common-base LNA 1

[Leiser]

Hi,

I am investigating the schematic of a FM common-base amplifier. It has two diodes that I do not know what are they used for.

D4 are two BAV99 diodes in a SOT323 package. I think they are used as an ESD protection. Does anyone know why are connected in such a way?

Pin 4 of the transformer is the RF input signal.

 

[VEBill]

The first thing to do would be to redraw the schematic in accordance with the circuit architecture. As drawn: the power is on the top (good) and the grounds are mostly on the bottom (good), but after that it goes haywire. The input(s) should be on the left, the output(s) on the right. If it's common base, then draw the transistor with the base terminal on the bottom.

Once the circuit is drawn "correctly" (<- my opinion), then the function of each component typically becomes obvious.

 

[Leiser]

Good advice!

I have done the layout according with a common-base configuration.

It's more clear now and easy to understand. After checking I thought these diodes could work as a power supply reverse polarity protection for Q1, but C9 confused me.

Well, if you have any comment they will be wellcome.

 

[VEBill]

Anything on the Base is very signal grounded (C13 & C14), thus leaving only DC bias. The upper D4 seems like protection as you've noted. Bottom D4 is less clear due to C9 exactly as you've noted. Mysterious diodes are sometimes used for temperature compensation.

Standby to see anyone else drops by the thread...

 

[BrianG]

Where's the d.c. path for the emitter? Unless it's through the RF In side of TR1 to a ground via a low resistance, then I can't see how this circuit will work.

 

[Leiser]

You are right. I have checked it and there is an inductor with a series resistor to ground at the RFin pin, as you said (just at the input of the transformer)That is the d.c. path for the emitter.

I have also removed D4 and the amplifier keeps amplifying, I think they are used as some kind of protection. Any ideas about what they are for?