# KHz range GBP for a transistor

Discussion in 'The Projects Forum' started by PaulieShaw, Feb 24, 2014.

1. ### PaulieShaw Thread Starter New Member

Feb 24, 2014
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Hi All

I'm making a Theremin for a project and I've tried a few different oscillators and I'm working on a Colpitts to function in the 500KHz range.

I'm using a L1 = 10mH inductor and a cap value of C1 = 10pF C2 = 470pF which gives me a operating frequency of 509KHz.

Now when I try to simulate it on Multisim it flat lines and won't oscillate, I am sure this is due to the gain bandwidth of the transistor not being enough at that frequency. I've had a look on the net and in some books but I can't find a suitable transistor that will oscillate at the frequency. As if I increase the values so that it will oscillate at 1MHz the simulation works fine, so I know the circuit is OK.

Does anyone know of a transistor that will work in that frequency range? 450KHz - 5550KHz

Any help will be greatly appreciated

Last edited: Feb 24, 2014
2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,754
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You need to search for RF transistors.

3. ### PaulieShaw Thread Starter New Member

Feb 24, 2014
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Thank you sir, from a quick search the BFDS60 works great a 450MHz o7

4. ### Papabravo Expert

Feb 24, 2006
10,144
1,790
A circuit at 450 KHz. hardly requires an RF transistor. A circuit at 450 MHz. probably does. I think you are unit confused.

450 KHz. = 450,000 Hz
450 MHz. = 450,000,000 Hz.

Three orders of magnitude is quite a difference -- which is it?

5. ### PaulieShaw Thread Starter New Member

Feb 24, 2014
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Hi,

I'm creating a Oscillator to work around 500KHz definitely Kilo Hz.

I used the equation 1/(2∏√LCt) for resonant frequency which i calculated at 500KHz.

The simulation will work if i change the values to create a 1MHz sine wave, my problem is that if i change the to what i want to create the 500KHz sine wave the simulation flat lines and will not oscillate.

I'm putting this down to there not being enough gain at that frequency for the transistor in the simulation. So I wanted to know if anyone knew a transistor that would have a good gain bandwidth product around the 500KHz frequency range.

Please see attached image for the circuit I'm using.

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6. ### crutschow Expert

Mar 14, 2008
13,013
3,233
Oscillators sometimes do not start in simulation and need a bump to start.

One thing that may help is to eliminate the initial operating point calculation (.uic) when running the Transient Response.

Another is a add small single pulse somewhere in the circuit to kickstart the circuit. In your circuit a voltage source in series with the base (not to ground) with, say a 100mV single short pulse, should help start the circuit. Once the pulse is done the voltage source looks like a short circuit and does not affect further circuit operation.

Apr 5, 2008
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8. ### Papabravo Expert

Feb 24, 2006
10,144
1,790
It might not be oscillating because it lacks some other requirement for oscillation. Besides the loop gain, is there not also a phase shift requirement?

9. ### PaulieShaw Thread Starter New Member

Feb 24, 2014
5
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the capacitors in series produce the phase shift to get the required positive feedback.

10. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
It's certainly a Colpitts topology. One of six possible for CB arrangement. I think C3 is superfluous.

11. ### PaulieShaw Thread Starter New Member

Feb 24, 2014
5
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Hi

I've found out why it wasn't working, I had not calculated my capacitors to allow enough feed back into the transistor.

I changed the cap values to 130nF and 40nF and now i get about 25% feedback into the transistor.

All fixed, thank you one and all for your feedback I appreciated it.