# Current through a transistor

Discussion in 'General Electronics Chat' started by sharanbr, Jun 19, 2015.

1. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
1
A basic question on the current flowing through a transistor.
If the Vdd is kept constant and channel width is increased, the current through the channel increases.

Does this mean that the resistance of the channel is decreasing with increasing channel width, since current = Vdd/R_channel.

Aug 21, 2008
2,664
634
yes.

3. ### WBahn Moderator

Mar 31, 2012
17,788
4,807
All other factors being the same, yes.

4. ### Bordodynov Active Member

May 20, 2015
643
188
Open channel transistor behaves like a resistor only at low voltages. Therefore, actually the current is smaller.
since current < Vdd/R_channel.

View attachment 87493

Last edited: Jun 25, 2015
satyapal likes this.
5. ### WBahn Moderator

Mar 31, 2012
17,788
4,807
That simulation doesn't address the TS's question. He very specifically asked about the effect of increasing the width of the transistor while keeping Vds constant. Your simulation keeps the width constant while changing Vds. The two situations are not at all similar. If you want to simulate the effect with discrete transistor models, fix Vds and then put multiple transistors in parallel and plot the results.

Also, your Ron serves no purpose since it's presence is completely masked by the 12V DC source.

satyapal likes this.
6. ### Bordodynov Active Member

May 20, 2015
643
188
I will give an explanation.Was really not an issue, and the assertion that if you increase the width of the transistor current is increasing.No doubt about it.
You can even argue that the current is proportional to the width(if you do not take into account the effect of the narrow channel).The argument was that current =Vdd/R_channel.But with this I disagree! Blue line just shows the value of the Vdd/R_channel.The green line shows the current of the transistor.I.e.what I showed refers to the topic.

Last edited: Jun 23, 2015
7. ### WBahn Moderator

Mar 31, 2012
17,788
4,807
Where are you getting the value of R_channel from?

Which channel resistance are you taking about, large signal or small signal?

8. ### Bordodynov Active Member

May 20, 2015
643
188
1. Where are you getting the value of R_channel from?

In datasheet it is given that a typical channel resistance at 10V gate voltage is equal to 2 ohms. I thought that the 5V voltage is about the same. I was wrong by 10%.It is yielding 2.2 ohms. I replaced the picture.

2. Which channel resistance are you taking about, large signal or small signal?

I use a large value for the signal (~ 0-300mV). Under these voltages resistance value of the channel for small and large signal are almost equal. At high voltages the dynamic resistance (dV/dI) begins to increase in comparison with static (V/I).

9. ### WBahn Moderator

Mar 31, 2012
17,788
4,807
So, as you can see, the channel resistance is a function of the gate voltage, so it makes no sense to use a fixed channel resistance as you change the gate voltage.

Your simulation is also not addressing the TS's question, which has to do with the channel resistance as you change the width of the transistor at a fixed gate voltage.