How to enhance output Voltages of this RF Power Amplifier

[nomi114]

TL;DR Summary

How I can boost the output voltage to the desired level i.e. 15~18Vpp.

Hello everyone, I’ve been working on modifying the RF Power Amplifier circuit, and I’ve made good progress (schematic is attached below). However, I’m now facing a small issue – I’m currently getting an output voltage of around 9Vpp, but I need it to be in the range of 15–18Vpp. I’ve tried changing the values of resistors R6, R7, and R8, but I haven’t been able to reach the desired voltage level. I would really appreciate any advice on how I can boost the output voltage to the desired level. Thanks in advance for your help.

 

[Baluncore]

Welcome to PF.

What is the application of this PA?
What simulator are you using? LTspice?
Attach your .asc file as a .txt file to your next post.

1. Increase Vdd.
2. Raise the output impedance of the band-pass filters.

 

[nomi114]

Welcome to PF.

What is the application of this PA?
What simulator are you using? LTspice?
Attach your .asc file as a .txt file to your next post.

1. Increase Vdd.
2. Raise the output impedance of the band-pass filters.

Thanks for your kind reply. This PA can be used for RFID Applications (Frequency range 7.7MHz ~ 8.7MHz). Yes, i am using LTSpice simulator, I tried to upload .asc file but fail. Sure, i will attached as text file for your perusal, pls. Thanks again Sir for your kind help.

 

[Baluncore]

Thanks, the .asc file works OK.

The filter should match the impedance of the MOSFET drain to the antenna.

Do you know the output impedance of the MOSFET drain?

Will your antenna really be 50 ohms, differentially fed, or will it be fed through two 50 ohm coaxial cables, to a 100 ohm loop antenna? That loop antenna could be tuned to advantage.

If you could combine the two MOSFET drains with a centre-tapped RF transformer, you would only need to build one band-pass filter, and feed one transmission line to the antenna.

Your BPF is smooth, but has about 10 dB of loss in-band, when driven with a 50 ohm source, and terminated in a 50/2 = 25 ohm load. I would be surprised if better matching of drain to BPF, and BPF to antenna, could not give you half of that loss back.

It is good practice to place the SI prefix in the place of the decimal point. That reduces the length of the text and eliminates some reading errors.

It is poor practice with LTspice to include the unit with the component value. One day you will place a one farad, 1F0 capacitor, that will have a value of 1x10^-15; a whole femtofarad. Units also make the use of evaluated {parameters} more difficult.

 

[berkeman]

This PA can be used for RFID Applications (Frequency range 7.7MHz ~ 8.7MHz).

What are the RF licensing and testing requirements for this type of device in your country? In the US, I believe I would need to do RF testing at a local test lab that has an anechoic chamber (or an open air test site out in the countryside) to ensure that my intentional radiator device does not exceed the transmit levels for in-band and does not have too high of a level of harmonics or other out-of-band energy. Will you be doing that kind of testing on your device?

 

[nomi114]

Thanks, the .asc file works OK.

The filter should match the impedance of the MOSFET drain to the antenna.

Do you know the output impedance of the MOSFET drain?

Will your antenna really be 50 ohms, differentially fed, or will it be fed through two 50 ohm coaxial cables, to a 100 ohm loop antenna? That loop antenna could be tuned to advantage.

If you could combine the two MOSFET drains with a centre-tapped RF transformer, you would only need to build one band-pass filter, and feed one transmission line to the antenna.

Your BPF is smooth, but has about 10 dB of loss in-band, when driven with a 50 ohm source, and terminated in a 50/2 = 25 ohm load. I would be surprised if better matching of drain to BPF, and BPF to antenna, could not give you half of that loss back.

It is good practice to place the SI prefix in the place of the decimal point. That reduces the length of the text and eliminates some reading errors.

It is poor practice with LTspice to include the unit with the component value. One day you will place a one farad, 1F0 capacitor, that will have a value of 1x10^-15; a whole femtofarad. Units also make the use of evaluated {parameters} more difficult.

Thanks for your prompt reply. The question you asked is quite interesting for me as well. Because I tried to calculate the output impedance of MOSFET, but unable to find exactly impedance. According to the datasheet the output impedance f IRF510 is 0.4 ohm. The output impedance seen by the FET PA is roughly calculated by V*V/2P.

In our case V=VDD==>24V, P = V x I ==>2W. So output Impedance will about 144 ohms (roughly estimated).

The output power of the amplifier is Po= Uo 2 / RL =16*2/144==>0.2222W.

These are my calculations according to my limited knowledge. I shall be grateful to you if you can check and verify these calculations and suggest me if I am wrong.

The input impedance of the antenna is 50 ohms, that’s confirmed. So, we I must need to consider output impedance must be 50 Ohms. Anxiously waiting for your kind and prompt response.

 

[nomi114]

What are the RF licensing and testing requirements for this type of device in your country? In the US, I believe I would need to do RF testing at a local test lab that has an anechoic chamber (or an open air test site out in the countryside) to ensure that my intentional radiator device does not exceed the transmit levels for in-band and does not have too high of a level of harmonics or other out-of-band energy. Will you be doing that kind of testing on your device?

Dear Sir, Thanks for your kind reply. There is no specific requirement as you mentioned. I Just would like to enhance the voltage level at the output and output power as high as good. In some books i found they achieve the output power about 20-30W. I wish i can get too such wonderful power at the output terminal. So, if you can suggest me something fruitful for this case, i should be grateful to you for this act of kindness.

 

[Baluncore]

When this PA is operated, in what country will it be used?

When you designed the BPF, I suspect you designed it for 50 ohm input and output. Was that the case, or what were the impedance specifications for design?

 

[nomi114]

When this PA is operated, in what country will it be used?

When you designed the BPF, I suspect you designed it for 50 ohm input and output. Was that the case, or what were the impedance specifications for design?

Yes sir, you are right, I have design it for the 50 ohms. its frequency response is good. so i think there is no issue with the BPF.

 

[tech99]

I cannot quite read the circuit diagram fully, but why do you have several resistors associated with the amplifier output?

 

[Baluncore]

I cannot quite read the circuit diagram fully, but why do you have several resistors associated with the amplifier output?

Download the .pdf file from post #1, to zoom in on a circuit schematic.
The IRF510 MOSFETs have AC gate coupling and 15R gate resistors to prevent oscillation. There are 4k3 bias resistors to pull the gate voltage gently up, while the current mirrors pull it down, into the active region. That maintains the drain voltage in mid-supply, so output RF can be maximum Vpp.

 

[tech99]

Thank you. Is the amplifier being fully driven?

 

[Baluncore]

Is the amplifier being fully driven?

Yes.

Attached is a heavily modified design.
The original 27 Vpp output has now become 72 Vpp.
Drain current is 6 amp maximum.

 

[berkeman]

When this PA is operated, in what country will it be used?

Kind of telling that the OP has chosen not to reply to this question of yours...

Dear Sir, Thanks for your kind reply. There is no specific requirement as you mentioned.

Yes there are. Remember that I am a Moderator here at PF, so I can see what town and province and country you live in. Although enforcement of RF Band Good Citizenship Laws is a little looser in your country compared to the US and Rest of World, it still exists. And I can pretty much guarantee that if you interfere with government band communications in your area, you will get a visit (by people with guns, as opposed to people with radios in my country). That also goes for any commercial systems that you sell and deploy based on the advice you get here.

Vaya con Dios*.

(*sorry for the mixed metaphor)

 

[nomi114]

Yes.

Attached is a heavily modified design.
The original 27 Vpp output has now become 72 Vpp.
Drain current is 6 amp maximum.

Thank you very much, Sir, for your generous assistance and valuable recommendations. I will proceed to download the file and diligently work on it. Should I encounter any ambiguities or issues, I will not hesitate to seek your guidance once again. Your kindness is greatly appreciated.

 

[Baluncore]

Since the drive is a 7 MHz to 9 MHz square wave, the output will have only higher-frequency, odd harmonics.

There is no need for a 50 ohm input, low-ripple, band-pass filter for signals. The next step may be to replace the 10 dB loss, band-pass filter, with a low-pass filter.

The primary aim for the LP filter design should be to minimise the in-band loss, while maintaining greater than 70 dB of attenuation, at and above 21 MHz.

 

[berkeman]

Since the drive is a 7 MHz to 9 MHz square wave

Square wave drive? I missed that.

Thread is done. We are not going to help an amateur engineer interfere with valid communications, even if the country they are in is looser in regulations compared to the US and EU.