[HBR] Gain in IF amplifiers

[Hopperdhh at aol.com]

Jim,
 
Thanks for your thoughtful reply.  Well, its been 40+ years ago so I  can't 
remember much of the details of what I tried as far as taming the  S-38.  
Hopefully I've learned something since then, but I do remember  trying everything I 
could think of at the time.  I must confess that I  haven't tried lately to 
duplicate my failure.
 
In addition to input and output IF transformers, Miller and others also  
supplied interstage transformers.  I know that the output transformers were  
closer coupled than the inputs because of the lower load impedance of the diode  
detector.  I wonder what the difference between the input and interstage  
transformers was.
 
Your reply certainly makes sense.  However, a voltage gain of 280  times is 
48.9 db not 43 db.  Assuming that the transformer is exactly  critically 
coupled, I think that the RL drops to one half of the unload  primary impedance.  
Usually the coupling is less than critical, but that is  nit picking.  Anyway, I 
think that a gain of nearly 100 db at one frequency  would require a lot of 
bypassing and shielding.
 
The point of my post was only to show that we have control over the gain of  
an IF stage by choosing the C and L of the transformer (and Q, too).   
Reducing the gain by increasing the capacitance is a step in  the direction of 
increased stability.  Any time we lower the plate  load impedance stability goes up, 
and of course some gain is sacrificed.   One somewhat extreme example of this 
technique is in the older R-389 and R-390  Collins receivers which used 1000 
pF capacitors in the IF transformers along  with resistors (100K on the 
primary and 470K on the secondary) to control  the Q.  These receivers had 6 IF 
stages at 455 KHz!  The versions I am  talking about are those before the R-390A 
which used mechanical filters for  selectivity.
 
Regards,
 
Dan K9WEK
 
 
In a message dated 7/24/2006 6:55:55 A.M. Eastern Daylight Time,  
N2EY at aol.com writes:

In a  message dated 7/23/06 5:04:21 PM Eastern Daylight Time, 
Hopperdhh at aol.com  writes:


> Here is the problem.  Low cost receivers with  only one IF  stage get 
> maximum 
> gain by keeping the  impedance as high as  possible.  This means using only 
>  about 100 or 125 pF capacitors and making  the inductance about 1 mH to  
> resonate 
> at 455 KHz.  Most IF transformers you   find in the junk box will be of 
this 
> design.  If you use these  transformers  for a 2 stage amplifier the gain 
> will go 
>  through the roof, so to speak.
> 
> If you look at schematics with  values for the capacitors in the IF  
> transformers, you will  notice that receivers with 2 IF stages usually have 
 
>  capacitances of 220 to 390 pF.  This is to lower the gain per stage to  
make  
> the receiver 
> more stable.

While this  sounds plausible at first, I don't think it's really the problem. 
Here's  why:

First, if you look at old parts catalogs, they do not make  different IFTs 
for 
1 and 2 stage sets. They do make "input" and "output"  versions, because the 
diode detector used in most sets loads down the last  IFT differently than 
the 
grid of a pentode.

Second, let's see how  much gain is really added:

The gain of a pentode amplifier can be  simplified to approximately gM times 
RL - if Rp is much bigger than some  other factors. Since the common pentodes 
used in IF stages typically have  Rp in the order of hundreds of thousands of 
ohms or more, the  approximation is pretty good.

RL is the load resistance - the resistive  part of the resonant circuit. In a 
455 kc circuit with 500 microhenries  inductance, we get:

XL = 6.28 x 500 x .455

XL = 1429  ohms

Assume a Q of 100, and we get RL = 142,900 ohms. In an IF  transformer, there 
are two tuned circuits. so the RL is actually about  70,000 ohms. This number 
will rise linearly with increases in Q or  L.

With gM of 4000 and RL of 70 K, the voltage gain of a stage is 280  times! 
That's about 43 dB. Two stages would be 86 dB.

I we keep the  Q the same and double L, the load resistance Rl will double. 
That doubles  the voltage gain  - 6 dB more per stage. 

That increase is dwarfed  by the increase caused by a second stage.

I have built receivers using  junk IFTs and more than one IF stage - with no 
regeneration. It takes  careful layout and bypassing, though. Pictures of one 
may be seen on the  HBR website. Note that the receiver pictured has an 
active 
crystal filter  *and* two stages of IF at 470 kc.!

I suspect that the real problem with  the hopped-up S-38 is that the layout 
and bypassing were simply not  adequate for the job of all that extra gain. 


73 de Jim,  N2EY
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