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simulation and hand-calculation on op-amp

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chu

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hi, i have a few questions here please help, thanks in advance.


Please check this 1 stage op-amp:

1) simulation gives differential voltage gain Av=36dB which is too far away from hand-calculation 58dB, why ?

NOTE : with current mirror load, Av=gm (Ron || Rop), gm depends on Ic (≈56uA) which is gm=0.00215 ; Ron & Rop depands on VA/Ic which give total Rout=388kΩ ; Av=824=58dB

2) does impedance matching for single-ended output and input of emitter follower important ? what is tolerance for this matching ?

3) when i switch between input terminal, i can still get the same stable result, why?

4) simulation for open-loop (removed resistor divider and follower), gain is just slightly higher , around 37.5dB. That means the loop-gain is 1.5dB, but calculation is around 21dB from AOL*β=37.5db*14k/(14k+11k), why so much different ?

thank you so much for helping
 

kevinj

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1. Rout is not 338K it's 1/gm because it's a emitter follower; gain = 1.

2. impedance matching is important for high freq. not for your design.

3. don't understand question.

4. look at the answer to 1.
 

zajbanlik

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I will try with question 4. For others I need some time to calculate, which I don't have now.
You simply cannot multiply amplification in dB with resistor divider transfer characteristik, it's not mathematicaly correct. In "decibell area" any multiplication is substitude with substraction. So it should be something like:
37.5dB+20log(14k/(14k+11k))=....
For first one. How did you calculate amplification?
 

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