I have the following circuit : enter image description here

It is meant to amplify signals in microvolt range and around 100 MHz. Now in the figure, the transistor is an ideal one. So it works fine and as expected. But as soon as I replace it with actual model, it doesnt work . I need either 2n3904, 2n2222 or bc547. And all three dont work. I tried tinkering around with capacitor values, but no use.

I have seen a few radio receiver schematics for FM ( 88 to 108 MHz). All of them make use of 2n3904 for RF Amplifier ; so it must work for my scenario as well, but it doesnt. (I referred to those Elenco radio kit manuals).

What am I missing ?

More info : Q - point : Vce = 6.3v --- Ic = 3.9 mA --- Vcc = 12V

Thank you.

  • \$\begingroup\$ I can't comment on the simulation, but when you try and build this imagine ~1pF of (stray) capacitance between the various nodes. Then look up the impedance of 1 pF at 100 MHz. (And you'll know why RF stuff always uses low resistance values.) (And why is C2 only ~ 2pF?) \$\endgroup\$ – George Herold Nov 14 '14 at 14:18

You wrote "All of them make use of 2n3904 for RF Amplifier".
But do they use it in the same way?

E.g. you are using a common emitter circuit but they may use e.g. a common base circuit. The maximum frequency both circuits are able to amplify are quite different (i.e. much higher for common base). In your case e.g. the Miller effect might be the problem.

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  • 1
    \$\begingroup\$ +1 In fact, this Elenco circuit (page 54/55) uses a common-base 2N3904 amplifier for the front end. \$\endgroup\$ – Spehro Pefhany Nov 14 '14 at 14:10
  • \$\begingroup\$ @Curd .Thanks. After an hour of effort, the CB configuration worked, although the max gain I am able to achieve with real model (2n3904) is only 10 or 20, that too with very low source resistance ( less than 5 ohm) \$\endgroup\$ – Plutonium smuggler Nov 14 '14 at 16:26

Just to add to the common base solution...

A classic way to increase voltage gain with an RF amplifier is an inductive load (L) in place of Rc.

The idea is that the L forms a parallel tuned circuit with the output and load capacitances and possibly a trimmer capacitor for adjustment. This tuned circuit makes a high load impedance at its resonant frequency, to increase the voltage developed by the collector current.

You may need a suitable R in parallel with this tuned circuit, to keep the Q low enough to cover the whole frequency band of interest. Instead, the original Elenco design tuned this circuit with one section of the tuning capacitor, so that its resonance was always at the frequency of interest.

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