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Books/references/tutorials about common emitter amps - RF Cafe Forums
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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
Top
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! :)
Top
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
Posted 11/12/2012 |
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