# Short in MOSFET between Gate and Drain

I try to understand a circuit, where this is a part of:

To me this looks like a short between the Drain and Gate in the pmos at the top and nmos at the bottom. The line from the top pmos to the right is used as the gate of some nmos gates, the line from the bottom nmos to the right is used as the gate of some pmos gates. (No shorts here)

My question now is: What is the behavior of this circuit regarding the "shorts"? Are they even shorts, or am I missing something obvious here?

• I'm normally more around in the field of computer science, so I have only basic knowledge regarding electrical engineering, so there is a high chance I missed something obvious. Jun 14 '21 at 22:58
• The schematic is probably simplified to imply current mirrors (the top & bottom transistors) and not shown as it is really implemented.
– qrk
Jun 15 '21 at 0:05
• When The Drain And Gate Of A MOSFET Are Connected Together, A Two-terminal Device Known As A ''diode-connected Transistor'' Results. if drain and gate is shorted then MOS will behave as a diode connected load and it will never go to triode region and can be used as resistive load offering a resistance equal to 1/gm. if drain and source is shorted then MOS capacitor in which SiO2 will behave as insulator. ie it is a resistor.
– Gil
Jun 15 '21 at 1:00
• Thanks @Gil a diode-connected transistor would also make sense in this context. You could also post your comment as an answer, so if someone stumbles upon the same question, they find your answer easier. Jun 15 '21 at 16:49

When The Drain and Gate of a MOSFET are connected together, A two-terminal device known as a ''diode-connected transistor'' results. if drain and gate is shorted then MOS will behave as a diode connected load and it will never go to triode region and can be used as resistive load offering a resistance equal to 1/gm. if drain and source is shorted then MOS capacitor in which SiO2 will behave as insulator. ie it is a resistor. Thanks for the suggestion, As you can see I just added it as an answer.

• Why are you capitalizing every word in your first sentence? Jun 16 '21 at 3:03
• Sorry no clue as to what happened. Me either. Somehow I missed it after I posted it, I always check what the post gets, they are not always the same as I thought I sent.
– Gil
Jun 16 '21 at 3:21
• No need to apologize, I just see people do that sometimes and I'm legitimately curious why. It's never made sense to me. Jun 16 '21 at 3:22
• @Hearth Ahhh, I ran into someone like that too and they said it was just habit and I was just like..."but...but why? That doesn't answer anything." Jun 16 '21 at 3:23

Addition to the answer of @Gil, the diode-connected transistor device also stabilizes the voltage gain as follows:-

You can visualize a diode - connected transistor as this

where M1 is the original transistor and M2 is the diode

Now, IN SMALL SIGNAL MODEL:

The voltage gain calculation:

As we can see the voltage gain depends just on geometrical parameters (width, W and length, L) of the MOS devices and it does not gets effected by some environmental or circuital changes like temperature, signal level and other changes. They can be considered as constant. Hence diode connected devices stabilizes the gain of transistor and that's the attractive feature of this device.

I am considering that you know the voltage gain of a Common Source MOS device which is -gm*(resistor connected between drain and ac ground).

In small signal model, 1/gm was the only resistor connected between drain terminal and ac ground.