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Basic question about transistor operation

Started by Imagineer 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 about that. 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 > about that. > > 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.