Part 2: LTSpice integrated circuit design: Diode connected MOS, current mirror


In integraded circuits you often need some defined currents. For this you can use current mirrors. The diode connected MOS is an easy circuit which will lead us to the current mirror.

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The diode connected MOS

In the diode connected MOS Gate and Drain are connected together so VDS and VGS are the same. There are two important things to know about the diode connected MOS. It is always in saturation and for any current ID VGS will adjust so this current can flow.

In our example we want to do a dc sweep simulation and sweep VDS to measure ID. The NMOS Geometrie is L=1u and W=2u.


NMOS diode connected

Diode connected MOS results

Here the resulting ID over UDS plot. Since we know that the mosfet is in saturation we can calculate the current ID with

(1)   \begin{equation*} I_{D} = \frac{K_{n}*W}{2*L} *(V_{GS}-V_{T})^{2}*(1+\lambda*V_{DS}) \end{equation*}

We also know that VGS = VDS so we see that the curve has a quadratic part and a linear part which is defined by the early Voltage 1/VEarly = lambda.

NMOS Diode connected plot

NMOS Diode connected plot

The NMOS current mirror

In the current mirror we have now two NMOS with the same VGS. Now we get a relation of mirror input current and mirror output current:

(2)   \begin{equation*} \frac{I_{out}}{I_{in}} = \frac{W_{out}/L_{out}}{W_{in}/L_{in}}*\frac{(1+\lambda_{out}*V_{DSout})}{(1+\lambda_{in}*V_{DSin})} \end{equation*}

What we see is that we can define our relation over the geometrie. We also see that we get some differences because of the early effect. We should use only W to define our relation and set L fix for both MOS, because the early effect changes with L a larger L means less early effect.

In our simulation we look at a 1/1 current mirror for 10uA where we load the mirror output with a Voltage source. This Voltage is dc sweeped and we look at the output current. We also do a paramter sweep for both Ls to see how the mirror characteristics change.


NMOS current mirror L sweep

NMOS current mirror result

In the plot we can see the different curves for the L sweep. For large Ls the early effect is not that dominant and we have just a small deviation from Iin (blue 4u and green 0.5u).

Another effect is that we have a higher treshold voltage with higher Ls.


NMOS current mirror L sweep plot

A better current mirror

We only get the perfect Mirror output current, when we have VDS = VGS. Because as you can see in equation 2 the early effect is cancelled. To achieve this we use a cascode to set VDS of the output MOSFET to its VGS. For this we need a cascode VGS Voltage of twice the threshold voltage for the input current.

In our simulation we will compare the current output of the cascoded current mirror with the normal current mirror. As before we load the current mirror with a voltage source.


NMOS cascoded current mirror comparison

A better current mirror results

 As we can see in the plot the current is more stable. This is because VDS of M3 is now fixed to a value where the early effect cancels. (Blue cascoded NMOS mirror).

NMOS cascoded current mirror comparison plot

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