CLapp LC oscillation

jason · Mar 19, 2005

Hello All

I am studying about Clapp LC oscillator.
Given below link is the circuit of interests
http://www.geocities.com/jason_class/ClappLC.jpg

May I know how do we calculate by hand if this circuit gonna oscillate
or not?

I learnt in lecture that I must find Av multiply by beta(feedback
factor). Av is always given by gm multiply by R efficient , which is
gmReff
Reff is the sum of series and parallel resistances of all emitter
resistance, ac emitter resistance , load resistance and also reflected
resistances from the tap of capacitors. Emitter is mentioned here
because I learnt about bipolar transistor configuration.

For the circuit in the link given, I do not understand why the same
approach is use?
Why we can use gmReff as the Av since it is a common drain
configuration? Is it because we have already sum all the net
resistances in Reff , therefore we can use gmReff as the Av?

So for other type of configuration of mosfet such as common gate and
common source? We can use the same Av=gmReff ?
If possible show me the correct way of checking if the circuit can
oscillate by manual calculation..at least the steps or methodology.
Really appreciate your help in advance.

Kindly enlighthen

rgds and thanks
Jason

Don Pearce · Mar 19, 2005

On 19 Mar 2005 06:09:08 -0800, "jason" <cheanglong@gmail.com> wrote:

Hello All

I am studying about Clapp LC oscillator.
Given below link is the circuit of interests
http://www.geocities.com/jason_class/ClappLC.jpg

May I know how do we calculate by hand if this circuit gonna oscillate
or not?

I learnt in lecture that I must find Av multiply by beta(feedback
factor). Av is always given by gm multiply by R efficient , which is
gmReff
Reff is the sum of series and parallel resistances of all emitter
resistance, ac emitter resistance , load resistance and also reflected
resistances from the tap of capacitors. Emitter is mentioned here
because I learnt about bipolar transistor configuration.

For the circuit in the link given, I do not understand why the same
approach is use?
Why we can use gmReff as the Av since it is a common drain
configuration? Is it because we have already sum all the net
resistances in Reff , therefore we can use gmReff as the Av?

So for other type of configuration of mosfet such as common gate and
common source? We can use the same Av=gmReff ?
If possible show me the correct way of checking if the circuit can
oscillate by manual calculation..at least the steps or methodology.
Really appreciate your help in advance.

Kindly enlighthen

rgds and thanks
Jason

First can you tell us the purpose of C1, C2 R5 and R6. The capacitors
are unnecessary, and the resistors will degrade the quality of the
oscillator.

d

Pearce Consulting
http://www.pearce.uk.com

jason · Mar 19, 2005

Dear Don

Thanks for the response.
C1 and C2 should be for coupling purposes. Then R5 and R6 should be for
high input and output impedance to and from the amplifier.
That's my guess. If wrong, kindly correct me.

My main question is why my lecturer can use Av=gmReff for both the case
of bipolar transistor and also mosfet for different
configuration(common gate, common drain, common source, EC, CB or CC)

Kindly enlighthen please

rgds and thanks
jason

John Woodgate · Mar 19, 2005

I read in sci.electronics.design that jason <cheanglong@gmail.com> wrote
(in <1111241348.593232.36370@g14g2000cwa.googlegroups.com&gt

about 'CLapp
LC oscillation', on Sat, 19 Mar 2005:

For the circuit in the link given, I do not understand why the same
approach is use? Why we can use gmReff as the Av since it is a common
drain configuration? Is it because we have already sum all the net
resistances in Reff , therefore we can use gmReff as the Av?

In oscillator circuits, it's not correct to use the terms 'common
drain', common gate, common source'. This is because the apparent
'common' electrode is determined by the *external* circuit, usually
where the earth/ground connection is made, and that doesn't affect the
conditions for oscillation.

So for other type of configuration of mosfet such as common gate and
common source? We can use the same Av=gmReff ?

Yes, if you choose to take for the 'input' and 'output' of the
amplifier, the points in the circuit which makes the equation true.

If possible show me the
correct way of checking if the circuit can oscillate by manual
calculation..at least the steps or methodology. Really appreciate your
help in advance.

An oscillator circuit is a loop, and you can break the loop at *any*
point in order to determine the loop gain. But the calculation is easier
for some places in the loop than for others.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

John Woodgate · Mar 19, 2005

I read in sci.electronics.design that jason <cheanglong@gmail.com> wrote
(in <1111243216.508324.254870@l41g2000cwc.googlegroups.com&gt

about
'CLapp LC oscillation', on Sat, 19 Mar 2005:

Thanks for the response.
C1 and C2 should be for coupling purposes.

There is no DC path through the Clapp network, so you don't need C1 and
C2.

Then R5 and R6 should be for
high input and output impedance to and from the amplifier. That's my
guess. If wrong, kindly correct me.

You don't need them either. The three capacitors in the Clapp network
are not only for tuning; they are also for impedance transformation. If
the inductor were tapped at two places, you would see it as a
transformer. Well, you can do (almost) exactly the same by connecting
two or more capacitors (three in the Clapp circuit) in series across an
untapped inductor and taking taps off between the capacitors.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

jason · Mar 19, 2005

Dear John

Thank you so much. You have taught so much I will never have in class
room.
But can you further explain about the followings?
I do not understand it

For the circuit in the link given, I do not understand why the same
approach is use? Why we can use gmReff as the Av since it is a common
drain configuration? Is it because we have already sum all the net
resistances in Reff , therefore we can use gmReff as the Av?

<<In oscillator circuits, it's not correct to use the terms 'common
drain', common gate, common source'. This is because the apparent
'common' electrode is determined by the *external* circuit, usually
where the earth/ground connection is made, and that doesn't affect the
conditions for oscillation. >>

The common gate drain or source is refer to the amplifier stage(the
simple mosfet in my case). Is that incorrect? Or you mean amplifer is
the external circuit?

So for other type of configuration of mosfet such as common gate and
common source? We can use the same Av=gmReff ?

<<Yes, if you choose to take for the 'input' and 'output' of the
amplifier, the points in the circuit which makes the equation true. >>

The main problem is I do not know which is the point of the "input" and
"output" that makes the equation true.
Can you explain about this for the picture I sent in the link?
Or if you have any online document thats explains this well and comes
with example.

Kindly enlighthen
Thank you all

rgds
Jason

jason · Mar 19, 2005

Dear All

Please help in this enquiry. Kindly explain in details as I am not IC
designer but a student taking IC design course. I am new in this.
Kindly explain in details if possible.
I know I am bold and ashame for asking this but I hope you can give me
a helping hand. Please share me me if u have any good online material
on this topic
Thank you so much in advance
My apology for any inconveniences caused
Thank you

rgds
Jason

Reg Edwards · Mar 19, 2005

All LC and crystal oscillators are the same.

Tubes or transistors

They all behave in the same way.

Changing the point of the ground connection has no electrical effect.
It merely changes the name.

The only way to understand how they work is to study the simple
elementary arithmetic involved. Otherwise one must suffer in
ignorance.

Unfortunately, in the Western World's schools, the standards of
education in elementrary arithmetic during the last 30 years has
fallen to abysmally low levels.

I once met a prospective member of parliament, no doubt with visions
of becoming the Chancellor of the Exchequer, who was unable to
calculate that 7 times 9 equals 63. I'm pleased to report he
forfeited his Ł500 deposit.

No chance of HIM ever understanding a Clapp or a Collpits or a
Hartley.
----
Reg.

Fabio G. · Mar 19, 2005

"jason" <cheanglong@gmail.com> ha scritto:

May I know how do we calculate by hand if this circuit gonna oscillate
or not?

Linearize it with the small signal analysis and write the expression of
the loop gain in the Laplace domain.

If a frequency in which the phase shift is zero exists, the circuit may
oscillate.
If at that frequency the loop gain is >= 1, the circuit will oscillate.
This is the basic methodology to analyze oscillators.

Bye

--
Per rispondermi via email sostituisci il risultato
dell'operazione (in lettere) dall'indirizzo

Jim Thompson · Mar 19, 2005

On Sat, 19 Mar 2005 21:03:17 +0100, Fabio G. <7+5@supereva.it> wrote:

"jason" <cheanglong@gmail.com> ha scritto:

May I know how do we calculate by hand if this circuit gonna oscillate
or not?

Linearize it with the small signal analysis and write the expression of
the loop gain in the Laplace domain.

If a frequency in which the phase shift is zero exists, the circuit may
oscillate.
If at that frequency the loop gain is >= 1, the circuit will oscillate.
This is the basic methodology to analyze oscillators.

Bye

480)460-2350 | |
| E-mail Address at Website Fax

Reg Edwards · Mar 19, 2005

May I know how do we calculate by hand if this circuit gonna
oscillate
or not?

======================================

Touch an RF point with a damp finger. If it stops oscillating then it
shows it's working and there's not much to worry about.

======================================

Linearize it with the small signal analysis and write the expression
of
the loop gain in the Laplace domain.

======================================

I do wish you guys would stop referring to Laplace. It's an indication
you are not fully aware of what you are waffling about.

It was Heaviside's revolutionary mathematics, 100 years later than
Laplace, which solved the trivial yet-to-appear problem of how
oscillators work - and a great number of other fundamental, physical,
hitherto insoluable engineering problems.

It was Laplace's relatively trivial piece of work, of not much
consequence at the time, which by chance provided the link between
Heaviside's masterpiece and the rest of the World of Mathematics. It
was the missing link, discovered more or less by accident by the early
practical communications engineers, needed to convince the university
academics and blockheads that the self-educated Heaviside's p and D
gave the right answers.

But it was the blockheads, confidence tricksters, obtaining money
under false pretences, not the engineers, who wrote the subsequent
text books. And so Laplace's s highjacked Heaviside's p and D.

Laplace, honoured, of the Napoleonic era, who survived Madam
Guilotine, is amongst the world's greatest mathematicians. No doubt an
honest man. He must be turning over in his grave at the injustice.

Heaviside, a self-educated young genius, died with little recognition
of the fundamental mathematical and engineering importance of his work
after many years as a lonely recluse in 1925 aged 75.

He will be remembered for his mathematical prediction of "The
Heaviside Ionospheric Layer" via which world-wide HF communications
were to blossom a few years afterwards.

Quotation (to the blockhead university professors) : "Shall I refuse
to eat my dinner because I do not understand the processes of
digestion? "
----
Sentimental Reg.

jason · Mar 20, 2005

I would like to thank people who gives constructive ideas. Thank you

Robert Baer · Mar 20, 2005

jason wrote:

Dear Don

Thanks for the response.
C1 and C2 should be for coupling purposes. Then R5 and R6 should be for
high input and output impedance to and from the amplifier.
That's my guess. If wrong, kindly correct me.

My main question is why my lecturer can use Av=gmReff for both the case
of bipolar transistor and also mosfet for different
configuration(common gate, common drain, common source, EC, CB or CC)

Kindly enlighthen please

rgds and thanks
jason

C10 and C14 do the DC blocking, so C1 and C2 are redundant.

R5 *decreases* the coupling (drive) to the LC network, thereby
increasing the feedback loss.
R6 is non-functional unless the value approaches 10% of R1 or more -
in which case it also would increase the feedback loss *and* add a phase
shift that can be undesirable.

CLapp LC oscillation

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