I have this circuit :

enter image description here

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)

enter image description here

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.

  • 2
    \$\begingroup\$ It's oscillating, and probably acting as a low power transmitter. Also, the first stage output should come from Q1 collector, i.e. before the choke, not after. The choke will block any signal at the frequency of interest. (Zoom in to find the actual oscillation frequency) \$\endgroup\$ – Brian Drummond Nov 16 '14 at 12:00
  • 2
    \$\begingroup\$ Are you sure that the observed output (first picture) is the amplified version of the input signal? The start of the output wave looks as if it is a self-sustained oscillation.(Brian Drummond was quicker by 30 sec). \$\endgroup\$ – LvW Nov 16 '14 at 12:01
  • \$\begingroup\$ @BrianDrummond . Thanks. Measuring output directly from collector in cascaded config solved the problem. \$\endgroup\$ – Plutonium smuggler Nov 16 '14 at 12:42

Is this much amplitude OK ?

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.

  • \$\begingroup\$ Just by tripling an inductor ? Then how can I be sure that by using 1 uH inductor in both amplifiers, it is indeed amplifying and not oscillating ? And that ground node is cut in pic ( the line going from first bjt's emitter has ground) \$\endgroup\$ – Plutonium smuggler Nov 16 '14 at 12:06
  • \$\begingroup\$ Tripling the inductor could cause the circuit of the 1st transistor to match the resonant tuned frequency of the 2nd transistor and the gain will sky-rocket at just one frequency. It's unclear what C15 is connected to in your measurement so I can't be sure of course. \$\endgroup\$ – Andy aka Nov 16 '14 at 13:00
  • \$\begingroup\$ It was at all frequency of interest though (88 to 108 MHz). C15 has voltage probe attached. Anyways, taking output directly from collector solved the problem. Thanks. \$\endgroup\$ – Plutonium smuggler Nov 16 '14 at 13:04

It's not amplifying, it's oscillating- that's why you get the slow build-up in the envelope.

It is possible to get higher than expected gains from an amplifier by the use of regenerative circuits-- in vacuum tube circuits a 'tickler coil' was used (cp. tickling the dragon's tail). The idea was to have the circuit close to oscillation, but not quite, so the signal was effectively amplified multiple times.

As in your oscillating circuit, the high steady-state gain would take many cycles to build up and approach the final value.


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