High Frequency Amplifier

[Johnny1010]

Hello everyone I am trying to built a discrete transistor amplifier using 2N3904 npn transistor. I am trying to achieve a 4Vpp output with 0.7Vpp (Sinusoid) input with a bandwidth of 120 MHz.

The amplifier that I have designed meets my requirement when simulated in Proteus but when implemented on PCB the gain starts to roll off after 1MHz. Any help would be appreciated. In my final design I used BFR93A (6 GHz UGB) still got upto 5MHz

 

[ronsimpson]

where is the output? Q1 collector?

 

[Johnny1010]

Yes

 

[ronsimpson]

I don't understand you amp.

 

[chemelec]

This is Bigger to See.
But not sure I like your design.

 

[ronsimpson]

I built your amplifier in spice. The gain is very high. That is why you don't like the frequency response.

First transistor has a gain of 1.
Second transistor has a very high gain.

Input=.7V, Output =4V, Gain=5.7 Is that what you want?

 

[ronv]

How is your layout? 50pf looks like 25 ohms at 120 Mhz.

 

[ronsimpson]

The 2N3904 only has a gain of 3 at 100mhz.
You should use that transistor at 50mA collector current if you want to get speed out of it.

 

[Johnny1010]

ronsimpson the thing is I used BFR93A(6GHz UGB) in my final design and the simulation works like charm at 100MHz. So that is what has got me bewildered that why isn't it working practically.
I have attached my pcb layout and also my simulation (even checked it on square wave) snapshot at 100MHz using BFR93A.

Red-output green-input.

 

[ronsimpson]

I would not have tried a 100mhz amplifier on a single sidded board.
I never use 6Ghz transistors with long traces. Your trace length is long enough to make inductors that will cause the transistors to osculate at 1ghz.
>your scope is not fast enough to see that.
This layout might be ok for audio not 100mhz and defiantly not good for 6ghz transistors.

 

[Johnny1010]

ronsimpson if you could please guide and give a well defined method for the pcb layout of my amplifier design will just shortening the paths resolve the issue. What else is to be taken care of?