# Basic question about transistor operation

Started by March 18, 2013
```Hi everybody,
could someone explain to me something very basic:
the simplest transistor has 3 legs: gate, drain and source. If I were to ap=
ply voltage to the gate (above threshold), the transistor "opens" and now c=
urrent between the source and the drain can flow. I have simulated it in LT=
Spice and also tested the same in a lab - and the drain current always prop=
ortional to the gate voltage. Is it suppose to be so and why?
```
```On 3/18/13 8:41 AM, Imagineer wrote:
> Hi everybody,
> could someone explain to me something very basic:
> the simplest transistor has 3 legs: gate, drain and source. If I were to apply voltage to the gate (above threshold), the transistor "opens" and now current between the source and the drain can flow. I have simulated it in LTSpice and also tested the same in a lab - and the drain current always proportional to the gate voltage. Is it suppose to be so and why?
>
There are several types of transistors which have slightly different
characteristics.  It sounds like you're talking about a FET transistor.

Eventually you'll hit the saturation point and no longer will a voltage
increase on the gate cause an increase to the source-drain current.

Also, if I'm remembering correctly (I've just learned this stuff a few
weeks ago), the relationship isn't entirely "proportional". I think it
is either a squared or a logarithmic relationship, but I could be wrong

Take my comments all with a grain of salt, I'm just learning this stuff
myself. I'm sure a more veteran member of this group will correct me if
I'm wrong, so you may wish to wait until they do ;-)

Hope that helps,
Daniel.
```
```On Mon, 18 Mar 2013 08:53:06 -0700, Daniel Pitts wrote:

> On 3/18/13 8:41 AM, Imagineer wrote:
>> Hi everybody,
>> could someone explain to me something very basic: the simplest
>> transistor has 3 legs: gate, drain and source. If I were to apply
>> voltage to the gate (above threshold), the transistor "opens" and now
>> current between the source and the drain can flow. I have simulated it
>> in LTSpice and also tested the same in a lab - and the drain current
>> always proportional to the gate voltage. Is it suppose to be so and
>> why?
>>
> There are several types of transistors which have slightly different
> characteristics.  It sounds like you're talking about a FET transistor.
>
> Eventually you'll hit the saturation point and no longer will a voltage
> increase on the gate cause an increase to the source-drain current.
>
> Also, if I'm remembering correctly (I've just learned this stuff a few
> weeks ago), the relationship isn't entirely "proportional". I think it
> is either a squared or a logarithmic relationship, but I could be wrong
>
> Take my comments all with a grain of salt, I'm just learning this stuff
> myself. I'm sure a more veteran member of this group will correct me if
> I'm wrong, so you may wish to wait until they do ;-)

For a classic FET the drain current is more or less proportional to the
square of the difference between the gate voltage and the cutoff
voltage.  With emphasis on the "more or less".

For a BJT the collector current is more or less proportional to the
exponent of the base-emitter voltage, at least until the internal
resistances of the emitter and/or base start coming into play.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
```
```On Mon, 18 Mar 2013 08:41:11 -0700 (PDT), Imagineer <antontravinsky@gmail.com>
wrote:

>Hi everybody,
>could someone explain to me something very basic:
>the simplest transistor has 3 legs:

We call them "leads" or "terminals".

> gate, drain and source.

That's a FET. Bipolar transistors have base, collector, and emitter.

> If I were to apply voltage to the gate (above threshold), the transistor "opens"

We'd say it "conducts" or "turns on." Open is the opposite of conducting.

>and now current between the source and the drain can flow. I have simulated it in LTSpice and also tested the same in a lab - and the drain current always proportional to the gate voltage. Is it suppose to be so and why?

For lots of enhancement-mode mosfets - the most common kind - with a constant
voltage applied to the drain, the Id versus Vg curve is pretty linear, between
threshold and full enhancement, rough numbers of gate voltage from, say, 2 to 10
volts. It can vary a lot among different parts. Sometimes the Spice models
aren't super accurate, so check the part data sheets.

Here's one:

http://www.onsemi.com/pub/Collateral/2N7000-D.PDF

Fig 2 is pretty much what you're doing.

Some fets, depletion mode parts, conduct with zero volts on the gate.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
```
```Thank you all, now I understand it beter.
It was indeed a MOSFET transistor.
```