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I'm confused about OP-AMP

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angbong

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Suppose OP-Amp is ideal and equivalent circuit of OP-amp is (a).
in case differential input is zero, output DC+AC level is zero. (a)
but real OP-Amp looks like (c), (d) and their output DC level is not zero.

1. in circuit (c) DC level of Vout is A = V(Iss)+Vov2+Vov4

2. in circuit (b) i think that Vref(also output of OP-Amp) should be B = V(R1)+Vbe2.
and if that OP-amp is made by (c), Vref should be V(Iss)+Vov2+Vov4 also.
What value is correct?

3. When i learning OP-Amp in a real circuit level, i thought OP-Amp operates in a small signal aspect.
(Output DC level is not concerned, only output small signal is important)
But when i learning OP-Amp as a black box(a), i thought OP-Amp is not a small signal amp.
What is correct?
제목 없음.png
 

1: True only if your power supply is symmetrical (-vdd +vdd).
Note that V+ - V- = 0, but V+ != 0 and V- != 0 else your opamp is not biased (again, for non symmetrical power supply, and will simply not work.

2: Seems correct for b.
For c, seems correct too, but like Vov4 and Vov2 depends on Vin.
In d, this is a small signal analysis model, M1 and M2 are not biased, and Vout (in open loop) depends directly on these gate voltages !
Try to simulate a simple opamp (like d) in open loop first and look at Vout, then in a "buffer" configuration (vout looped on V-, input on V+, and add a dc source on V+, you'll learn a lot !)

3: It depends what you are looking for !
If you want frequency analysis or loop stability analysis, small signal (ie ac analysis) is ok. But it does NOT mean that the small signal variation are centered around 0 V !
If you want distortion analysis, slew rate, etc, you'll need spice model (or equivalent time model)

Hope this will help you
 

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