IF.455K:why tap stabilize the IF Amp

@6/18   2012

 At the pass weekend, I fight with the oscillation  in the I.F. amplifier.  there is the rule:What oscillator does not and amplifier will, that oscillating.  ( IF BJT 2 Stage with AGC )

At first glance, it look like old traditional 455 khz I.F. amplifier, but there is several different. 

1.  T1, T2 are not taped, I pick up my favorite IF transformer and start to use it, which without tap in the primary.

2.  AC signal at Q1, Q2's emitter is not totally bypass to GND ( i want to do that but it ruining into oscillating )

3.  other  differences is minor, ignore them while we talking the stability of tuned I.F  amplifier.

Initially, i want to built a amplifier present in the Fig 2. The emitter is AC bypass directly to the Ground.

But, I  haven't an IF CAN with taped, and I paid for this, despite some I.F. amplifier did use IF CANS with no Tap,

but you should read carefully about that transformer's turns ratio: Pri:Sec. a typical yellow IF cans Pry:Sec is around 22:1~12:1 . and my project use yellow slug IF cans for both T1, and T2 in the Fig1. Pri:Sec = 12:1 (the following cute yellow cans is what I'm talk of.)

But every time i Adding bypass cap C1,C4 in Fig1, the I.F amplifier  go to oscillation or just very noisy.  and the band pass like middle line in the following Fig.  which indicate the amplifier is not stable or is  oscillating.

I try everything: add resistor in base or collector,  reduce Q value by parallel primary with resistor, shield, low beta transistor.  the best result i've got is  a noisy  amplifier with bypass emitter to ground.  i not try neutralisation, because the transistor 9018 Ft is high and Ccb is low, according most articles i can get, NPN like this should work stable without neutralisation( with considerable gain). but i STILL FAILED to get it to work.

After talking with guys on forum, http://www.crystalradio.cn, some one  suggest me to try a taped version IF coils(the following green slug IFT), and finally, i come to the only way left , a taped Coil, which 160uH in the primary,  Primary: secondary= 36:1, tap:primary= 5.5:1 .  the result  surprise me so much, it stop oscillate and very stable , make it free to adjust bias current, off-course emitter is AC bypassed to ground.  i never got a IF amplifier so quiet and many stations indoor with loop antenna.  

But Why?

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refer to Mouse IF cans data:

Fig.6 show  the AC path of the tuned amplifier's( include input transformer).   At low frequency compare to the resonate frequency , LC tank impedance  is inductive.  Plus Ccb, it's almost a Hartley oscillator.

So when GAIN is high enough,  it's start oscillate.  there is a important way to  prevent the oscillation, the  neutralisation.  neutralisation is the (only?)  way  to prevent oscillation not by decrease the overall gain.  Here is a key point to understand it:  the GAIN is what the BJT (collector) seen, not the overall gain to next stage.   Fig.8, the BJT seen Gain  on load of  L1-2, not the gain delivering to R1 by L4-5.

Refer to <<Principles of T ransistor Circuits>> by   S. W. Amos,BSc, CEng, MIEE   , M. R. James, BSc, CEng, MIEE

where Rb and Rc are the resistances effectively in parallel with the base and collector circuits .  and c is the internal base-collector capacitance(Cob). 

Why taped primary transformer look like easily to get a stable Amplifier?

the taped transformer make it stable but, why?

 the key point is : taped IF.T ( IF transformer) present the BJT collection a fairly lower Impedance, then   gm*2*pi*f*c*Rb*Rc  would be less than  2 compered to non-taped IFT in the same circuit.

my green slug IFT given a very low Rt, it's the major reason make my  circuit stable.  but tapped transformer still make it stable by decrease so much the collection Load of BJT. while Rc decrease to 1/n*n and Gain decrease to 1/n.

Suppose my Yellow IFT also have a primary tap with tap ratio 4:1 in secondary stage(most of tapped IFT use around this ratio.)

NOTE:

if no-tap version tuned to stable,  Ie <0.5, the max gain is:

20*log(30000*0.5/26/12.8) = 33dB      ( stable factor just reach to oscillation)