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Charl
Post subject: Books/references/tutorials about common emitter amps Posted: Mon Jan 15, 2007 1:08 pm

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hello everyone,

Well, it's a bit embarassing to ask, but I am fairly clueless about doing calculations on a common emitter amplifier with a transformer as load. I don't know how the reflected impedance from the secondary will "combine" with the impedance of the primary coil at the operating frequency.

I've searched the ARRL handbook, Radiotron, and the web for a good tutorial on this, but haven't been able to find one. Although I could post the previous paragraph as the question, I think it'd be better to get some comprehensive reading material on the subject. Any suggestions?

Thanks in advance,
Charl


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fred47
Post subject: transformer couplingPosted: Mon Jan 15, 2007 2:54 pm

General


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi!
Transformers are some of the technically ugliest components. In order to handle the ugliness, people use simplified models which they hope are "good enough".

So as not to give too much detail, could you let us know what frequency range you're looking at?

Some of the books on the subject of transformers are out-of-print, and so it would also help to know if you have access to a university engineering library, or if I should restrict my recommendations to the more-readily-available stuff.

A short summary:

The simplest ("ideal") mental model ignores the internal impedances and the coupling factor of the transformer entirely. The secondary impedance reflects to the primary as the square of the turns ratio. You're probably not there...

The second mental model assumes unity coupling (which is usually approached only with toroids, and not even always then - size, frequency, and number of turns enter in), but uses primary and secondary inductances only - then those values simply appear in parallel with the primary and secondary terminals of an ideal transformer. This second model ignores capacitances. You might be here, depending on the frequency.

The third mental model assumes that the capacitances can reasonably be viewed as "lumped" - that is, occurring at one physical point. The parasitic or "stray" capacitances appear in parallel with the primary and secondary, and between the primary and secondary.

Of course, at high-enough frequencies, you can't consider the capacitances as lumped at all - then you've got a difficult problem.

Good Luck,
Fred


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Charl
Post subject: Re: transformer couplingPosted: Mon Jan 15, 2007 3:09 pm

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hi fred47,

The frequency is fixed at 10.7MHz, thankfully. I have access to all national academic engineering libraries.

Basically, what I don't know is this. Let's say that the transformer reflects my secondary load as an impedance of X ohms. Let's also say that the primary coil in the transformer has an impedance of Y ohms at the operating frequency with no secondary load (Y = jwL(pri) + R(wire)). How do these impedances X and Y relate to the impedance Z that my transistor sees as its collector load? X + Y = Z?

Kind regards,
Charl

PS. neither the primary or secondary are tuned.


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fred47
Post subject: TransformerPosted: Mon Jan 15, 2007 4:46 pm

General


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi Charl,

Assuming that when you say "the transformer reflects my secondary load as an impedance of X ohms" that you're referring to a theoretical impedance and not an actual measurement, then the primary inductance appears as a parallel inductance to the (ideal) primary, and the wire resistance (which I would think would be negligible) would appear in series with the parallel inductances. Likewise, for the secondary, the secondary inductance appears in parallel with the (ideal) secondary, and the wire resistance in series.

I hope I've understood you correctly.

My books are at home, and I'm at work - I'll post references later.

Good Luck!
Fred


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Charl
Post subject: Posted: Mon Jan 15, 2007 5:34 pm

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hi fred47,

I shouldn't have skipped my caffeine fix today. What am I thinking, transformer coupling not in the Radiotron? It has been used in nearly every tube amplifier ever built! So, looking in the correct chapter of the book, there it is:

[code]The basic circuit of a single Class A triode is Fig. 13.1. The load resistance (R2) is normally connected to the secondary of a transformer (T) whose primary is connected in the plate circuit of the valve. The load resistance (R1) presented to the valve is given by:
R1 = (N1/N2)^2 * R2[/code]

No mention of the inductance of the primary coil at all. Well, at least that's clear. But it leaves me wondering: how do I pick the size of the transformer? I know what the ratio between the turns should be, but what is the difference between a transformer with, say, Np=50/Ns=20 and a transformer with Np=25/Ns=10?

Thanks for your time,
Charl

PS. I'd still like those book references - I need some structure! :)


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fred47
Post subject: TransformersPosted: Tue Jan 16, 2007 3:25 am

General


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi Charl,

A classic book is "Transformers for Electronic Circuits" by Nathan Grossner. It was published in 1967 by McGraw-Hill. The diagram you want is on page 3. It certainly appears elsewhere - it's a classic circuit diagram - but I don't remember where else I've seen it. Grossner references "Magnetic Circuits and Transformers" by the MIT EE Department (McGraw-Hill 1943). I have the Grossner book, but not the MIT book.

As far as choosing the absolute number of turns is concerned, you need to make sure that the inductive reactance of the primary at the operating frequency is at least 10x the operating (AC) resistance at the primary. For example, if you need a 100:400 Ohm transformer, the primary inductive reactance should be at least 1k. ZL = 2 pi f L, so L = 1000/(2 pi f) = about 15 microHenries. The secondary, then, would have an inductance of 60 microHenries.

But we're not done yet. Attemping to get a 60 microHenry inductance at 10.7 MHz introduces the issue of parasitic capacitance. Here, how you wind the coil comes into play - as does the core material. (I'd recommend a Nickel-Zinc ferrite, as they have high resistivity). Toroids are good for reducing leakage inductance.

It's getting past bedtime, so I bid you a good night and a good day!

Fred


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Charl
Post subject: Re: TransformersPosted: Tue Jan 16, 2007 4:01 pm

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hi fred,

fred47 wrote:
As far as choosing the absolute number of turns is concerned, you need to make sure that the inductive reactance of the primary at the operating frequency is at least 10x the operating (AC) resistance at the primary.


Just what I needed. Thanks!

Kind regards,
Charl






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