A Basic MOSFET question

S

Saran

Guest
Hello Everybody:

Good morning. I have a couple of basic MOSFET questions that I would
like to clarify. I have studied a lot of articles, asked a few people
about this doubt but could not find a satisfactory explanation. I am
hoping somebody will take the trouble of clarifying my doubt.

MOSFETs, as I understand, are voltage-controlled devices. Doesn't that
mean that they need a charge (voltage) applied to the gate to make the
device conducting? If this is the case, in some circuits I have found
in the process of learning analog VLSI, what is the meaning of a "gate
of a nmos/pmos transistor being driven by the output of a current
mirror?" I have read that the output current of the current mirror is
used to bias a next stage. What exactly does this mean?

Secondly, if the MOSFETs are voltage-controlled devices, why do we most
of the time deal with currents when talking about VLSI systems?

Hope, I am clear with my questions. Thanks in advance.

Regards,
Saran
 
So, let me restate what I understood. The current mirror's output
current is used to charge the gate of the transistor. So, depending on
the magnitude of the current, the gate voltage increaser faster or
slower (sorry about my layman language, I just want to be very very
clear).

If I understood this correctly, then:
1) If a current mirror is charging the gate of a transistor, or in
other words, changing its Vgs, the maximum Vgs value is determined by
the saturation voltage for the mirror.
2) We are controlling the rate at which gate is charged in a circuit.

Have I understood this correctly? Thanks in advance.

Saran
 
Thanks a ton, John. It is conceptually clear now.

I have one question, how do we know how much current we have to
generate from a current mirror to bias a transistor. I know this is a
vague question, what I mean is, the gate capacitance determines the
rate at which the gate will be charged, right? So, are the gate
capacitances values standard for different transistors? Where are they
available from?

For instance, in an example, if the author says that a current mirror
outputs constantly 200nA to the input of the gate of some transistor
M1, he means that the gate voltage is being maintained constant
(unchaged) with this small current. The exact values, I assume, can be
obtained from the specific data sheets are manuals.

Is this right?

Thanks once again.
 
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