how to choose impedance matching transformer?

[simpsonss]

hi all,

i would like to construct an experiment on coupling two stages of Common emitter amplifier by using transformer coupled method. But i dont know which ratio to choose or how many resistance for the primary and secondary winding.

I get the idea from here.



Anyone have any idea on how to calculate what is the spec i need to buy for this circuit? How is the calculation? After some calculation i get Zin for the 2nd stage is 1.06kohm (correct me if i'm wrong). then how about the Zout for the 1st stage?

Hope someone can help me on this.

thank you.

 

[carbonzit]

I take it that's an audio circuit? Where on earth did you find that?

We could help you, but you ought to know that using interstage transformers with transistors is totally antiquated. It went out, oh, about 60 years ago (not exaggerating). There are much better ways to do this.

(I should hasten to add that there are a few exceptions, like high-quality transformers for impedance-matching microphones, but other than that, they just ain't used anymore.)

 

[Nigel Goodwin]

Why would you want to? - it's an extremely pointless (and expensive) way of doing it.

It was done in the late 50's early 60's when they didn't know any better.

Impedance matching provides maximum power transfer, but that isn't what you want, you want voltage transfer - FAR more efficient.

 

[simpsonss]

i do this because it is consider a basic learning on understanding different type of coupling for multistage amplifier and it is an assignment for my school. so i have to find out the calculations and prove it with the practical part.
Hope i answer y'all doubts~

and is it possible to use a ferrite core to substitute the matching transformer part?

thank you.

 

[carbonzit]

Well, here's an audio transformer available from Digi-Key that might be useful for experimenting.

I'm a DIYer, but I'm not sure I'd recommend winding your own transformers, unless you really insist on doing so. By the time you get all the stuff you need (core, magnet wire, tape, bobbins), you'll be better off just buying one ready-made.

There are lots of other transformers available out there. You can probably even salvage some from discarded electronic equipment. (Anything with a phone connection is likely to have at least one 600Ω isolation transformer in it.)

 

[RCinFLA]

Since you have bypass caps on emitter resistor the input impedance on common emitter BJT is roughly (beta +1) x (0.026/Ie).

Your two stages have Ie of about 0.9 mA. Assuming an AC beta of 75 then input Z is 2200 ohms.

If your transformers are 5:1 turns ratio then the collector load is about N^2 x 2.2k or 55k ohms on first stage, 25k ohms on second stage.
The 5:1 might be impedance ratio, depends on how transformer is spec'd.

The gain will be very high, probably overdriving second stage.

 

[The Electrician]

These:

**broken link removed**

will probably do what you want, and the price is right!

Another source is to buy old transistor radios on eBay and salvage the interstage transformers from them.