High Frequency Amplifier

[Johnny101]

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.

 

[Arouse1973]

A circuit diagram and picture of the PCB layout would be good.
Adam

 

[Johnny101]

Adam thanks for your reply here are the schematics. In my final design I used bfr93A (smd with 6 GHz UGB) still could get only 5 MHz with the PCB.

 

[(*steve*)]

Where did you get the design from? I'm always wary of high spec designs where the person drawing them doesn't orient basic components in a conventional manner.

This is high frequency RF, you'll need to have careful layout of the board. All those long looping connections will have significant inductance and capacitance at 150MHz.

 

[Johnny101]

Actually it was my first time designing a pcb I simply used the Proteus auto-routing option. Would be glad if you could guide on the board design.

 

[davenn]

agree with Steve

VHF RF design is a pretty specialist field

BE VERY wary of producing signals on 120MHz ... that's in the middle of the aviation band
you are likely to get into LOTS of trouble if you get caught
Authorities take a really dim view of illegal transmitters in that area

cheers
Dave

 

[Johnny101]

So, is there no defined method to follow or will I have to improve the layout and then hope for the best (simple hit and trial).

 

[(*steve*)]

This might be an interesting read.

 

[Johnny101]

Guys I have tried improving my pcb design please point out any problems you might observe in it.

 

[KrisBlueNZ]

If the three-terminal regulator mounts on the same side as the components, then it's drawn backwards. When you view a positive three-terminal regulator (78xx) looking at the markings with the leads pointing down, the input is on the left.

Your Q1 and Q2 markings seem to be swapped (compared to the schematic in post #3). I haven't checked for any other errors.

Where did you get the design from?

You said it's only working up to 5 MHz. Have you fixed that?

Where is the output taken from? It looks like it's supposed to be taken from the bottom end of R7 but there is no connection for it.

What do you hope to use this circuit for? What is the project?

Please answer ALL of those questions THOROUGHLY.

 

[Johnny101]

I was actually looking for errors regarding the pcb design that would cause the stray capacitances and other complex impedances to kick in at high frequency as mentioned by other members.
This was a simple project in our university we got the design from our seniors(was working at only 1MHz) and I am trying to make it work at high frequency we want to switch a simple LED with the output.
The output is taken from R7 right now I am just working on the amplifier so that's why you see no connection.
As I have mentioned that the simulation is working at 100MHz but the pcb was working at 5MHz and since I came to know from here that the pcb is faulty so I just another design trying to improve it wanted to know what improvements still could be made in this design of mine.


MHz not Mhz