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Old 3rd October 2011, 08:51 AM   #1
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Default how to choose impedance matching transformer?

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.

Ok.Now i have the first stage CE amp with Zin= 1.6kohm and Zout= 3.9kohm
second stage CE amp with Zin =1.7kohm and Zout=3.9kohm

so i will use an impedance matching transformer to coupled the first stage's Zout to the second stage Zin. then i stuck here. what ratio and resistance of matching transformer should i buy?

Can someone help me on this?thank you
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Old 3rd October 2011, 09:36 AM   #2
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the first stage CE i have a 3.9kohm resistor connected from Vcc to Collector pin. so when i want to use a transformer to coupled the signal should i replace the 3.9K resistor with the primary side of the transformer?

thank you.
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Old 3rd October 2011, 10:31 AM   #3
AndrewT is offline AndrewT  Scotland
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The CE stage is usually designed to drive a high impedance load.

If you have Zout=3k9 then normally the Zin of the next stage is much higher than 3k9.
What is the minimum impedance that your CE stage can drive and still meet it's performance specification?

Once you know the minimum required load impedance you can then determine the turns ratio of the coupling transformer.

Let's suppose your CE stage can drive 10k, in addition to the 3k9 load on the collector.
Your second stage is actually 1k7. Impedance ratio is ~7:1.
The turns ratio required is sqrt(impedance ratio). sqrt(7) ~2.6.
The input has say 260T the output has (260/2.6) ~100Turns.
The voltage delivered to the second stage will be reduced to 1/2.6

Now I'm at my limit. If you replace the 3k9 collector load with the input winding of the transformer, then you are driving a quite different load by that first stage. Let's see what other replies you get.
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regards Andrew T.

Last edited by AndrewT; 3rd October 2011 at 10:40 AM.
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Old 3rd October 2011, 02:46 PM   #4
DF96 is offline DF96  England
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The transformer can replace the collector resistor, or it can be used via capacitor coupling in parallel with the collector resistor. The two situations are quite different, so you would need to decide which method you want to use. The intended application is relevant. What type of signal? How much gain? How are you biasing the stages? Is this a paper exercise or do you intend to build it and test it?
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Old 4th October 2011, 09:27 AM   #5
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Hi,

Actually i got the idea from here.

5.JPG
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Old 4th October 2011, 10:37 AM   #6
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anyone have any idea on how to calculate what is the spec i need to buy for this circuit? How is the calculation. i can see that the Zin for the 2nd stage is 1.06kohm (correct me if i'm wrong). then how about the Zout for the 1st stage?

thank you.
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Old 4th October 2011, 01:13 PM   #7
DF96 is offline DF96  England
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How did you calculate 1.06k? There does not seem to be enough information for that. Similarly Zout for the first stage. Both depend on the transistors to be used. It is possible you are confusing Zout with Zload. I assume this circuit is from a textbook.
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Old 4th October 2011, 03:40 PM   #8
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Sorry for the wrong answer. i had made the correction as below. (i'm using 2N3904)

Zin(2nd stage) = rb || (Beta)(re)
= 3.33k || (160)(0.0167)
= 2.67ohm

i get this circuit from google search. Another question is can i use a ferrite core to substitute the transformer if i cant get an impedance matching transformer?
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Old 4th October 2011, 04:03 PM   #9
DF96 is offline DF96  England
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Where did you get 0.0167 for re? What do you calculate for the current in stage 2?

Whether a ferrite cire is suitable depends partly on frequency. What is the intended application of this circuit? At present it looks like a standard textbook circuit - nothing like what is commonly used in the real world.
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Old 4th October 2011, 09:01 PM   #10
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I see you guys are struggling with this.

Now, should I chime in here and explain it all, or do we suspect that this is a student cheating on their homework?
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