Power Amplifier Designed for 8.2MHz

Thread Starter

Mike2526

Joined Nov 10, 2020
30
hi, Good evening,

I have designed a power amplifier for RF applications. it will operate at 8.2MHz. But at out i can't get good results, i think the components values are not correct/adjusted. somebody can help me in this regard then highly appreciated. My requirements are as under;


1. Two inputs square waves (VP1 & VP2) are used which are 180 out of phase with each other.
2. The configuration of amplifier is like as differential amplifier (two outputs at a same time), means at the same time we can obtain same signal with same magnitude and power but 180 degree out of phase.
3. At output (for each) we need about 30 V, and power about 7W~8W for each output.

Anxiously waiting for prompt response please and .asc file attached for your reference please.


Thanks
 

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DickCappels

Joined Aug 21, 2008
7,424
For those who might not be able to open a pdf file, such as those using some cell phones.1605110016331.png

You might improve bandwidth slightly by biasing the 2n3904's and 2n3906's into class A-B like this:
1605110334104.png
 

Thread Starter

Mike2526

Joined Nov 10, 2020
30
For those who might not be able to open a pdf file, such as those using some cell phones.View attachment 221993

You might improve bandwidth slightly by biasing the 2n3904's and 2n3906's into class A-B like this:
View attachment 221995

Dear DickCappels, Thank you very much for your help and suggestion, please suggest me what should be the values of R1, R2, Cin & Cout ?? Waiting for your Kind and prompt reply.
 

DickCappels

Joined Aug 21, 2008
7,424
Based on Bordodynov's simulation above, it is apparent that the existing input buffer works just fine. So it is best to leave it as it is.
 

Ian0

Joined Aug 7, 2020
2,183
Didn't he say it worked in the MHz region? But how is he keeping the bias stable as those MOSFETs warm up?
Sorry - I forgot - things don't warm up in SPICE.
 

Bordodynov

Joined May 20, 2015
2,865
With those gate divider ratings that I have set, transistors work in class "C". The constant displacement on the gate is less than the threshold voltage. The power on the transistors is less than 10W. Need a heat sink. I took an old transistor that has a normal safe working area. Unfortunately, more modern transistors do not have this quality. And to use specialized transistors, designed for linear operation is very expensive at a price!
 

Thread Starter

Mike2526

Joined Nov 10, 2020
30
Dear Bordodynov,

I did work the power Amplifier circuit as suggested , as per my retirement i have added a bandpass filter with PA. This filter works very well for the frequency of 7.5MHz ~ 8.7MHz, verified with some other circuits & Software. But when i attached the BP filter with PA, at output i can get only mV and power uW across the load resistance (R11 & R12) , while i need about 20V and 15W at each output terminal (Across R11 & R12).

Screen shot is attached for your reference please.

Anxiously waiting for your kind and prompt reply.

Thanks
 

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Thread Starter

Mike2526

Joined Nov 10, 2020
30
I advise you to recalculate the filter - expand the bandwidth and shift the center frequency to the bottom.
Test from the beginning using the Test1 circuit.
Dear Bordodynov,

Thank you very much for your kind reply. I will made changes as suggested. if i meet with some issue/problem i will turn back to you. I am really thankful to you for this act of kindness.

Regards
 

Thread Starter

Mike2526

Joined Nov 10, 2020
30
Dear Respecetd Bordodynov,

Hope you are doing well. I have designed circuit as per your kind suggestions. circuit works well, and full fill all requirement as per my requirements. But i found at out power level is not stable, There is variation at power level as you can observe in the attached screen shot. Please help me in this regard, how can i get a stable output power level.

Thanking you for your kind cooperation and help.

Regards
 

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