I have this circuit :
Two common base amplifiers for amplifying frequency in FM range (88 to 108 Mhz).
The Problem is, as seen in the figure, I am getting an output of 1.5 V for an input of 10 uV. That means a gain of nearly 150,000, which really bothers me.
Firstly, I would like to ask is whether this much gain or amplitude (1.5 V) is normal or is it too high. The schematic is the front end for an FM receiver and this output will be fed to a mixer for producing intermediate frequency.
Second and more importantly is the following sequence of events :
1) When I had a single common base configuration instead of two, the circuit was same (meaning I was taking output from only 1 transistor and second one was not in the picture), except the inductor in collector leg was 1 uH instead of 3 uH.
In this situation, the output was of the order of 500 uV or so ( See below)
2) Next I combined the two transistors as in the schematic above. This gave me an output of 1 - 3 mV depending on frequency. So far OK I guess.
3) Now as soon as I change even one inductor to 3 uH, the gain shoots up dramatically ( first pic). More importantly, this does not happen in EVENT 1, if I replace the 1 uH by 3 uH.
Can someone explain this, and more importantly address my first concern; Is this much amplitude OK ?