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 ?
Thank You.
Best Answer
It depends on how much amplitude you want of course. Asking if this much amplitude is "too much" presupposes that someone will know what signal is required by whatever follows the circuit in your question.
Regards the gain of 150,000 this is extremely unlikely given the circuit shown. My guess is that the circuit is self-oscillating - look how the waveform rises from nothing to its final value - why does it gradually rise like that and then level out? I've seen intentional LC oscillators do that after power is applied and my gut is telling me you have positive feedback and have "made" an oscillator.
When you had your first stage, you had a gain of 50 and this seems reasonable. Your second stage is virtually identical so I'd expect, at best a total gain of no more than 2500. This tells me you have inadvertently made an oscilator.
I'll also add that your circuit is not clear - where is the 0V attached and could it be that the nodes marked R3(1) and R8(1) are not connected to perfect voltage sources - I'm no expert in this software suite so this may be a red-herring but the 0V point needs to be clearer on both diagrams to fully understand this.