IF.455K:OLD MW Radio BJT IF

* Tuned Amplifier, cascade by IFT (IF transformer, IF-cans)

* Single Tuned or DTC

* Common Emitter

* Simple AGC deploy, typical via control the first IF stage quiescent current.

* Most of them first stage quiescent current use 0.5mA , second stage use 0.8-1mA. refer to Fig2

Typical Gain achieved

Most article you can get might told you, single tuned stage [transistor] given about 30dB Gain/per Stage. neutralisation or not.  Actually , today's  new transistor might has very small Cob (<2pF, or even smaller),  high Ft, and very cheap.  proper designed IF-cans make 45dB per stage possible and fairly stable. [even more gain]

455khz IF strip is absolutely not dead, it's easy to implement narrow band, high gain amplifier and had many advantages. refer to WN5Y: The Advantages of the 455 kHz IF Strip

Advantage:

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* mass product,  proved moderate performance

* simple, low frequncy, easy tamed, easy get  BFO and so on

* Q-Multiplier deploy-able

* easy to get high gain and fairly stable

* easy implement narrow band

* quiet, compare to integrated circuit

Limitation:

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* Simple Diode detector 's input impedance vary with input signal[1N60 with 0.2V bias, around 500ohm@500mV RF], strong signal severely damping the output stage's IF-can.

* BW/Gain interference:  make High Gain , more Ie, make lower input impedance then might impact the pre-stage.

* Strong signal make high AGC voltage change first stage BJT quiescent current so much, make BJT output/input impedance changed, then impact the IF can filter's damping, reduce the BW.

* Strong signal changed AGC also change the BJT's Cob, make band pass not symmetry.

If you want check if an IF strip stable or by inspect output via oscilloscope or other device, please PULL DOWN THE INPUT. or you get wrong result,  play around  IF-B0: BJT 2 Stage with AGC  teach me this.

And a sweep generator for 455khz is definitely good tool to aliment and check the stability.

Fig 5 Unstable IF strip symptom

Why An tuned IF strip oscillation?  Refer to <<Principles of T ransistor Circuits>> by   S. W. Amos,BSc, CEng, MIEE   , M. R. James, BSc, CEng, MIEE

Any way, we got oscillation condition:

c: the Cob

Rb: resistance present to base of BJT  [should include  the input resistance of BJT if Rp of tune circuit is comparable]

Rc: resistance part of the impedance given to collector

The Design consideration to prevent Oscillation:

* proper design the tuned filter, present  selected Rb, Rc, make ω*c*Gm*Rb*c < 2/n  [n>1]

* neutralization for get more gain. 

Practical for build technical:

* choose a IF transformer which had tap in the primary, refer to why primary tip is very important to stabilise the tuned IF amplifier.

* select low β BJT: which present the low Rb in single tuned amplifier

Not recommend but it works, junk box built suitable:

* series few tens to few hundreds ohm resistor to the the base of BJT  [ if for prevent high frequncy spurs, this might recommend]

* parallel 10k to 100k resistor with the if can. [decrease Q and the lower the Rp]

Experimental Circuit

Superheterodyne: BJT AM receiver    : my first superhet AM broadcast receiver

IF-B0: BJT 2 Stage with AGC :  the first IF stage in the RF experimental system

IF.B0: Gain Vs Stable:  [in the designing while this article be written @7/10 2012 ]

* Band Width/Gain/stability

The Gain design should conform to the stability condition.  Gain controlled by   L1-2: L4-5. meanwhile  BW control by the ratio of L1-3:L4-5  and L1-2:L1-3. 

ω*c*Gm*Rb*c < 2/n  [n>1] 

* work point  consideration

For a 2 stages traditional IF strip, first stage's BJT bias normally  to chose the 0.4mA-0.6mA, for better AGC performance.  the Secondary Stage BJT chose 0.8mA to few mA per design.  here are some practical method for tune the bias:

1. if tuned in/away the station with "whistle" it might because  too much bias current  in the Mixer

2. if tune in/away station with much "hiss" , it might because first stage has too much quiescent current. or too narrow BW.

* IF transformer consideration

from mixer output to detector output IF-cans,  transformer primary:secondary ratio  reduced in every stage.  this improve the AGC performance and prevent over load next stage.

* typical gain design

Value:   RF (10-15dB)  --> Mixer( 30-35dB)--> IF (36-40dB) --> detector( 12-17dB)

BJT:       RF(-5 to -8dB) --> Mixer( 22-26dB)--> IF( 60-65dB) --> detector( 15-20dB)

Add on circuit to IF strip

* tune indicator

* AFC

Improvement  to traditional IF strip

   *DTC

   *cascade  amplifier

   *delay  AGC

   * detector buffer

   * impendace consideration