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# how to analyse this circuit?

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#### wjxcom

##### Full Member level 5
Hi, all: this circuit can generate bias voltage, but I do not know what is the role that some MOS transistor plays.

In this circuit, I want to know what is the role that M18, M51, M20 play? at the same time, what is the role that resistance R3, R2 play? what is the role that M56 and M57 play?

This circuit can generate bias voltage for an ADC, so I think this circuit is important for me. Help me, Please!!!

Hi,

M18 is a mere source follower. It´s source will follow the gate voltage by 1 Vt.
I think it´s use here is a level shifter.

The circuit M18-M51-M53 is a feedback loop that will try to equalize the currents (or a certain ration of currents) between M51 and M53.

R3 and R2 together with M58 make a Vgs multiplier : the voltage across the drain source of M58 = (R2 + R3) / R2 * Vgs.

M56 and M20 I can´t find in your circuit.

M57 together with M58 make a voltage reference, with value = (R2 + R3) / R2 * Vgsm58 + VgsM57.

Br, JT

Hi, JT: I am sorry to disturb you again! at first, I acknowloedge you for your help.
At the same time, this circuit is certainly has no M20, but there has M56. In fact, M56 is on the right side of R2 and R3, whose gate connects with R2 and R3. M58 is on the right side of M56, whose gate connects with the drain of M56, and M57 is underneath of the M56.
So I think I can know the role of M56 from you. Help me, please!!

wjxcom:

Firstly, i want to say that you will no waste others points when you ask question by downloading your attachment.

In this circuit, I am not clear about the function of VB3.
It is a feeback signal, but i can not find the feeback loop.

I guess:
The bias may be used for biasing thegain-boost fold-cascode full differential amplifer.
VBP1,VBP2 bias the upper load current mirror;
VBN1,VBN2 bias the bottom load current mirror;
VB1 & VB4 bias the gain-boost amplifer.

VBP & VBN bias the amplifer current source.

M18-M51-M53 is a feedback loop which is used to make the VBP is insensitive.

Just use the Vgs voltage difference betwwen the M56 & M57 and the M58 to get a voltge which drop on the resistor R4 to get a bias VB3 which is propotional to variation with threshold of MOS.
Therefore, decrese the sensitivity of power supply .

Hi, jiangwp: can you tell me which book explains the circuit that composed of M56, M57, M58 and R2, R3, R4?

Hi, all: in this circuit, why we not replace the R4, R2, R3 by three MOS transistors?

Hi, jiangwp: because the vds of M25, M24, M60 and M61 is variable, so we must define VB3 to confirm the VB1, VB2 and VB4

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