# What is the purpose of resistor R in this circuit?

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#### pthoppay

##### Member level 5
Purpose of Resistor R

Hi,

Can anyone please explain the purpose the resistor R, in the below circuit.

Regards,
Prakash.

#### willyboy19

##### Full Member level 3
Re: Purpose of Resistor R

Set the DC gain of this stage. The two diode-connected transistors ease the DC bias of the following stage.

#### yolande_yj

##### Full Member level 2
Purpose of Resistor R

When I see this circuit, I remember something. Is it for linearity improvement? I am not sure.

#### Davood Amerion

Purpose of Resistor R

This is a non linear amplifier.
because M3 and M4 isnt curent sources or linear resistors.
I think R1 can be adjust nonlinearity and voltage gain.

#### eldo

##### Member level 1
Purpose of Resistor R

R is nothingto do with linearity. It is used to lower the gain.

#### jjsnail

##### Junior Member level 1
Purpose of Resistor R

i have seen R in the common souce of M1 M2

anyone can tell me what is that used for?

thanks very much

#### pthoppay

##### Member level 5
Re: Purpose of Resistor R

Do you mean at the source of M1 and M2, the purpose of such resistor is to linearize the current and voltage dependence.

Which in turn will make the gaina constant after a certain level of input voltage.

Prakash.

#### pixel

Re: Purpose of Resistor R

In the case od circuit with R1 upper mosfets M3 and M4 are still on while one of the input mosfet M1, M2 is off. It should be faster. It also enables smaller sizes of M3 and M4, because of they have smaller maximal circuit.
vout=gm1*(0.5*R1||1/gm3)(vg1-vg2)

##### Member level 2
Re: Purpose of Resistor R

pixel said:
In the case od circuit with R1 upper mosfets M3 and M4 are still on while one of the input mosfet M1, M2 is off. It should be faster. It also enables smaller sizes of M3 and M4, because of they have smaller maximal circuit.
vout=gm1*(0.5*R1||1/gm3)(vg1-vg2)

in what applications could i use this type of circuits according to the above explanaition? i mean why could i need M3 and M4 to always on ? why not only one of them?

#### pixel

Re: Purpose of Resistor R

This circuit has low gain, which probably means that it was used for high speed design.When you turnon/off mosfet you usually need some additional time.

#### teteamigo

Re: Purpose of Resistor R

R1 link the gates of two saturated MOS (ALWAYS saturated). The load of differential pair presents simetry guaranteed by this resistor.

For R1=∞ , there isn't link between gates. The load of differential pair offer no guarantee of simetry.

Remember the simple case of two different drain resistors-->gives common gain.

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