Mosfet, Mesfet vs Bipolar transistors in RF circuits

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I was wondering in general what types of semiconductors are used in most solid state RF power amplifiers like the ones at cell base stations and elsewhere. Do they use mosfets etc which can only switch a square waveform or are bipolar ones also used that can output sinusoidal outputs that can be directly fed into an antenna?

Do mosfet amplifiers have an output filter much like the lower frequency class D audio amplifiers or does the antenna function as a filter at such high RF frequencies?

I imagine for example an RF amplifier outputting a QAM modulated wave into an antenna so that the signal at the antenna looks like higher and lower amplitude half periods of a sine, but if the amp works on mosfets, does the signal at the transistor output is the same amplitude but square ?


One of the reasons I ask this is because I have had limited experience with RF circuits and I am building a est device where I would want to excite an RF cavity that has no capacitance but only inductance associated with it and I would like to drive it with an RF signal that is sinusoidal so I am wondering do bipolar transistors come with high enough power levels at RF frequencies and aren't they too inefficient due to the sinusoidal operation mode vs the on/off of the mosfets etc?


PS. at RF frequencies is there any place where a square wave is used or is it all just sinewaves and modulated sinewaves?
I guess one cannot even achieve a square wave radiating from an antenna but how about some special purpose RF circuits?
 

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  • #2
Baluncore
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Do they use mosfets etc which can only switch a square waveform or are bipolar ones also used that can output sinusoidal outputs that can be directly fed into an antenna?
Can you please provide a reference to that square/sinewave restriction at RF ?
 
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  • #3
tech99
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I think all devices can operate either in a linear or switched mode. If you have a signal which relies on amplitude variations, such as AM or QAM then the device must be operated in a linear mode. This usually means adjusting the bias so it is slightly "on" with no signal. If the signal is just on or off, or maybe is frequency modulated, then the device can operate more efficiently by using it in a switched mode, or something approaching that (such as Class C). If you use a switched mode, the output filter (or tuned circuit) will remove the harmonics, so we still end up with a sinusoidal carrier. However, we are unable to amplitude this carrier. Your cavity will remove harmonics, so the waveform will be sinusoidal even if the cavity is driven with a square wave carrier.
 
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  • #4
davenn
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I was wondering in general what types of semiconductors are used in most solid state RF power amplifiers like the ones at cell base stations and elsewhere.
MOSFETS or variations there-of are the common devices used

Do they use mosfets etc which can only switch a square waveform
This is completely incorrect ....
Not sure where you got your idea from ?

Many/most transmitters and receivers these days ( and for quite some years)
use MOSFETS and and perform very well with analog signals
ALL my transceiver gear has MOSFETS for the RF power amplifier stages




cheers
Dave
 
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  • #7
davenn
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. I've never heard of a power JFET.
Did you not read any of the data in Alan's link ?
They show many high power, GHz frequencies JFETS :smile:
here's just a few of them......

Clipboard03.jpg
 
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@Baluncore and @davenn Well I think I misspoke, I did not intend to say that mosfet driven amplifiers can only output square waveforms but I assume that internally the very transistor itself works in a on/off square wave fashion does it not? Because after all that is the very reasoning behind saving energy and heat dissipation.


But like @tech99 said the output filters or reactance of rf cavities or antennas will "round off" a signal so that eventually it resembles a sine.
 
  • #9
DaveE
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Did you not read any of the data in Alan's link ?
They show many high power, GHz frequencies JFETS :smile:
here's just a few of them......

View attachment 271779
Oops, yes you're correct. I tend to put the GaAs, GaN, and SiC devices in a different category, but yes they are JFETs. I was thinking of Si devices.
 
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  • #10
Baluncore
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Well I think I misspoke, I did not intend to say that mosfet driven amplifiers can only output square waveforms but I assume that internally the very transistor itself works in a on/off square wave fashion does it not?
No it does not. It is more like a voltage controlled variable resistor. A vacuum tube is just a FET with a pilot lamp. They both do the same jobs. Vacuum tubes worked well in linear circuits for 50 years.
It takes only about 0.5 volt to switch a BJT between on and off. A MOSFET requires close to 10 times that gate voltage. I see no reason why you would want to use a MOSFET for square wave switching at RF. MOSFETs have a higher gate or miller capacitance than the base of a BJT or the grid of a VT, so turning MOSFETs all the way on then all the way off at RF, would be quite inefficient.
 
  • #11
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@Baluncore hmm I see your making a point here. Well vacuum tubes were used for both non linear and linear operation just that in linear operation like a class A amp the lamp current was never allowed to go below a certain level so that the lamp is always on. In the Mosfet analogy to be honest I've never heard of a mosfet amplifier circuit where the mosfet is always on because the mosfet very function is such that it's either conducting or not as inbetween it has a higher internal resistance which would dissipate alot of heat if used under high power. I've even learned this from experience when using mosfets in power supplies and the gate voltage goes "bad" and turns the fet only partially on.
Sure a mosfet can be always ON like in a ssd lamp switch but then it's either always off or on not in between.


I do see your point about the gate capacitance, because the amount of energy required to charge/discharge that gate capacitance is a function of the capacitance itself and the number of times it is discharged/charged so when the frequency gets really high one would need a substantial power put into "driving" many such parallel fets , is this your point?

PS. I feel like I don't know something here because when speaking about mosfets i'm typically thinking the larger power types used in smps and other switching circuits and all of them work in the fully on/fully off manner, but from your language I suspect that there might be something different in RF circuits with mosfets, can you elaborate?
 
  • #12
DaveE
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the mosfet very function is such that it's either conducting or not as inbetween it has a higher internal resistance which would dissipate alot of heat if used under high power.
This is true for any type of device when operated as a linear amplifier. That is the point, you turn it partly on and it will dissipate heat. This has nothing to do with the structure of the device (BJT, FET, Tube, etc.) it is the design intent in the circuit.
 
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  • #13
DaveE
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I once designed a very large linear power supply which was made of 48 large MOSFETs (SOT-227) in a series/parallel arrangement. It was water cooled and could (was supposed to) dissipate 5KW of heat. It was very reliable (with N+2 redundancy) and is still in production. Trust me, you can operated MOSFETs as linear devices.

One issue I had was that I designed for a 200KHz bandwidth, but had to deal with some parametric (i.e. sharing) oscillations at frequencies as high as 200MHz with these devices, which were not intended for RF applications.

edit: BTW, there are some large MOSFETs with multiple die that do not work well in high power linear applications. This is about device construction and selection and not intrinsic to the MOSFET dice.
Sabre_Passbank 002.jpg
 
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  • #14
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yes sure @DaveE, I somehow got confused , the mosfet can be operated below saturation just that the current through it must be limited I assume otherwise it could destroy the device , so one probably wouldn't want this mode of operation in a typical power supply or am I wrong ?
 
  • #15
Baluncore
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PS. I feel like I don't know something here because when speaking about mosfets i'm typically thinking the larger power types used in smps and other switching circuits and all of them work in the fully on/fully off manner, but from your language I suspect that there might be something different in RF circuits with mosfets, can you elaborate?
When it comes to digital on/off power switching, the power MOSFET is best for switching high currents occasionally, while the BJT wins for high frequency switching.

That is because to saturate a BJT requires excess base current flow continuously, for as long as it is on. A saturated BJT will have a Vce of about 20 mV. PN junctions do not do well in parallel since the negative temperature coefficient causes the hottest patch take the greater current, which, due to positive feedback, results in a hot spot.

To saturate a MOSFET only requires the gate voltage change once, but that change takes time and current to charge the gate capacitance. A saturated power MOSFET will have a Vds of about 2 mV. MOSFETs work well in huge parallel arrays because channel resistance shares the current fairly.

With linear and RF circuits, the BJT and the MOSFET generate similar amounts of heat.
FETs are used in low level RF amplifiers because FET circuits can have lower noise characteristics than BJT circuits.
 
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I guess the best way to put it would be to say that a mosfet can also work in linear fashion but within just a fraction of it's maximum rated (usable at room temps) current capacity which can only be fully used in the non-linear aka switching operation mode.

@Baluncore so I've heard long ago about the bipolar transistor tendency to form hot spots and thermal runaway etc, but how about mosfets , say if operated within a linear region with rather high currents, can't they also break down from similar problems ?
 
  • #17
DaveE
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Every part of the data sheet for devices was written for a reason. You must use devices within their intended limits. A common rookie mistake: not reading and understanding ALL of the data sheet.
 
  • #18
Baluncore
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... but how about mosfets , say if operated within a linear region with rather high currents, can't they also break down from similar problems ?
MOSFETs work well in huge parallel arrays because channel resistance shares the current fairly.
The low ON resistance of a power MOSFET array comes from hundreds of MOSFETs, all in parallel. Their individual channel resistance is high, but once in parallel the total is low.

A power MOSFET is designed and rated for switching maximum OFF voltage, and for maximum ON current, but only one maximum at the time. The package decides the power rating.
A BJT is rated for maximum power in a linear application. That may have coloured your misconception that MOSFETs are only good for switching high currents.
 
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  • #19
DaveE
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MOSFETs work well in huge parallel arrays because channel resistance shares the current fairly.
Yes! As you said, in saturation.

In linear high power operation the gate characteristics also matter (primarily Vgth and it's TC). They work well in linear applications when the gate parameters are matched; of course, you get this for devices paralleled on the same dice (or wafer?). You often don't get good matching for multiple die.

At low frequencies, MOSFETs have an advantage over BJTs for linear, high power, moderate to high voltage applications because they (mostly) lack the second breakdown limits of the SOA. But be careful here, MOSFETs not having second breakdown is a myth, they do, it's just normally much better than BJTs; I think because of the parallel cellular resistive current sharing you described. How this applies in RF designs isn't something I ever looked into, but I think it's the same because it also explains rugged switching performance.
 
  • #20
DaveE
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I guess the best way to put it would be to say that a mosfet can also work in linear fashion but within just a fraction of it's maximum rated (usable at room temps) current capacity which can only be fully used in the non-linear aka switching operation mode.

@Baluncore so I've heard long ago about the bipolar transistor tendency to form hot spots and thermal runaway etc, but how about mosfets , say if operated within a linear region with rather high currents, can't they also break down from similar problems ?
Time to learn about the SOA part of the data sheet, I think:
file:///C:/Users/4dave/Downloads/application_note_en_20180726.pdf
 
  • #21
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Do they use mosfets etc which can only switch a square waveform or are bipolar ones also used that can output sinusoidal outputs that can be directly fed into an antenna?
I think that both bipolar and mosfet can also be used for square wave input or sinusoidal input, just pay attention to the impedance matching at the input.

Do mosfet amplifiers have an output filter much like the lower frequency class D audio amplifiers or does the antenna function as a filter at such high RF frequencies?
Generally speaking, the output of the RF power amplifier should use filters, including the LC filter and the influence of resonant frequency/bandwidth of the antenna itself.

I imagine for example an RF amplifier outputting a QAM modulated wave into an antenna so that the signal at the antenna looks like higher and lower amplitude half periods of a sine, but if the amp works on mosfets, does the signal at the transistor output is the same amplitude but square ?
The carrier of the radio signal must not be a square wave, it is impossible to allow an infinite bandwidth radio signal be transmitted into space, but the modulating signal can be approximated as a square wave, and the mosfet can of cause be used as AM amplifier.


One of the reasons I ask this is because I have had limited experience with RF circuits and I am building a est device where I would want to excite an RF cavity that has no capacitance but only inductance associated with it and I would like to drive it with an RF signal that is sinusoidal so I am wondering do bipolar transistors come with high enough power levels at RF frequencies and aren't they too inefficient due to the sinusoidal operation mode vs the on/off of the mosfets etc?
Bipolar and mosfet can also achieve high power, please check the link https://www.mouser.com/Semiconductors/Wireless-RF-Semiconductors/Transistors-RF/_/N-ax2e7

Regardless of the inductance or capacitive reactance of the RF cavity, as long as this parameter is included in the design of the antenna output matching/filtering circuit.

If you are very concerned about efficiency, you can consider using class E amplifiers, but it seems that class E power amplifiers are still difficult to achieve high operating frequencies, such as higher than 100 MHz.

https://cdn.macom.com/applicationnotes/AN4001.pdf
"A class E power amplifier operating at 81.36 MHz has been designed and built using MACOM MRF151A power MOSFET. Using a 48V power supply, the amplifier yielded 300 watts of output power with better than 82% efficiency .."


In general, I think the design of high-power and high-efficiency RF amplifiers is not easy. In practical applications, you may find that it is more complicated than designing small-signal RF amplifiers.
:smile:
 
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  • #22
DaveE
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In general, I think the design of high-power and high-efficiency RF amplifiers is not easy. In practical applications, you may find that it is more complicated than designing small-signal RF amplifiers.
I think this maybe the understatement of the century. These are VERY DIFFICULT designs, IMO.
 
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  • #23
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Well ok so far so good, but I have another question that might come up as a limit to what I would like to make.
Say I have a bunch of small planar rf coils all stacked in a line some 1 meter long, each coil has it's own small (mosfet or bipolar doesn't matter for the argument) transistor that drives the coil. Then each of the transistors would have to be connected in parallel and driven from a larger transistor.
Assume the transistors are capable of for example 10 Ghz operation, so each coil is small and each transistor is right next to it so far so good, but then the traces connecting all the transistors in parallel are rather long, how would this affect the chance of achieving high frequencies ?


I guess I am asking , doesn't the paralleling of devices cause problems in terms of frequency limit if the traces used to parallel the devices get too long? as I would effectively create an antenna which is not part of the plan.
 
  • #24
Baluncore
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I guess I am asking , doesn't the paralleling of devices cause problems in terms of frequency limit if the traces used to parallel the devices get too long?
Yes they do. No matter what you do at 10 GHz, (λ = 3 cm), with coils spread over 1 metre, you will have an antenna array. What frequency or waveform do you want to transmit?

You need to explain why you need to arrange a 1 metre long, linear array, of loop antennas. Then we can describe a transmission line length independent solution for your problem.
 
  • #25
berkeman
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doesn't the paralleling of devices cause problems in terms of frequency limit if the traces used to parallel the devices get too long?
Are you familiar with how RF splitters/combiners and transmission lines work? The biggest issue is attenuation going through the splitters and loss in the TLs.
 

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