two amplifier outputs connected to the same load

[praveen450]

two amplifier ouputs connected to same load

Can anyone tell what are the possible effects when two amplifier outputs are connected to same load?.

Thanks

 

[FvM]

Re: two amplifier ouputs connected to same load

Connected how? Through power combiner?
Both amplifiers fed by the sample input signal?

 

[KlausST]

Re: two amplifier ouputs connected to same load

Hi,

Two voltage amplifier outputs connected together wil most prbably fight againt each other.
No good situation. High current, high heat dissipation...

Klaus

 

[praveen450]

Re: two amplifier ouputs connected to same load

Connected how? Through power combiner?
Both amplifiers fed by the sample input signal?

They are not connected by power combiner. I have seen a documnet by TI where they connected output without power combiner. Below is the link for the document.

https://www.ti.com/lit/ug/tidu002/tidu002.pdf

I just wanted to know if it works for RF amplifier if we are not concerned about loss of power at the output.

 

[KlausST]

Re: two amplifier ouputs connected to same load

Hi,

can you give me a hint where in the document are "two amplifier outputs connected together". I don´t want to read the complete document.

Klaus

 

[praveen450]

Re: two amplifier ouputs connected to same load

Hi,

can you give me a hint where in the document are "two amplifier outputs connected together". I don´t want to read the complete document.

Klaus

Section 2 explains the load sharing by two opamps.

 

[KlausST]

Re: two amplifier ouputs connected to same load

Hi,

I think I´m blind: I can´t find it.

My section two just is named: "2 Signal-Chain Considerations".
No further text.

Did you upload the correct document?

Klaus

 

[praveen450]

Re: two amplifier ouputs connected to same load

Hi,

I think I´m blind: I can´t find it.

My section two just is named: "2 Signal-Chain Considerations".
No further text.

Did you upload the correct document?

Klaus

Hello,

I uploaded the document. Please let me know if you have any issues opening the document.

Thank you

 

[mtwieg]

Re: two amplifier ouputs connected to same load

In principle if their inputs are also the exact same signal, it can work. But in practice it will lead to awful performance if there's any mismatches. Splitters and combiners are used to do this while isolating the inputs/outputs from each other.

 

[praveen450]

Re: two amplifier ouputs connected to same load

In principle if their inputs are also the exact same signal, it can work. But in practice it will lead to awful performance if there's any mismatches. Splitters and combiners are used to do this while isolating the inputs/outputs from each other.

Thank you for the information. So, the performance degradation is interms of power dissipation and efficiency?

 

[KlausST]

Re: two amplifier ouputs connected to same load

Hi,

now it is the correct document.

And you can see that the outputs are connected using resistors "Rs"

The document says:
In high voltage swing applications, where an operational amplifier is pushed to drive close to
its supply rail, it is possible to drive several identical operational amplifiers in parallel and
combine the outputs to achieve higher bandwidth and lower distortion. The remainder of this
design guide develops this concept of load sharing and demonstrates how this technique can
be used to reduce current sourcing and sinking requirements towards the amplifier.

Also they speak about amplifiers in the 60MHz region. What else do you want to know?

Klaus

 

[praveen450]

Re: two amplifier ouputs connected to same load

Hi,

now it is the correct document.

And you can see that the outputs are connected using resistors "Rs"

The document says:
In high voltage swing applications, where an operational amplifier is pushed to drive close to
its supply rail, it is possible to drive several identical operational amplifiers in parallel and
combine the outputs to achieve higher bandwidth and lower distortion. The remainder of this
design guide develops this concept of load sharing and demonstrates how this technique can
be used to reduce current sourcing and sinking requirements towards the amplifier.

Also they speak about amplifiers in the 60MHz region. What else do you want to know?

Klaus

This is the first time I'm seeing this kind of architecture without power combiners. I'm doubtful to implement it in practice. Do u know any similar architecture as mentioned in the document (load sharing with amplifiers).

Thanks.

 

[FvM]

Re: two amplifier ouputs connected to same load

I think the references to OPs are basically inappropriate because OPs are voltage sources in contrast to RF amplifiers that implement impedance matching with a defined output impedance, e.g. 50 ohm. The paper is about load sharing of OPs, performed with lossy resistive networks. It should be discussed under Analog Circuit Design rather than RF and Microwave.

Paralleling 50 ohm RF amplifiers results in halving the output impedance to 25 ohms, which may be intended in special cases, but usually requires an impedance transformation to match a 50 Ohms load. A power combiner (e.g. Wilkinson) does the transformation with additional isolation of amplifier outputs against each other.

If RF amplifiers can be paralleled without harm is also a question of power level. In case of high power amplifiers, you often use circulators to isolate the load and avoid possible amplifier damage by reflected power. They also make parallel operation easier.

 

[praveen450]

Re: two amplifier ouputs connected to same load

I think the references to OPs are basically inappropriate because OPs are voltage sources in contrast to RF amplifiers that implement impedance matching with a defined output impedance, e.g. 50 ohm. The paper is about load sharing of OPs, performed with lossy resistive networks. It should be discussed under Analog Circuit Design rather than RF and Microwave.

Paralleling 50 ohm RF amplifiers results in halving the output impedance to 25 ohms, which may be intended in special cases, but usually requires an impedance transformation to match a 50 Ohms load. A power combiner (e.g. Wilkinson) does the transformation with additional isolation of amplifier outputs against each other.

If RF amplifiers can be paralleled without harm is also a question of power level. In case of high power amplifiers, you often use circulators to isolate the load and avoid possible amplifier damage by reflected power. They also make parallel operation easier.

Thank you for the details. For example, I'm using low power amplifiers that has output power of 4 dBm in CMOS 180 nm technology and I don't want to install external components such as power combiers and circulators at the output. Does this application falls under the special case if the power amplifiers are not damaged?

 

[FvM]

Re: two amplifier ouputs connected to same load

4 dBm is pretty low, but CMOS voltage rating may be exceeded though. CMOS transceivers often implement differential output with balun matching network, another way to parallel amplifier outputs.

 

[biff44]

Re: two amplifier ouputs connected to same load

I guess it could work. I have seen spatially combined amplifier parallel stages that worked just fine. But each amp was making the SAME signal (i.e. same frequency, waveform) and with the proper phasing per stage to make it all power combine. Also the act of spatial combining gives some modicum of isolation between the stages, which keeps them from blowing each other out.

You could hard connect two amps to the same load, and if you used ferrite isolators between each stage and the load....that would keep one from frying the other. AND you might be able to violate the phase coherency requirement.

 

[BigBoss]

Re: two amplifier ouputs connected to same load

Thank you for the details. For example, I'm using low power amplifiers that has output power of 4 dBm in CMOS 180 nm technology and I don't want to install external components such as power combiers and circulators at the output. Does this application falls under the special case if the power amplifiers are not damaged?

You can combine them but there will be issues in practice in term of voltage and current phases.If the phases are different, one of the amplifier will sink or source the signal.Also, the output impedance of each amplifier will charge the other, impedance mismatch will occur.
So briefly, it's not a preferable method..