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RF Amplifier @ 200MHz

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zachi

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bf199 amplifier

Hi ...

I want to build a 30 dB RF amplifier for VHF range (from -40dBm to -10dBm), so I use two stages of 15dB CE transistor amplifier(BF199) .

Every stage work great @ 200MHz , but when I connect them together..the signal is distorted !!!!!!!!

I try to connect many capacitors (1p - 1mm) between them, but I get the same results.

If I connect them, and check @ 50 MHz - they work great and I get more then 30 dB gain.

what is the best way to solve this issue :?:
Thanks in advance,
 

mr_ghz

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bf199 spice

Hi zachi,

what happens exactly to your signal: When you take both transistors do
you have distortion even with very small input signals? When you take
one transistor and more input power (15dB more, so it will act like the 2nd
transistor in your full design) do you have also distortions?
What are the input/output impedances of the both amps?

Bye
 

Borber

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bfr96 amplifier

Give us schematics of amplifier. I think it's better to use BFR90 if you want to make widebandwidth amplifier. BF199 has ft=550MHz. What do you mean by distorted.
 

BigBoss

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bf199 spice model

zachi said:
Hi ...

I want to build a 30 dB RF amplifier for VHF range (from -40dBm to -10dBm), so I use two stages of 15dB CE transistor amplifier(BF199) .

Every stage work great @ 200MHz , but when I connect them together..the signal is distorted !!!!!!!!

I try to connect many capacitors (1p - 1mm) between them, but I get the same results.

If I connect them, and check @ 50 MHz - they work great and I get more then 30 dB gain.

what is the best way to solve this issue :?:

Thanks in advance,

It's very normal..

When you apply -40dBm+30dB(Gain) then
Output Power becomes -10dBm

When you apply -10dBm , Output Power will be +20dBm..

Have you considered the operating point of the amplifiers ? This is very important because of P1dB compression point. If you have set the operating points of the amplifiers being as quite low, the transistors go into deep saturation and create much harmonics and spurious.

Instead of this , use MMIC Gain Block that is very well characterized and with good behaviour.

Or use transistors but simulate before do it..

I think BF199 is very very old transistor and is not good choise.Because if you want to work -40dBm , Noise figure will also be very important.

Regards
 

Vulcan

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bfr96 spice model

hey all....
i'm zachi's partner working with him on the same project.
We used the RF Generator to give us a -30dBm @ 200MHz as an input test signal.the Output was measured using a spectrum analyzer.

after playing with the values of the resistors and capacitors, we got the following configuration (Schematics Attached) which gave us the best results, meaning a 10-15dB Gain.
but when we connected a second stage (cascaded two amp) the reading on the spectrum analyzer showed Loss instead of Gain.
 

molloy

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bf199 rf

Vulcan said:
hey all....
i'm zachi's partner working with him on the same project.
We used the RF Generator to give us a -30dBm @ 200MHz as an input test signal.the Output was measured using a spectrum analyzer.

after playing with the values of the resistors and capacitors, we got the following configuration (Schematics Attached) which gave us the best results, meaning a 10-15dB Gain.
but when we connected a second stage (cascaded two amp) the reading on the spectrum analyzer showed Loss instead of Gain.
You spoke about the gain, but what about the P1dB? You are sure you optimized the P1dB? And what about the matching network (input and output i mean)?
 

Borber

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bfr90 tv amplifier

You can do nothing with BF199 if you want 30dB gain at -10dBm input. Allso supply voltage is too low to achieve -1dB comp. at 20dBm output.
You should use at least BFG198 for out stage and BFR96 for first. Use emmiter follower for output.
Do you need wide bandwidth amplifier or...?
 

zachi

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bfr96

I think there is some misunderstanding, so I will start from the beginning:

1. I want to build a wideband (7MHz), VHF (200MHz) ,30dB gain amplifier
2. The input level is -40dBm .
3. I want to use two levels (15+15) of amplification using two BF199 transistors
4. The Transistor is connected in a CE mode .
5. The Amplifier is working great @ 50MHz
6. The amplifier doesn't work @ 200MHz (working as a attenuator not as an amplifier)
7. If I check each transistor alone, I see no problem.

So again, how do i solve this problem
 

flatulent

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rf amplifier 200mhz

It looks like a loading problem or an out of band oscillation problem. To check for the oscillation, use some form of detector like a very high frequency spectrum analyzer or a detector diode to see if the stages oscilllate when connected in tandem.

The loading is harder to check for in a working circuit unless you have expensive test equipment. One trick is to use a third transistor as an emitter follower between the stages and see if that helps things out. The other thing to do is use a simulator where you can "mesure" the current and voltage in the wire between the stages to calculate impedance.

It might be a slew rate limiting problem. Try measuring the gain at much lower signal levels.
 

Borber

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bfr96 amp

Your amplifier can not have wanted gain in band 7MHz to 200MHz because the ratio of collector and emitter resistors resistances is not enough. For 15dB of gain it should be about 5.6. Your case is 350/250=1.4. Peak of gain at app 60MHz is caused by overcompensation by emitter condensers 100pF. Output impedance so as input, I believe, should be 50Ohms.
I made few corrections and simulations on your circuit with MC7 but result is poor frequency response (BF199 ft=550MHz). I tried allso BF959.
Finally, I sugest circuit that can satisfy your needs. Resistors R17 R18 and L1 compensate for return loss.
 

VSWR

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bf199 in am circuits

Simulating one stage of your amplifier in GENESYS 2003.03 gave the result below. Note S11 is positive, indicating an unstable amplifier design. You must match the transistor input and output to 50 ohms (using L's and C's), then check the K-factor for stability. Before even thinking of cascading the amplifier stages...

I picked the BF199 SPICE parameters from

https://www.semiconductors.philips.com/models/spicespar/data/BF199.html

and used the Genesys 'Link to SPICE model' feature in the simulation.
 

zachi

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bf959 pspice model

Hi Borber ,

first of all - thanks !!!!

you send me two response graph.....what is what ???

Can you please tell me the guides line for designing an amplifier ??

you talk about...."ratio of collector and emitter resistors" ...it the first time I hear about this thing (please explaine)

What software did you use for this simulation ?

Zachi
 

Vulcan

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bfg198

thank you for your reply...!!...

a little misunderstanding i have to correct...

the gain is needed along a 7MHz band on a center frequency of about 203MHz (meaning from ~ 199.5MHz to 206.5MHz)

and not from 7MHz to 200MHz...sorry if you were mislead.
 

flatulent

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transistor 200mhz

If you want a narrow band of frequencies, use a tuned amplifier. Shunt the load resistor with an inductor. This will make the gain bandwidth product depend upon the tuned bandwidth and not the band from DC to your operating point.

After looking at your circuit, the second stage is loading the first stage both real and capacitive impedance loading. You need an emitter follower between them.

A further trick is that when operating well below maximum gain the bandwidth is limited by the input capacitance (c sub pi of the hybrid model) to much lower than the gain bandwidth product divided by the gain (sometimes by over a factor of 10-100) The trick to minimize this problem is to add some unbypassed emitter resistance. this feedback effect raises the input impedance which makes c sub pi seem smaller to the outside world.
 

jwei363

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200 mhz rf amplifier circuit for

why not try gain blocks from mini circuits,siranza and so on
 

flatulent

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bf199 output impedance

frequently the cost difference between block modules and discrete designs is enough on the side of the discrete design that it is economically necessary for mass produced products. This is especially true in countries that charge import tax on foreigh electronic devices.
 

Borber

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bf199 output resistance

In circuit.zip there are:
circuit.cir simulation file for Micr@cap7
circuit.gif suggested amp
BF199 freq responce.gif is freq responce of your circuit
I appologize. Corrected version is uploaded.
Quick simplified way to explain Rc/Re:
Emitter current so as collector current depends on input voltage on base of transistor and value of emitter resistor. Input voltage will apear on emitter resistor reduced by 0.9 factor because of h11. Because emitter and collector currents are allmost equal, the AC voltage drop on collector resistance will depend on value of it as long as stage is linear operation.
So gain of such stage is allmost equal to Rc/Re.

You are actually attempting to make 200MHz TV channel amplifier. Am I right? In this case you must use double tuned filters on the input so as on the output of amplifier. In that case out and inp. impedances must be 75Ohm.
 

zachi

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bfr90+soice model

yep..that is he case

I'm building a tv transmittter and I need to amplify the power from the modulator
 

sammy

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bf199 spice modell

What do you think about a monolitic amplifier like mav.11?
 

brian39

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+s11 is positive +simulation

Hi , I am a newer for amp designing.
Here is a problem , flatulent said that "the second stage is loading the first stage both real and capacitive impedance loading. You need an emitter follower between them. " why to place a emitter follower between the first and second stage?
 

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