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class AB op-amp specification and design rule

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r.mirtaji

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The following figure shows a class amplifier.
If ه want to use this amplifier as a buffer, how much should we design the output resistance of the amplifier?(R11 II R10)
How much quiescent current (i11 &i12) should we design the flow?
Assume the circuit specifications are as follows
UGB:100Mhz CL=5pF RL=1k slewrate=>50 V/us

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The specification does not carry sufficient information to fully advice how to design.

Btw, where is i11 and i12 in your schematic?

Capacitive load and slew rate requirements dictates roughly what current you need to push/pull.

In order to find the required output impedance of your buffer, you need to know the gain and also what losses you can tolerate due to the impedance ratio. Not only in offset/gain, but also in nonlinearity.

A tip is to set up a simple RC+vcvs model of the amplifier and start from there to find the required values.

The following figure shows a class amplifier.
If ه want to use this amplifier as a buffer, how much should we design the output resistance of the amplifier?(R11 II R10)
How much quiescent current (i11 &i12) should we design the flow?
Assume the circuit specifications are as follows
UGB:100Mhz CL=5pF RL=1k slewrate=>50 V/us
View attachment 176138
Hello,

I do recommend you to read the book of " Introduction to CMOS OP-AMPS and comparators" by Roubik Gregorian, at the end of the book the author gave a complete design example based on this circuit.

Generally, to to drive a 1 k ohm resistor you should specify the voltage swing for this load, if your out put voltage needed to be for example 1 Vp-p then we are talking about a peak current of 1 mA,
The 1 mA is not required to be available at the steady-state since it is the concept of the class AB amplifier, which implies that your steady state currents should be enough to satisfy the GBW ac requirement and the slew rate due to the compensation capacitors. The load capacitors will be definitely out of slew rate constraint since they are driven by M11 M10 heavy driver.

in order to hand determine the GBW you need to estimate the compensation capacitors in your circuit. just be sure that the value of this capacitor depends also on how much the pole from the M11 and M12 is far which depends on the gm11&gm12, its a matter of power now to push them further toward reducing the C.

I suggest you to read and rewark the example from the book. then back to me to see your problem, I have worked in similar circuit a lot and I can share with you my experience. Thanks at the time to all my friends here whom they helped me to understand it

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