# Colpitt's Oscilattor

Discussion in 'Homework Help' started by daveo91, Aug 21, 2013.

1. ### daveo91 Thread Starter New Member

Aug 21, 2013
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Hey guys, I was hoping someone here wouldn't mind giving me a little bit of help on a part of a question I am having trouble with.

Below I have attached a screenshot of the full question, as well as my attempt at the answers. However I can't for the life in me work out how to do part d), what is more annoying is I am sure it is something really obvious that I am missing.

Thanks in advance for any help or guidance.

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2. ### Brownout Well-Known Member

Jan 10, 2012
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Loop gain has two components: the gain of your feedback network consisting of C1, C2 and L1 and the gain of your common-base amplifier. So, the loop is the votage gain from say the collector, through the feedback network, and back throught the transistor to the collector. Multiply the gains of the loop components (network and amplifier) to obtain the loop gain.

3. ### daveo91 Thread Starter New Member

Aug 21, 2013
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Ahh, thanks - I have a feeling I knew that as well but my brain is just having an all out failure of a day.

Any suggestions on how to calculate C1 and C2?

4. ### Brownout Well-Known Member

Jan 10, 2012
2,364
1,000
OOps, I was helping with the last part. C1 should be 2-4x larger than C2. Use that and the equation you already have to calculate the values.

5. ### daveo91 Thread Starter New Member

Aug 21, 2013
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Thank you very much, gives me a bit of a fighting chance with it now

6. ### Brownout Well-Known Member

Jan 10, 2012
2,364
1,000
I'm going to back off just a little on what I wrote. The ratio of C1 and C2 need to be such that loop gain > 1. What I wrote earlier was just a rule of thumb. You can have C1 = C2 or any other ratio as long as loop gain > 1.

7. ### LvW Well-Known Member

Jun 13, 2013
769
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Hello Dave,

perhaps the following helps a bit to solve the task:

* You should be aware that - due to the resistance Rp in parallel to C2 - the resonant frequency is NOT equal to 1/sqrt(LC). It is somewhat smaller because the resistance of Rp is rather small (Rp=R3||re) with re=26mV/2mA=13 ohms.
* More than that, the oscillation criterion requires a loop PHASE of 0 deg. at the oscillation frequency (not only a loop gain of somewhat larger than unity).
* Note that in your circuit this phase is NOT achieved at the maximum gain.

(A short comment to the task: It is said that the gain of the common base stage is 20. You have interpreted this value as current gain. May be that this was the intention of your teacher. However, it is more correct to say that the current gain of the transistor is 20 (rather than the gain of the stage).

8. ### daveo91 Thread Starter New Member

Aug 21, 2013
4
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Thank you for taking the time to help me.

It might be just my lecturer being bad but he quickly checked over the working and seemed to think it looked correct, but he was in a rush and didn't really pay much time.

However my exam is tomorrow so I have asked to quickly see him properly so I will pass on what you have said to him and see what he says, thanks again.

Jun 13, 2013
769
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Good luck!!!