# Weird Current Mirror

I just encountered this circuit and I'm a bit confused by it I see that it's an NMOS current mirror. At first I thought it's a cascode current mirror due to M3 on the right but it isn't. I have two questions

1. What is the point of M3 in this circuit? I mean it's going to have some factor times Iref through it?
2. What is the point of R1? I mean surely I_REF will push that much current through it no matter what so?

If somebody could just give me an insight on how it works, that would be great.

• As for R1 read this wiki.analog.com/university/courses/electronics/text/… And for M3 do you know what channel length modulation is? Basically, M3 eliminates channel length modulation in the M2 transistor. – G36 Nov 8 '18 at 16:45
• Yes, I know channel-length modulation. How exactly is M3 doing that? Also is M3 and M2 together not just a cascode current source? – AlfroJang80 Nov 8 '18 at 22:14

## 2 Answers

I believe both the resistor and the cascode are techniques for keeping $$\v_{DS}\$$ constant to avoid short-channel effects.

### Resistor $$\R_1\$$

A regular diode-connected transistor ($$\R_1 = 0\$$) will also change $$\v_{DS1}\$$. Increasing $$\I_{REF}\$$ will make $$\v_{GS1}\$$ larger, which in turn also causes $$\v_{DS1}\$$ to increase as they are short-circuited.

If $$\R_1 > 0\$$, then as $$\v_{GS1}\$$ increases with $$\I_{REF}\$$, $$\R_1\$$ will push $$\v_{DS1}\$$ down compared to $$\v_{GS1}\$$. If you choose $$\R_1\$$ carefully you can make it compensate the change of $$\v_{GS1}\$$ such that $$\v_{DS1}\$$ remains approximately constant.

### Cascode M3

Cascodes are a technique that employ negative feedback to keep the current constant.

If we try decreasing $$\v_{DS2}\$$ by sinking extra current to ground, then $$\v_{GS3}\$$ increases which makes M3 conduct more current. As M3 injects more current into the node between M2 and M3, it counteracts the decrease of $$\v_{DS2}\$$ (negative feedback).

$$\v_{DS2}\$$ variation is reduced by M3, reducing short channel effects.

• Very nice explanation. What exactly do you mean by "sinking extra current to ground"? – AlfroJang80 Nov 9 '18 at 11:15
• Can be anything really, like M2 conducting more current or adding another path through which current can flow out of that node. – Sven B Nov 9 '18 at 12:32

Yes. It is just a cascode current mirror that:

• uses a separate voltage bias source for the cascoding transistor.
• Reduces the Vds of the bias transistor with a resistor.

Both choices actually make it worse than a standard cascode, as the channel length modulation might not simply match on both sides of the mirror.

It seems to me more of a homework exercise than a practical circuit.