# 3 stage cascaded amplifier (common emitter)

Discussion in 'Homework Help' started by nvr_mnd, Apr 25, 2007.

1. ### nvr_mnd Thread Starter New Member

Apr 25, 2007
1
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how to solve this kind of circuit...having 3 stage cascaded....how to solve the voltage gain on the stage A?,,,,thnks....

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2. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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Simply ignore everything to the right of Q1 to solve for gain of stage A. Solve gain seperately for each stage as though the other two did not exist. Multiply the gains together for overall circuit gain.

3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
He needs to consider the input impedance of the following stage when calculating gain.

4. ### NavjeetSingh New Member

Feb 8, 2009
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This is something what we call Darlington Pair? Am I right?

The gain of this is equal to the product of the individual gains of the three transistors. It provides a very high gain of the order of many thousand times.

Can anyone tell me what are those capacitors doing over there?

5. ### Skeebopstop Active Member

Jan 9, 2009
358
3
First thing I recommend is you learn how to learn.

second is to find the input impedance to the second stage and work with it like it is Rload in BJT common emitter amplifier design.

6. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
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This isn't going to work. The DC bias is all wrong. The transistors have no voltage across them to speak of.

7. ### AliceT Guest

The net effect is 3 gains of -3.4 and two gains of .63 so
Gain=-3.4*3.4*3.4*.63*.63=-15.6

This is just a quick estimation using rule-of-thumb equations. You really need to understand the limitations of the quick formulas that I used here because if you change values some assumptions break down.

However, The Electrician is correct that the biasing is wrong since collector bias current is about 2 times too high and the transistors are in saturation. You need to increase the 16.3K bias resistors to about 36K, or double your Vcc supply voltage to 24V.

Last edited by a moderator: Feb 8, 2009
8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
There seems to be little point in analyzing the original circuit, as the post is almost 2 years old. Taimour basically hijacked (or maybe hitch-hiked onto) an old thread. He has a different problem.

9. ### mik3 Senior Member

Feb 4, 2008
4,846
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At midband frequencies you can ignore the loss of voltage across the coupling capacitors. Find the AC gain of the first stage using a transistor model or in any way you want and then raise it to the power of 3 because you have 3 similar stages. This is valid for the voltage gain, if you want to find the current gain then things change. You can't find the current gain of the first and raise it to the power of 3 but you need to find the current gain for each stage individually.

10. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Except for the fact that the last stage has no load, while the first two are loaded by the input impedance of the succeeding stage.

11. ### Dave Retired Moderator

Nov 17, 2003
6,960
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I have restored the original posts that were relevant to this thread and have undeleted posts that were deleted as a result of the move.

Apologies for the confusion, and thanks to Ron H for pointing out the errors.

Dave