# Calculating output current in a pseudo current mirror circuit

1. ## Calculating output current in a pseudo current mirror circuit

Dear All

Recently I faced a problem in calculating the output current in a circuit which is similar to current mirror circuit. I wanted to ask for your ideas on how to solve this circuit problem . In this circuit we supposed Vdd as Vreference.
the schematic is as below :

2. ## Re: Calculating output current in a pseudo current mirror circuit

It looks like an erroneous schematic. Presumed Q1 is a regular NMOSFET as claimed by the schematic symbol, drain and source terminals are interchangeable, and in this case, just drawn wrongly. After fixing the symbol, you get a regular current mirror with an additional resistor R2 draining part of the reference current.

Wonder if the circuit has been created to confuse students?

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3. ## Re: Calculating output current in a pseudo current mirror circuit

Originally Posted by FvM
Wonder if the circuit has been created to confuse students?
Very well possible!

In a 3-terminal NMOSFET the p-substrate is connected to the source, so the transistor isn't symmetric any more. In this schematic, the parasitic anti-parallel diode - composed by the p-substrate and the n-drain - simply would act as a standard silicon diode biased in forward direction:

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4. ## Re: Calculating output current in a pseudo current mirror circuit

Hi,

Assuming that the Q1 transistor is an NMOSFET (if its PMOSFET, it switches off as the Source is tied to its gate), the Current in the R1 will have to accomodate the currents of both the Q1 & R2. However the Iout will be not be equivalent to Iref, infact slightly greater (current flowing ) due to the higher Vds across Q2 when having the same R1 as its load res.

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5. ## Re: Calculating output current in a pseudo current mirror circuit

In a 3-terminal NMOSFET the p-substrate is connected to the source, so the transistor isn't symmetric any more. In this schematic, the parasitic anti-parallel diode - composed by the p-substrate and the n-drain - simply would act as a standard silicon diode biased in forward direction
Current mirror is an IC design topic, thus I rather expect an incomplete MOSFET symbol (substrate terminal omitted) than a discrete MOSFET with substrate connected to source. "Pseudo current mirror" isn't a common technical term, in so far it's hard to guess what it wants to mean, but placing a substrate diode is really far from any kind of current mirror.

6. ## Re: Calculating output current in a pseudo current mirror circuit

thank you all for your answers and guidelines , as I studied Behzad Razavi CMOS design book , I have written one voltage division on R2 from Vref and then written one KVL in the loop between R2 and Vgs of the Q2 and finally calculated the current flowing through the Q2 , I don't know if it is the correct approach or not , because as I know , there is no element between transistor pairs of a current mirror and according to CMOS design book of Mr. Razavi , if there is any element between two transistor pairs of current mirror, then we cannot call it a current mirror because the current which is flowing through Q2 won't copy the reference current.

Thank you all

7. ## Re: Calculating output current in a pseudo current mirror circuit

You didn't yet answer to the discussion questions.
- What's the purpose of "inverted" MOSFET?
- What's the purpose of R2`
- What do you want to achieve with this specific circuit?

8. ## Re: Calculating output current in a pseudo current mirror circuit

Originally Posted by FvM
You didn't yet answer to the discussion questions.
- What's the purpose of "inverted" MOSFET? I believe this is a mistake (as mentioned with PMOS the CKT doesnt work)
- What's the purpose of R2` (R2 can be estimated as a Fab defect or Parasitic due to poor layout techniques)
- What do you want to achieve with this specific circuit?
(Its just a Current mirror with a parasitic resistance on the Gate)

Hi Omidrey, can you pls confirm if my above assumptions are in sync with you

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9. ## Re: Calculating output current in a pseudo current mirror circuit

Hi Mr. Chaitung , yes that resistor is a parasitic resistor on the gate causing current drainage.

10. ## Re: Calculating output current in a pseudo current mirror circuit

Hi dear FVM , sorry for replying late ...

the purpose of inverted Mosfet: in fact a this is only a schematic symbol misunderstanding and as i asked from the professor, the Q1 is a normal nmos transistor.
the purpose of that R2 is to causing current drainage so that the the Q2 transistor will lack in copying the current as a current mirror circuit.

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