[HBR] Gain in IF amplifiers

[N2EY at aol.com]

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