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FINFET BASED DIFFERENTIAL AMPLIFIER

MOHAMMAD ALEEM

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I have designed a finfet based self biased differential amplifier .Amplifier without feedback is working fine but giving distorted result with both capacitive and resistive feedback using HSPICE. Any possible reasons?
 

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sutapanaki

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I think a schematic and some simulation results will be helpful.
 

sutapanaki

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You say you get distorted results and yet you only show AC response. And still no schematic. If you want some one to help you, then help him/her help you.
 

MOHAMMAD ALEEM

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sorry for all the inconvenience as i am new to the group.my schematic and graphs are posted here.I have applied 2.5 common mode voltage for biasing at input but in transient graph inpiut voltage is showing as 3.4754 volt but if without feedback then this voltage is accurately 2.5v with sinusoidal swing .
 

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sutapanaki

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Is it your intention to connect this circuit in a positive feedback configuration? Also, I'm not sure but is the DC component in your SIN voltage source 2.5V?
 

dick_freebird

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I don't know what finFET technology you're using but
the one I've been exposed to (GF LPP14/LPP12) will not
handle 2.5V even using the "EG" (thick oxide) devices.
 

MOHAMMAD ALEEM

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Is it your intention to connect this circuit in a positive feedback configuration? Also, I'm not sure but is the DC component in your SIN voltage source 2.5V?
DC component i have used is Vdd+Vcc/2 and it is 2.5V .Problem is with 2.5V common mode voltage at both end i am getting gretaer then 2.5V at inverting terminal which is the common mode voltage applied.Also with feedback resistors i am not getting gain in ratio of feedback resistances.
 

MOHAMMAD ALEEM

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I don't know what finFET technology you're using but
the one I've been exposed to (GF LPP14/LPP12) will not
handle 2.5V even using the "EG" (thick oxide) devices.
PTM 7 nm model i am using. Should i change the common mode voltage value?
 

sutapanaki

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Well, if you don't have a sine signal, just its common mode, then you will have 2.5V+2.5V if I read your netlist correctly. On the other input looks like you only have 2.5V.

You didn't say if you intentionally configured this circuit in a positive feedback.
 

MOHAMMAD ALEEM

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Well, if you don't have a sine signal, just its common mode, then you will have 2.5V+2.5V if I read your netlist correctly. On the other input looks like you only have 2.5V.

You didn't say if you intentionally configured this circuit in a positive feedback.
You can check the netlist.i have given both common mode and sinusoidal signal Vd. I just want to check my amplifier works as gain amplifier by giving feedback resistor R1 and R2 and checking dc gain which is not even close to R1/R2.How to sort out this issue?
 

sutapanaki

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Again. Didi you intentionally configured the amplifier in a positive feedback?

In addition there is 1.25V difference between the DC values of the two inputs. That's a huge offset.
 
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MOHAMMAD ALEEM

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Again. Didi you intentionally configured the amplifier in a positive feedback?

In addition there is 1.25V difference between the DC values of the two inputs. That's a huge offset.
I havent done it intentionally.Can you please tell me more how to be sure that applied feedback is negative or positive?
 

sutapanaki

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Well, it is easy. Your output is at 6 and node 5 is inverting because it is the input of inverter. 4 is then the non-inverting input.

But if you have that 1.25V difference in the DC bias between the two inputs, I don't know what this can cause. You have to simulate. Make the sine wave amplitude equal to 0 and see what happens.
 

MOHAMMAD ALEEM

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Well, it is easy. Your output is at 6 and node 5 is inverting because it is the input of inverter. 4 is then the non-inverting input.

But if you have that 1.25V difference in the DC bias between the two inputs, I don't know what this can cause. You have to simulate. Make the sine wave amplitude equal to 0 and see what happens.
When i am stimulating same circuit without feedback bias difference is 0.Might be feedback is adding some error.
 

sutapanaki

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Don't you have this from your netlist? Here the feedback resistors play no role.

1617944565489.png
 

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