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455kHz IF Transformers

There are usually 4 RF transformers in a AM radio with colors of Red for the LO oscillator, Yellow for the mixer or 1st IF, White for the 2nd IF, and Black for the 3rd IF. I notice the impedance of these transformers ranges from

50K : 500 for the 1st stage (yellow), 30K : 500 for the second (white), and 20K : 5K for the 3rd (black) stage.

I think I understand the 500 ohm secondary impedance which matches the input impedance of the next stage, but I can't figure out why the primary impedance is different for the 3 stages?

The 5K secondary impedance of the black (3rd stage) is probably higher to produce a higher voltage for the detector.

What exactly does a primary impedance of 20K mean and how would that be setup with a bipolar transistor?

-Bill

Reply to
Bill Bowden
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I haven't seen a radio built like that in decades. Are you referring to a particular model you're restoring?

Reply to
a7yvm109gf5d1

I have a 1960 app note describing the design of an early AM receiver using germanium transistors. This was a two band receiver, 540-1640KHz and 155-280KHz.

They used a pair of 28K:800ohm transformers and a detector transformer whose impedances were not specified, merely described as 1.85:1 turns ratio.

28k:800ohm was the 470KHz output/input impedances of the particular transistor used, (Mullard OC45, running at about 1mAdc).

They did note that the output impedance of the mixer stage in the 540-1640KHz band generally exceeded 35k at 470KHz, but they used the 28k:800 transformer simply for economy. So that might explain your 50k mixer output transformer.

The impedances around the detector are not specified, but the OC45 is running at a higher current so this might explain an apparent lower output impedance.

--
Tony Williams.
Reply to
Tony Williams

Each stage runs at a different collector current, resulting in a different optimum collector impedance. In those days they tried to get maximum gain out of each stage, so they took extra care, even if it required different transformers for each stage.

As it turns out, the exact impedances are not all that critical, as power transfer only varies as the square-root of the mismatch, and those old transistors had very loose specs to begin with.

Reply to
Ancient_Hacker

The "standard" (tube) configuration was known as the all-american five, meaning 5 tubes used counting the rectifier for the B+. 4 RF transformers has been common for tube and transistor radios, but the impedances were different as necessary to match lo-Z transistors.

Reply to
Robert Baer

I'm not sure how to calculate the output impedance of the transistor, but I think it's related to the supply voltage and bias current. And I see these same transformers used in a variety of radios running on different battery voltages.

So, if the supply voltage is say 6 volts, the bias current for the 50K transformer would be around 6/50000 = 120uA

Seems a little low and the transistor gain may be higher at greater bias currents?

Suppose we use a higher bias current to obtain a higher transistor gain. How would that effect operation?

-Bill

Reply to
Bill Bowden

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