Resistance /impedance looking into the source / drain ?

1. Resistance /impedance looking into the source / drain ?

1) Looking into the source or drain- is it impedance or resistance ? I think its resistance because there are no capacitors in effect ( mid band). Is this correct ?

2) when we calculate the resistance looking into the source or drain, we use a test voltage source . Is this test voltage dc or ac ?

3) when we use test voltage source , we ground the independent voltage source and open the current sources. Then how is the transistor in saturation region when we calculate the resistance?

Thank you so much

2. Re: Resistance /impedance looking into the source / drain ?

1, If you measure a simple resistor with an ohmmeter you will see its impedance, which has no reactance. Everything is impedance measure. When you measure source/drain impedance you should see the resistive part of the impedance, so basically it is resistance, but reactances always have some negligible effects, and you should never forget what you see is still impedance.
2, AC, small signal
3, independent sources not physically shorted or opened, just in the small signal equivalent model. These are used to simplify AC calculations like gain, but not real or viable models. Under real measuring test signal has to be AC coupled, which doesn't modify the operation region of the device.

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3. Re: Resistance /impedance looking into the source / drain ?

Hi,

1, If you measure a simple resistor with an ohmmeter you will see its impedance
Most Ohmmeters work with DC excitation. With those Ohmmeters you just can measure resistance, but not impedance (which is considered to be an AC parameter)

Klaus

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4. Re: Resistance /impedance looking into the source / drain ?

True, DC ohmmeters and multimeters have some considerations, non-linear device measure is not possible for example, but I have learnt that Z = R + j*X, and an ideal resistor has no X part, but it has got Z. That was my point.

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5. Re: Resistance /impedance looking into the source / drain ?

Hi,

but I have learnt that Z = R + j*X, and an ideal resistor has no X part, but it has got Z. That was my point.
Seen from the Ohm-meter:
* With DC excitation it will never know about X. Itīs not possible to measure X, thus it can only show R

Seen from the "ideal resistor".
True, an ideal resistor has no X thus Z = R.
But for a FETīs drain-source ... it is no ideal resistor ... there is a C. It is small, thus one may ignore it at low frequencies, but one need to consider it at high frequencies or high dV/dt.

Klaus

6. Re: Resistance /impedance looking into the source / drain ?

Originally Posted by frankrose
1, If you measure a simple resistor with an ohmmeter you will see its impedance, which has no reactance. Everything is impedance measure. When you measure source/drain impedance you should see the resistive part of the impedance, so basically it is resistance, but reactances always have some negligible effects, and you should never forget what you see is still impedance.
2, AC, small signal
3, independent sources not physically shorted or opened, just in the small signal equivalent model. These are used to simplify AC calculations like gain, but not real or viable models. Under real measuring test signal has to be AC coupled, which doesn't modify the operation region of the device.

Thank you so much .. a couple of more questions :
1) Since the capacitance effects are negligible in mid band, can we use dc test voltage source to get the resistance looking into the drain or source ?
2) why do we use time constants to get low frequency and high frequency response ?

7. Re: Resistance /impedance looking into the source / drain ?

2)Since the capacitance effects are negligible .. can we use the dc test voltage source to get the resistance ?

3) you mean that In spice when we want to get the resistance looking into the drain or source , we only ground the signal source not the dc sources ?

8. Re: Resistance /impedance looking into the source / drain ?

Wow, your questions fall apart. Please think about them before multiple post, otherwise you confuse us and yourself.
1, how do you want to AC couple a DC voltage? I have already answered this. AC, small signal is required by coupling.
2, what? For why? I don't know what do you want to ask.
2nd 2, Still no, we cannot use DC.
3, We not connect sources to ground anyhow, that hasn't got any useful result. Unfortunately I don't understand what you want with this either.

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9. Re: Resistance /impedance looking into the source / drain ?

Hi,

a agree with frankrose: it is getting confusing.

Please post a draft of the test circuit you have in mind, so we can discuss about it.

Klaus

10. Re: Resistance /impedance looking into the source / drain ?

Originally Posted by Robotduck
2)Since the capacitance effects are negligible .. can we use the dc test voltage source to get the resistance ?

3) you mean that In spice when we want to get the resistance looking into the drain or source , we only ground the signal source not the dc sources ?

No, you can not use a DC source to measure input/output impedances. These are ac (i.e. incremental), small signal parameters and thus measured with an ac source. Remember, these resistances/impedances are equivalent parameters, not necessarily physically present in the circuit. For example, the drain output resistance ro is not physically there in the transistor but it models and effect of its behavior, namely the slope of the Id-Vds characteristic in saturation.

When they say remove the independent sources while measuring impedances, they mean remove the incremental signal sources. DC biasing is not considered a signal. For example, Vdd is part of the circuit bias but in ac small signal analysis we consider it ac ground.

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