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Old 28th April 2004, 04:21 PM   #1
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Default Low gain x circuit

What do you think of this design? I wanted it only using single differential, Xcct, and push-pull VAS/Output stage.
The differential is working at low ccs, only 2mA, output stage is working at 10mA, due to modified current mirror.
Can this be classified as low gain design? Since I think the gain is coming from the differential + modified current mirror only.
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Old 28th April 2004, 05:10 PM   #2
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This circuit is rather suitable for you to simulate and for trying different component values. Download the freeware LTSpice and start messing around.
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Old 28th April 2004, 05:22 PM   #3
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If current mirrors you must have, why not simply use one across the differential? And why the difference in currents from one side to the other?
Or, put another way, I don't see that Q4, Q6, Q7, and Q8 actually contribute much towards your goal.
Low gain? Loading a gain device with a current source--which represents a pretty high impedance--generally yields a lot of gain. Unless you were planning on burning it off with feedback, which isn't the same thing at all.

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Old 28th April 2004, 05:26 PM   #4
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Actually, on looking at it again, you're taking the output for each phase from different points on each side.

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Old 29th April 2004, 02:29 AM   #5
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This is what I think when drawing this design. First, I'm much inspired by low gain design (that will lead to low feedback). The current mirrors can be replaced by folded cascode, but I choose to use current mirrors.
The differential is Q1 and Q2, fed by 2mA ccs. Each leg is connected to current mirrors, so I have 1mA flowing down at Q4 to Q7 and also from the most right transistors. The function of this Q4-Q7 (the same with the most right transistors), is to shift the current to Q8 and the left down transistor (which Re is 200ohm).

I wanted push pull output. I can replace Q8 by CCS, but that will lead to single action only by Q6. If Q6 is "push" then I wanted Q8 tobe "pull", not steady like ccs. That is why I have Q3-Q4-Q7-Q8 for shifting the "pull" action from the other leg of differential.

I modified the current mirror Q5-Q6 so I can have bias of 10mA from 1mA differential leg. If I use the same 2k for Re, then the bias is too low, only the same as differential, 1mA.

Maybe the drawing is confusing, because I first I draw it single ended, but Jam mentioned something about X cct. So I put additional current mirror and output stage to have X cct. Using current mirror is good in this design, because I can

1.Level shift the single differential outputs to have push-pull action .

2.I can still use single differential to have X cct.

I really hope this is qualified as low gain design. And with good output for X cct qualification.
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