What does it mean by a small-signal resistor? What is meant by Diode-Connected load in MOSFET? Does it mean that MOSFET act like a diode connected with a load? Then why it is called as small signal resistor?
2 Answers
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MOSFET is a unidirectional device that varies its output drain current as a function of the input gate-source voltage, thus maintaining a relatively constant current. Jokingly, this is a “transistor-connected transistor".
If we connect the drain of the transistor to its gate, we force it, through the negative feedback mechanism, to vary its “output” gate-source voltage as a function of its “input” drain current, thus maintaining a relatively constant voltage like a diode (i.e., we reverse it). Figuratively, we can call this a “diode-connected transistor” due to the similarity of its IV curve to that of a diode.
Therefore, the similarity of this transistor device to a diode is limited to their IV characteristics.
answered Sep 15 at 17:47
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1\$\begingroup\$ @Kelvin, I meant "nearly constant", "roughly constant", "approximately constant"... \$\endgroup\$ Commented Sep 16 at 18:59
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\$\begingroup\$ Why do they say this diode-connected load act as a square root operator? \$\endgroup\$– KelvinCommented Sep 21 at 3:46
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1\$\begingroup\$ @Kelvin, they say that in the forward conduction region, the voltage-current relationship is quadratic. I can't explain exactly why that is, as I'm not an expert in MOSFET technology. \$\endgroup\$ Commented Sep 21 at 6:27
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simulate this circuit – Schematic created using CircuitLab
A MOSFET whose gate is connected to it's drain is called diode-connected.
Edit: The VI characteristics of a diode connected MOSFET is kind of similar (not exactly same but similar) to a diode. That is why the name. When you pump some forward current into a diode, the voltage will be loosely speaking clamped to VF. Similarly, when you pump some current into the diode connected MOSFET, the voltage will be kind of clamped to VTH of the MOSFET.
Small signal resistance is defined as dV/dI where V is the voltage between terminal 1 and terminal 2. I is the current flowing from terminal 1 to 2. This component is not really a resistor but, around the bias point, we can calculate the small signal resistance dV/dI. It is an approximation done to help us calculate small signal gain of say an amplifier where such a diode connected load is used. In the diagram below, M3, M4 are the diode connected load for the differential pair M1-M2. Now, because we know the small signal resistance of the diode connected load is 1/gmn, it becomes easy to calculate the small signal gain as gmp/gmn. So, this is the approximation.
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\$\begingroup\$ Then what does the phase diode-connected load means? As it will always be in saturation, giving contstant current, as V>Vth, like a diode's threshold voltage, hence being similar to a diode. Having a small resistance, being similar to a load. So is it named as diode-connected load? \$\endgroup\$– KelvinCommented Sep 15 at 9:10
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1\$\begingroup\$ Yes. The VI characteristics of a diode connected MOSFET is kind of similar (not exactly same but similar) to a diode. That is why the name. When you pump some forward current into a diode, the voltage will be loosely speaking clamped to VF. Similarly, when you pump some current into the diode connected MOSFET, the voltage will be kind of clamped to VTH of the MOSFET. \$\endgroup\$– saiCommented Sep 15 at 9:19
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\$\begingroup\$ I don't think a circuit with two diode-connected transistors wired up this way makes a whole lot of sense. For a current mirror, you usually only need one transistor in diode configuration. \$\endgroup\$ Commented Sep 15 at 14:27
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1\$\begingroup\$ @Circuitfantasist, in a 2 stage opamp, this is very often used as the first stage. I did not give the current mirror example because the small signal resistance of the diode is not too critical there but, here it decides the gain \$\endgroup\$– saiCommented Sep 15 at 14:52
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1\$\begingroup\$ Yes indeed. The diodes plus the second stage input pairs are 2 current mirrors. \$\endgroup\$– saiCommented Sep 16 at 1:40