High Frequency Amplifier

J

Johnny1010

New Member

• Jul 13, 2014

• #1

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

Well-Known Member

Most Helpful Member

• Jul 13, 2014

• #2

where is the output? Q1 collector?

 

J

Johnny1010

New Member

• Jul 13, 2014

• #3

Yes

 

ronsimpson

Well-Known Member

Most Helpful Member

• Jul 13, 2014

• #4

I don't understand you amp.

 

chemelec

Well-Known Member

• Jul 13, 2014

• #5

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

 

Last edited: Jul 13, 2014

ronsimpson

Well-Known Member

Most Helpful Member

• Jul 13, 2014

• #6

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

Well-Known Member

Most Helpful Member

• Jul 13, 2014

• #7

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

 

ronsimpson

Well-Known Member

Most Helpful Member

• Jul 13, 2014

• #8

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.

 

J

Johnny1010

New Member

• Jul 13, 2014

• #9

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

Well-Known Member

Most Helpful Member

• Jul 14, 2014

• #10

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.

 

J

Johnny1010

New Member

• Jul 14, 2014

• #11

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?

 

Last edited: Jul 14, 2014