Oscillator Design - Transistor not Biased properly?

Hello. I'm playing around on a Clapp Oscillator circuit, doing simulations in Multisim 10 and building the physical circuit on a breadboard. In simulation, I have a nice stable 100 Khz sine wave. In my physical build, I have a 100 MHz sine wave. Could someone please take a look at it and see if there's something I'm missing?

Thanks in advance.

Possible causes are stray inductance due to poor layout and inadequate decoupling. The 10uF will have a poor high frequency response.

Keith

Except for C5, also series inductance of C2 and C3 must be suspected to create parasitic resonances. An effective method to suppress high frequency oscillations is a base series resistor of 50 up to several 100 ohm.

I agree with FvM that a base resistor can suppress high frequency oscillations. However it seems quite strange that you circuit can oscillate over parasitics if layout is "reasonable", the circuit you built is identical with respect to the drawing (I mean no errors are introduced) and components are not broken or shorted (mainly the 3.5 mH inductor). This because the transistor has a poor gain at 100 MHz (GBW is 300 MHz) thus Q factor of parasitic resonances has to be much higher than the main resonant tank. Please check also the power supply.

The setup had been described as breadboard, in so far high frequency oscillations won't be surprizing. But there should be a 100 kHz signal, too.

A 2N3904 running at 140 uA isn't much gain. Also put a 0.1 uF bypass cap on collector.

Only likely reason sim works and circuit doesn't is 1) Q of coil not in model, 2) Rs of 10 uF electrolytic not in model.

Thank you gentleman. I will look into your suggestions and get back to you.

I took this to a professor who stated it looked like I was using a too low frequency transistor, and suggested a higher one would be good. However, the datasheets all said the transistors I'm using have frequencies from 40 to 250 MHz, way less than this.

I don't think is matter of transistor frequency in fact the spurious oscillation occours at 100 MHz. However, from the data-sheet, the small signal gain seems to be optimized for a collector current of about 10 mA than much higher than your set point. Probably the better thing is to substitute the transistor with one having an higher hfe to low currents f.i.: BC107B. I think you can also try to increase the collector current of the 2N3904 changing R1, R2 and R3 respectively to 100k, 2.2k and 1k.