# Why inductor is placed like this?

Discussion in 'Homework Help' started by screen1988, Mar 21, 2013.

1. ### screen1988 Thread Starter Member

Mar 7, 2013
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I don't understand why the inductor is placed in the gate of NMOS. Could anyone explain the reason for that?

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2. ### WBahn Moderator

Mar 31, 2012
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What if the inductor weren't there?

What would the voltage on the gate be in response to an AC input signal?

3. ### screen1988 Thread Starter Member

Mar 7, 2013
310
3
With AC signal, inductor behaves like a resistor, I only know this and get stuck here

4. ### WBahn Moderator

Mar 31, 2012
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My questions asked what if the inductor weren't there. If it's not there then it doesn't matter how it behaves.

5. ### screen1988 Thread Starter Member

Mar 7, 2013
310
3
Oh, sorry,I didn't notice. I don't see any problem if the inductor weren't there. VGS will bias for the transistor. Can you give some hints?

6. ### WBahn Moderator

Mar 31, 2012
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The second question IS the hint.

7. ### screen1988 Thread Starter Member

Mar 7, 2013
310
3
With AC signal applied, then there is a fraction of input voltage drop in inductor.
$V_{in}= V_{GS} + V_{L}$
Do you mean that it used in AC signal to put AC signal to gate of transistor?

8. ### WBahn Moderator

Mar 31, 2012
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You've basically got it. Without the inductor, the gate voltage is simply Vgs and the input signal can't affect it. Whatever current is pushes/pulls through the capacitor goes straight to the Vgs source. The inductor let's the high frequency signals from the input get to the gate while letting the low frequency signals from the bias source get their too.

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9. ### screen1988 Thread Starter Member

Mar 7, 2013
310
3
Without the inductor, both Vin and Vgs are connected to the gate with different voltage. Then how do you know the gate voltage is Vgs not Vin?
And is there a short circuit that happens in this case?

10. ### t_n_k AAC Fanatic!

Mar 6, 2009
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What do you mean by short circuit?

11. ### Sue_AF6LJ Member

Mar 16, 2013
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The AC would be shorted to ground in the bias source.
The inductor, or choke provides a high impedance to the AC source as well as providing a low resistance path for the gate bias voltage.

Without it there would be no AC signal on the gate.

12. ### screen1988 Thread Starter Member

Mar 7, 2013
310
3
I meant as the picture bellow. Vin and Vgs are connected to the gate and they have the different voltage.
I mean what happen when a AC source and a DC source are connected in parallel without any resistor.

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13. ### screen1988 Thread Starter Member

Mar 7, 2013
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Can you explain why thre is no AC signal on the gate without the inductor?

14. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I was actually hoping the OP would clarify this statement. Clearly you know what the answer is but the OP clearly doesn't.

15. ### Sue_AF6LJ Member

Mar 16, 2013
45
27
Yes I can...
There is no AC signal because the gate bias supply will shunt the AC to ground unless the bias is decoupled by some means a choke is not the only way to accomplish this you could also use a high value resistor. with the end connected to the gate bias source bypassed with a cap...

Sorry I wasn't up here earlier...

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16. ### screen1988 Thread Starter Member

Mar 7, 2013
310
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Thanks Sue.
I really feel it is hard to get it right. How DC bias cancel AC signal? Could you explain a little more or give me a link to it?

17. ### WBahn Moderator

Mar 31, 2012
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There are a number of ways of answer this question.

A1) You don't know the gate voltage precisely because you have two sources tied together, and hence all bets are off and you can neither analyze the circuit nor predict its behavior.

A2) There really isn't a short -- remember the input capacitor. So you have a source tying the gate to Vgs and the input signal just charges and discharges the capacitor.

A3) The gate voltage will vary to some degree with the signal since the Vgs source has some output impedance associatated with it. In the case of a gate bias network, the impedance might be considerable. But it is also unknown. Similarly, the signal source has an output impedance -- possibly also considerable and definitely also unknown. So while the circuit may well work reasonably okay, we still can neither analyze it nor predict it's behavior.

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18. ### WBahn Moderator

Mar 31, 2012
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At the end of the day a voltage source is a voltage source and at any moment in time the outputs combine via the properties of the circuit network connecting them to produce the voltage at any paritcular point in the circuit.

Mar 7, 2013
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