Language:
To browser these website, it's necessary to store cookies on your computer.
The cookies contain no personal information, they are required for program control.
the storage of cookies while browsing this website, on Login and Register.
 Storing Cookies (See : http://ec.europa.eu/ipg/basics/legal/cookies/index_en.htm ) help us to bring you our services at overunity.com . If you use this website and our services you declare yourself okay with using cookies .More Infos here: https://overunity.com/5553/privacy-policy/If you do not agree with storing cookies, please LEAVE this website now. From the 25th of May 2018, every existing user has to accept the GDPR agreement at first login. If a user is unwilling to accept the GDPR, he should email us and request to erase his account. Many thanks for your understanding

Custom Search

### Author Topic: Hartley oscillators (inductances)  (Read 8432 times)

#### watari

• Newbie
• Posts: 16
##### Hartley oscillators (inductances)
« on: May 15, 2013, 01:57:39 PM »
Hi everyone,

I ended up in this forum (that seems pretty interesting, btw) looking for information about Hartley oscillators. I'm new in this field that is electronic, and to tell the truth I don't know so much about it. So here is (one of)  my question.

If I want an output frecuency for a Hartley oscillator, let's say 1MHz, I have a given equation that can provide the parameter L and C for the tank circuit. That is:

f= 1/ (2*Pi*sqrtLC)    => LC = 1 / (2*Pi*f)^2

now I can get LC for that frecuency (1MHz), and according to my maths it is

LC= 2.53*10^-14

so at a given C (let's say 4nF) I can get L, that is L= 6.325 uH ~ 6.33 uH.

But, because L= L1+L2, now I have to give some number to those two coils.  Using a N3904 transistor, I take its beta (at 40 mA current collector) as 200. The gain for a Hartley oscillator must be:

beta > or = L2/L1

So to solve this problem I deciced to stablish a system of equations (or unequestions):

L1+L2= 6.33

L2/L1<or=200

So my results are    L1= 0.032 uH and   L2<or=6.3uH

But because  (beta=200),  200 >or= L2/L1 this means 200 is the top limit, therefore whichever proportion between L2 and L1 which result is less than 200 (always respecting that L1+L2=6.33uH) will be ok to make the Hartley oscillator oscillate at 1MHz. So for instance, if I chose:

L1= 1.032uH and  L2= 5.3uH will be ok, as

L1+L2= 1.032+5.3= 6.332 uH      and

L2/L1= 5.3/1.032= 5.136  which is less than 200

These two conditions are satisfied, but if I chose any other L1 and L2 which satisfies this as well, it will be ok too. So finally, here is my question: Even though I can satisfy those two conditions with differents values of L1 and L2, How can this affect to the gain of the circuit? I mean, the closer this proportion L2/L1 is to 200, the better gain will I have and the better the circuit will run? Or how does it work instead? For example, if I get a gain (L2/L1) =1.000000000000001, will it be as good as having a gain like 199.9999999 to get the circuit working ok?

Thanks!!

#### gyulasun

• Hero Member
• Posts: 4139
##### Re: Hartley oscillators (inductances)
« Reply #1 on: May 15, 2013, 08:51:54 PM »
Normally the gain of the circuit does not depend on the ratio of the two inductances.  Rather, the gain mainly depends on other components, namely collector impedance and on either dynamic emitter resistance (rd=26mV/IE) or the inserted emitter resistance if you use bipolar transistor (like you referred to, 2N3904).
The collector impedance may include a normal resistor or a choke coil but you have to consider the load resistance value too, so gain becomes the ratio of the collector impedance divided by the emitter impedance. If this value is say 200 then you may wish to choose the ratio of L2 and L1 to be also around 200.   You may know that the gain normally should be slightly higher than 1 to maintain a positive feedback for the continuous oscillation but component losses and changes in temperature may also influence gain so it should be chosen higher than 1.
(I assumed a decent Beta for the transistor of course, in the some hundred range, and feedback should be used to lower it.)

#### watari

• Newbie
• Posts: 16
##### Re: Hartley oscillators (inductances)
« Reply #2 on: May 18, 2013, 02:48:28 PM »
First of all, thanks for answering, gyulasun and also i apologize for taking so long to respond you back.

So, if i did not understand wrong, the gain depends on the ratio collector impedance/emitter impedance (for a BJT which is what i have) and once i get this, i have to chose a ratio L2/L1~ 200.

Am i right?

I'll try to work on this and i'll report my results

Thanks!!

#### watari

• Newbie
• Posts: 16
##### Re: Hartley oscillators (inductances)
« Reply #3 on: May 18, 2013, 02:53:11 PM »
I forgot!! L2/L1 ~200 if i get a ratio collecto impedance/ emtter impedance=200.

By the way, is this valid for every kind of transistor configuration or just for common emitter one?

#### gyulasun

• Hero Member
• Posts: 4139
##### Re: Hartley oscillators (inductances)
« Reply #4 on: May 18, 2013, 03:14:16 PM »
Well the 200 for the gain is not 'obligatory', any smaller number will be good as long as it is higher than 1 and just compensate for the losses in the components. Depends also on the supply voltage whether it is changing and also on the expected ambient temperature change.
Practical consideration for the gain also includes the inductance value for the coils because if the smaller coil becomes too low value due to a higher gain number, then it may not be practical to wind (you wish to use at least 5 or 10 turns rather than much smaller number of turns) so you can avoid this by thinking in advance.

This is valid for grounded emitter and grounded base configuration only, (provided there is no resistor feedback between collector and base).

#### watari

• Newbie
• Posts: 16
##### Re: Hartley oscillators (inductances)
« Reply #5 on: May 27, 2013, 07:16:54 PM »
Hi, I'm back again. I've been searching for more Hartley oscillator circuits in order to compare them with mine and check if it is right, also I took into account (and also I appreciate) your advices. gyulasun

First of all I would like to say that beta=200  was chosen looking at the  N3904 datasheet, given a 40mA current collector. Which, if i'm not wrong, 200 is its typical value.

I understand what you say about how i have to get a ratio L2/L1 close to beta,  and how a proper value of this beta has to be big enough to compensate loses. But in my search on the net, I found a website where they show you how to build a Hartley oscillator. The values of the inductances are; L1=1.2uH and L2=6.8uH and the ratio L2/L1= 5.67, what is bigger than 1 but clearly far away from every minimum value of beta given in  N3904 datasheet (that is what it uses), even in graphics. I did not calculated any other parameter yet, but, for what i know (that is not so much) either this does not fit well enough or what i know is simply not right.

If anyone of you wants, I can give you the link to that website in order to check it out by yourself.

Once more, thank you all.

#### gyulasun

• Hero Member
• Posts: 4139
##### Re: Hartley oscillators (inductances)
« Reply #6 on: May 27, 2013, 07:45:02 PM »
In fact I did not really imply the Beta and the L2/L1 ratio should be connected and be 200, I implied the gain of the amplifier stage i.e. the collector and emitter impedance ratio which is to be related to the coils ratio (in my Reply #1 above).
Please forget considering the Beta,  when you choose the transistor type (2N3904), you already decided on Beta, right?

Do you really need to run the transistor at 40mA collector current? For LC oscillators in general the current is normally less than say 10-15mA, otherwise frequency stability suffers a lot. I would prefer any current between 5-10mA or so.

Gyula

#### watari

• Newbie
• Posts: 16
##### Re: Hartley oscillators (inductances)
« Reply #7 on: June 08, 2013, 01:41:47 PM »
Hey guys!! I'm sorry for being absent for so long. I'm very busy these days and i have no time neither for playing with circuits nor for this forum (sigh). I don't know when i'll be able to pay attention to them. Anyway, as soon as i can i'll be back to work on this and i'll keep posting my results (and question).

regards

#### watari

• Newbie
• Posts: 16
##### Re: Hartley oscillators (inductances)
« Reply #8 on: September 30, 2013, 04:53:14 PM »
Hi again!

I've been traveling for almost four month and I've been completely absent for that. I've had time enough to think over this project i want to do. And I came out with the conclusion that having an oscilloscope will help me a lot. But I don't know so much about them and I don't have any so I started a post right here in this forum  asking for advise to buy one. Until I dont get the oscilloscope I won't keep going omn with my porject.

I would like to invite you to this new post I started to give advise.

http://www.overunity.com/13842/best-oscilloscope-choice/msg372082/#msg372082

Once more, thanks!!