# Finding the resistance between the drain and the source - transistors

Discussion in 'Homework Help' started by trappy, Mar 27, 2008.

1. ### trappy Thread Starter New Member

Mar 27, 2008
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0
Hi guys ,

My first post so please go easy - what a good site !

Right i have a question im struggling with .

I have a transistor (type not known) with a resistor conected to the emitter,
the base of the transistor is connected to the source which is 5 volts and the collector is connected to ground.

im told Vt is 1 volt and K = 0.5 mAVsquared Vd is 0.1volt

im asked to calculate the effective resistance between the drain and the source?

what does this mean ? are they asking for the current through the resistor ?

Any formulaes / solutions would be greatly appreciated.

thanks in advance

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
172
Source and drain implies FET; collector, emitter and base implies BJT. Which one are we looking at here?

Do you have a circuit diagram? That might help clear things up.

Dave

Mar 27, 2008
7
0
4. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
That's a misdrawn FET.

With 4.9 volts dropped across the resistor and .1 volt across the FET, the FET resistance is 49x that of the resistor.

5. ### trappy Thread Starter New Member

Mar 27, 2008
7
0
thats the only diagram given in the question i have , hence my confusion !?

6. ### veritas Active Member

Feb 7, 2008
167
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Why do you think that is a misdrawn FET, beenthere? I've seen that symbol used for it dozens of times in my classes.

(sorry to post somewhat off-topic)

7. ### veritas Active Member

Feb 7, 2008
167
0
to clarify for trappy, since you have Vd, you can solve this just as though it were a voltage divider with two resistors => Vd = Vcc * Re / (Re + Rd), assuming that Re is the effective resistance of transistor from source to drain.

Apr 26, 2005
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9. ### Dave Retired Moderator

Nov 17, 2003
6,960
172
The nMOSFET operating in the triode region (which it it in this example) the resistance is given by:

$r_{ds=$$\left[k(W/L)(V_{gs}-V_{t})\right]$

This would calculate the effective resistance as a function of Vgs (which is the controlling parameter in the triode region. Given the information you have (you are not given the transistor dimensions although you can calculate it), the voltage divider explanation given above is appropriate.

Dave

10. ### Dave Retired Moderator

Nov 17, 2003
6,960
172
Incidentally, the representation of the transistor in the diagram is a valid symbol for a nMOSFET, where the arrow denotes the source. Switch the arrow direction and you have a pMOSFET. It is a more common representation here in UK.

Dave

11. ### trappy Thread Starter New Member

Mar 27, 2008
7
0
guys you have been a BIG help to me , thankyou for your time