I've got difficulties to solve this electronic exercise - diyAudio
 I've got difficulties to solve this electronic exercise
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diyAudio Member

Join Date: Nov 2002
Location: Grenoble, FR
I've got difficulties to solve this electronic exercise

Hi all,

This is a part of an exercise I try to solve,
I have to draw the "equivalent small signal schematic" for this amp, and calculate the voltage gain from this small signal schematic

I'm totally lost. For a single transistor it's ok, but for a cascode I can't find how to calculate the gain
Attached Images
 image1.jpg (21.3 KB, 133 views)

 4th June 2004, 10:29 AM #2 Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4 It's easier than you think. Just forget the cascode and pretend that you only has RC and a plain transistor stage. Ic*40*Rc = gain... approx.=> 1 mA, 1 kohms = 40 in gain Put the schematic into LTSpice and their you have the answer at least. __________________ /Per-Anders (my first name) or P-A as my friends call me PA03 LM4780 amplifier group buy, SIGN UP HERE for the group buy 0 boards left. 110 paid.
 4th June 2004, 10:36 AM #3 diyAudio Member     Join Date: Nov 2002 Location: Grenoble, FR And here's the equivalent circuit I find finding the ic for T1 is straightforward: ic=B*ib=B*Ve/h11 but now, what about T2? How can I calculate the voltage across h11? (h11 from T2)
 4th June 2004, 10:43 AM #4 diyAudio Member   Join Date: Nov 2003 Location: Brighton UK Well as a pointer : The purpose of a cascode transistor T2 is to prevent the voltage swing at T1's collector that would normally occur, thus preventing the miller effect reducing the bandwith of T1. T2's base is clamped by the voltage source and the voltage swing appears on T2's collector. As the voltage source should have very low impedance the miller effect is less consequential here. As T2's base is clamped so is its emitter effectively. I gave up on equivalent circuits, finding them more confusing than helpful beyond a certain point but they are important. As stated the gain is near enough identical to a single transistor stage. sreten.
 4th June 2004, 10:53 AM #5 diyAudio Member     Join Date: Nov 2002 Location: Grenoble, FR The purpose of a cascode transistor T2 is to prevent the voltage swing at T1's collector that would normally occur, thus preventing the miller effect reducing the bandwith of T1. agreed T2's base is clamped by the voltage source and the votage swing appears on T2's collector. As the voltage source should have very low impedance the miller effect is less consequential here. As T2's base is clamped so is its emitter effectively. What voltage source? Remember, my answears have to be based on the equivalent circuit.
diyAudio Member

Join Date: Nov 2003
Location: Brighton UK
Quote:
 Originally posted by Bricolo [B What voltage source? Remember, my answears have to be based on the equivalent circuit. [/B]
R1, R2 and Cd, at AC this is a low impedance voltage
source, a divided [R2/(R1+R2)] version of rail voltage E.

sreten.

diyAudio Member

Join Date: Nov 2002
Location: Grenoble, FR
I just realized that I forgot to post the picture

That's what I get for the equivalent circuit. It is correct?
I explained how to fond ic and ib for T1, but now how to calculate it for T2???
Attached Images
 dscf0003.jpg (28.3 KB, 106 views)

 4th June 2004, 11:49 AM #8 diyAudio Member   Join Date: Nov 2003 Location: Brighton UK T2 divides T1's current by the ratio of its Beta. So ib2 + B2.ib2 = B1.ib1 ib2(B2+1) = B1.ib1. ib2 = B1.ib1/(B2+1) B2.ib2= B2.B1.ib1/(B2+1) Output current is very near a single transistor for high Beta. Note also this is one of those very poor circuits only found theory textbooks, as voltage gain is dependent on B1, this sort of thing should only be used inside a feedback loop. DC output voltage is also very poorly defined. sreten.
 4th June 2004, 12:29 PM #9 diyAudio Member     Join Date: Nov 2002 Location: Grenoble, FR Thank you, that makes sense. So, for a high beta (180 in this case) ic is nearly the same as with a single transistor ib1 is also very close to ib2, so V' is equal to Ve am I right? So T1 isn't really running at constant Vce, but if the voltage gain is important, the Vbe variations can be considered null. BTW, what would be a better circuit, with no (or lower) beta dependance, and DC stability? (one that wouldn't require feedback)
diyAudio Member

Join Date: May 2002
Location: Switzerland
Quote:
 BTW, what would be a better circuit, with no (or lower) beta dependance, and DC stability? (one that wouldn't require feedback)
Simply add an emitter resistor to T1. And use a proper voltage divider for its base.

Regards

Charles

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