 

[Garbonzo]

Other thoughts to add:

1. Output match to get maximum power is not going to be where you get a good match. You probably know this, but it wasn't clear in your question. I just want to make sure that you understand the difference.

2. Quarter wave sections will be big, but give good broadband matching.

3. 15 Ohms is quite close to 12.5, so consider using an 4:1 transformer (2:1 turns ratio). At these low frequencies they can be gotten pretty cheaply and low loss. If indeed you need a 15 Ohm load line the final L-section to go from 12.5 to 15 Ohms will be pretty low-Q and pretty broadband.