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Some questions about transistor datasheet

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baby_2

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Hi,
I'm going to design a 100mhz amplifier.I have learned S parameters and amplifier design but I have some questions (because I've read designing procedures but don't have design anything)

1)when should we design with s parameters and when should we design according to pi transistor model?
2)how can find the maximum frequency of a transistor? is it ft? does we have any control of current to get our desire beta (beta*Ic=Ft)
3)what is bandwidth in transistor datasheet? does it define maximum frequency operation of transistor? or something else?
4)does we have any exist component for amplification between 30mhz to 120 mhz?(RF amp ICs)?

thanks
 

Beta*Ic=Ft? I don't know where you got that, but, no. Ft is the frequency at which beta=1.
There many high frequency opamps available.
 
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    baby_2

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Hi,
I'm going to design a 100mhz amplifier.I have learned S parameters and amplifier design but I have some questions (because I've read designing procedures but don't have design anything)

1)when should we design with s parameters and when should we design according to pi transistor model?
2)how can find the maximum frequency of a transistor? is it ft? does we have any control of current to get our desire beta (beta*Ic=Ft)
3)what is bandwidth in transistor datasheet? does it define maximum frequency operation of transistor? or something else?
4)does we have any exist component for amplification between 30mhz to 120 mhz?(RF amp ICs)?

thanks
1-You design an amplifier either using with s-parameters or PI equivalent circuit that is valid relatively low frequencies only.Because some RF components which will appear in higher frequencies are not covered in PI equivalent circuit .Therefore we use s-parameters while designing an RF amplifier in order to characterize the transistor up to mmWave .
2-There is no "max. frequency" of a transistor, there is only Transition Frequency of a transistor where at Current Gain drops to unity and your equation is not correct.
3-Bandwidth isn't related to the transistor, instead it's related to your design.
4-Many amplifier can be found from many manufacturers if you search google.
 
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    baby_2

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Hi,

I think that as with everything, derating a max. value is common sense, not sure how much for a transistor's operating frequency. Possibly things like input resistance and output resistance could affect the actual frequency, along with any unforeseen capacitance.

I think analog/amplifier design books explain BJT fT in detail including the calculations.

I recollect reading that the datasheet parameter is not something to expect to achieve in an application (unless your final design incorporates a test laboratory :) ).
 
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    baby_2

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Hi,
Thanks for your explanation
Yes,I've read more about Ft and you are right that ft define a frequency where beta is equal to 1 but someone confused me with his definition about his designing rule where Ic*beta=Ft. So at which frequency(max) we can find linear beta ( approximately a fix value in a wide range of working frequency) according to datasheets info? because I don't see any hfe-ft diagram
03483.png

1)I want to know in what frequency range we can use pi model and design and when should we use s parameters?( because I've heard and read that pi model is valid up to ft/5 but I think it should not be correct because for a high freq transistor with ft=25ghz ,@ f=5ghz transistor behavior is so complex)
2)I think for f<100mhz we can use pi model for designing an amplifier easily. However, I see different design with y and S parameters and I can't see anything about Pi design near 100mhz. can you introduce me a good reference to design a LNA with pi model?
 

Forgetting about the High-frequency aspects of the design for a moment, you seem to be very concerned about constant beta. That is not the way to approach this. Manufacturers specify a range for beta, not an absolute value. And that's why you need to use feedback. Your feedback network will establish the constant gain you are looking for (as long as beta remains high enough over your bandwidth).
 
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Thank you barry for your explanation and your contribution
Yes, but for feedback network to have a specific and constant gain "loop gain" value should be big enough that for 100mhz is so complex and big ( for example for have a large band-width and gain we should have different cascode stage and different stages)
 

existing amps? Piece of cake:

https://www.rfmd.com/store/downloads/dl/file/id/28594/sbf4089z_data_sheet.pdf

There are all sorts of DC coupled gain blocks out there. Your goal is to find a junky old design that does not go too high in frequency, so that layout will be trivial. If you instead pick one that has gain from DC to 8 GHz, you can easily turn it into an 8 GHz oscillator if you do not understand microwave layout techniques.
 

It will help to know what sort of gain and output power do you require?
 

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