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Operational design without current reference

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ejcmos

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Hi all,

I am designing a operational amplifier and using negative feedback in my application,
So V1=V2=Constant voltage, the input voltage is fixed most of the time.
Since input is fixed, I can control the OP consuming current by adjusting input MOS. Can I remove current reference stage as type (B).

Comparing the type A and B, what's the advantage and disadvantage and consideration ?

Thanks in advance
 

Schematic B won't give you differential output. The long-tail pair needs a common resistor joining the emitter legs (that is a resistor at least if you don't use a transistor/mosfet).

The aim is to make the input transistors interdependent in the sense they need to respond to each other. Increasing current through one reduces current through the other (and vice versa).
Schematic A gives you this.

I'm also not sure you need the current mirror at top. Resistors may be sufficient. However my simulations usually require a lot of adjusting to find a good operating point. Real op amps contain features that do this automatically. I admit to gaps in my knowledge.
 

Schematic B won't give you differential output.
First circuit neither, these are asymmetric or single ended circuits.
The aim is to make the input transistors interdependent in the sense they need to respond to each other. Increasing current through one reduces current through the other (and vice versa).
The current mirror load compares the currents of the 2 branches. This is a respond to each other.

Both circuit can work, but 2nd circuit (called quasi-differential pair) will be very non-linear, the current consumption can be incredibly high and Vcm dependent, and the common mode rejection will be very bad. Only advantage is the output swing is higher a bit.
 
What you show is only the first stage of a classical op
amp (delivering well less than full output swing and nil
output drive).

The second stage must track and match the first for
any decent Vio performance (else systematic errors).
So you want a rack of matched current sources.

Now you mention "reference" but really the one shown,
and the others I mention, are "repeaters" from a not-
shown reference.

While bandgap-style is most popular (leading to low
voltage and temperature sensitivity) I have seen low
power "programmable" op amps with an external
set-resistor biasing the current mirror rack. If you
are on the hook for the signal chain and nobody
cares how you set the biasing, this could be a simple
work-avoidance, schedule-friendly play.
 

.............I'm also not sure you need the current mirror at top. Resistors may be sufficient. However my simulations usually require a lot of adjusting to find a good operating point. Real op amps contain features that do this automatically. I admit to gaps in my knowledge.
The current-mirror at the top gives a high-gain, differential to single-ended output.
And you don't use resistors in an IC unless there's no other choice.
 
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