What is the difference between the concept/meaning of output resistance, ro, and finite output resistance Ro in context of a circuit with MOSFETS (ie. a MOS current mirror)?
thanks
mosfetresistancetransistors
What is the difference between the concept/meaning of output resistance, ro, and finite output resistance Ro in context of a circuit with MOSFETS (ie. a MOS current mirror)?
thanks
For the short answer:
The rules of the FET operation override his circuit figure when applicable. Perform circuit analysis, but use his rule for \$ i_{gate} = 0 \$ and his rule \$ i_{d} = V_{gs}*g_m \$. When he said "looking into the gate" he is taking the perspective of the current at the gate. When he said "looking into the source" he is taking the perspective of the current at the source. He must specify because the currents are different at each terminal even though they share a common voltage between them based on the FET rules.
For the long explained answer:
The author is referring to the concepts of thevenin or, equivalently, norton theorems and how they apply dependently on which node you look at. This dependence is based on a set of rules the author uses for describing a FET. Keep in mind impedance is a complex resistance that can be purely resistive or frequency dependent.
See wikipedia's articles (he also explains it in a earlier Sedra and Smith chapter): http://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem
To add some context to the discussion, we cannot create a FET out of just normal linear elements like resistors, capacitors, and independent sources. However, we can create a model that "acts" like a FET (in a small linear operating region) by adding a dependent current source, and making this source dependent according to the rules of a FET. These rules simplify how a FET really works, but they let us approximate its behavior with normal circuit elements. Sometimes the rules are assumed or taken for granted, and the author has done this somewhat in that figure by using the rules to override our intuition of how the circuit drawn works. In a sense, the resistor he shows is an illusion that results from the FET rules. You will see later in the chapter that he draws a more intuitive version of this circuit where the gate is floating to imply 0 current in the gate. Here, he simply uses an algebraic rule to do the same thing.
In your first figure, the gate terminal is connected directly to the resistor with a value of "1/gm". Intuition would say if any voltage were applied between gate and source, a current will flow through this resistor, and that current should obey KCL with superposition, such that current from Vgs in one node should be equal to current out of its other node. You might then think intuitively that the impedance from gate to source looks the same as from source to gate, its just that resistance between them. However, one of the rules he drew is that gate current = 0, and so you must always follow this rule when it is given since it is a rule that models FETs, even if it is not intuitive to the circuit drawing. To understand why, you need to study the physical design of a FET and the author only assumes you've accepted this rule. Keep these "model rules" aspect in mind.
Let's now go back to the ideas of thevenin theorem and "looking into" circuits. Just like any circuit, we can use ohm's law to describe how it behaves or responds for these FET models. When a known voltage is applied to 2 nodes in any circuit, an amount of resulting current will flow between those 2 nodes through its impedance. Equivalently, a resulting voltage will be impressed on those nodes from a known current flowing through its impedance. We don't really care what kind of circuit is behind these 2 nodes actually because it can all be described by its impedance we "see" without knowing what's inside.
The reason he has to specify what part of the FET he's looking into is because, depending on what terminal of a FET you look at, it will behave to the only "FET rules" that apply to that terminal and do not necessarily apply to the others.
When he says "look in", he really means we are applying an input signal (either a known voltage or known current), and seeing either how much current flows or how much voltage is impressed as a result based on the rules for that terminal. When we say the "impedance seen from", we usually mean the output impedance, or that we are looking at an output signal and seeing how much current is flowing out of it at a known voltage output.
For example, take his second claim "that the resistance between the gate and source looking into the gate is infinite". If we use thevenins theorem and apply any input voltage from gate to source, and then use Ohm's law, we can see what he means:
\$ R_{input} = \dfrac{V_{input}}{I{input}} \$ Where Vgs = Vinput, and Igate = Iinput.
But his rule for FET gates overrides that Igate = 0, and so R will be infinite for any voltage applied from gate to source - no current will flow!
It is tricky because even though there is a voltage applied and no current is flowing between gate and source, a current from the drain could still be flowing into the node where all 3 current paths meet, because the drain's own rule with the dependent current source says a current flows through it. Since Igate = 0, any of the drain current goes through the node and all of it exits out of the source terminal (from KCL). Since that current doesn't flow in the gate, its not a part of "looking into the gate".
Now we can take his first claim "that the resistance between the gate and source looking into the source is 1/gm". As was just mentioned, even though there is no current from gate to source (infinite impedance), a current can still flow in the source because the drain's dependent current source is always equal to the voltage between the gate and source times its transconductance gain factor gm:
\$ i_d = V_{gs}*g_m = i_s\$
So now, we must again use an ohm's law equation to determine the equivalent impedance "looking into" the source to gate.
First we apply the voltage across the 2 terminals we wish to find the impedance of. Again it is Vgs. This time, however, since we are looking into the source, the current is not 0, and so we already can see there will be a difference in impedance compared to looking in the gate before.
Since the drain current is dependent on this applied Vgs voltage, the current from the drain is going to be:
\$ V_{gs}*g_m \$
Now again, using KCL at the junction node, all of this current from the drain must go through the source since igate = 0. We already know enough now to find the impedance.
If, the voltage applied is Vgs, and the current we see at the source is Vgs*gm, then:
\$ R = \dfrac{V}{I} => R = \dfrac{V_{gs}}{V_{gs}g_m} = \dfrac{1}{g_m}\$
So, it is actually an algebraic coincidence that the R = 1/gm, even though it is drawn in his figure like a real resistor between the gate and source. This is not a real resistor, just a circuit model with enough rules to make it act like a FET instead!
By doing this, we can gain insight to how the FET model is working and how it simulates a real FET in saturation mode. Any voltage applied to Vgs will not draw a current from Vg to Vs, but it will force a current through the drain to source by the FET rules, and this current is proportional to the amount of voltage we have at Vgs.
If we make the transconductance gain factor gm very large, then we only need a tiny amount of voltage at Vgs to create a large current through the drain to source, and as a result the resistance 1/gm will approach 0 so that it looks like there is no impedance from gate to source (only when looking from the source's point of view because of the rules!). This shows how a FET in saturation is much like a voltage controlled current source.
When you want to measure the output resistance, you replace the \$i_{in}\$ current source on the left with an open circuit, and you put a \$v_s\$ signal generator at the output and measure the \$i_{s}\$ that is drawn out of it. If you do this, the gates of \$M_2\$ and \$M_4\$ do not move and are small signal grounds. Since the source of \$M_2\$ is also a signal ground, \$v_{gs2}\$ (small signal) is always zero.
This means that \$M_2\$ behaves like a resistor of value \$r_{ds2}\$. Let's call this \$R\$.
So you have a simple transistor with its source connected to ground through a resistance and its gate at signal ground, and you want the output resistance.
\$i_s\$ come in from the output and will be flowing down \$R\$, thus the source of M4 will be at a voltage of $$ v_{s4} = i_s R $$ This means that $$v_{gs4} = 0 - v_{s4} = - i_s R$$ and thus that you have a downwards current in the transistor of $$i_{d4} = g_{m4} v_{gs4} = - g_{m4} i_s R$$ You also have a current in \$r_{ds4}\$, which in the downward direction is $$i_{r_{ds4}}=(v_s - v_{s4})/r_{ds4}$$ These two currents, summed, are equal to \$i_s\$. $$i_s = - g_{m4} i_s R + v_s/r_{ds4} - i_s R/r_{ds4} $$
$$i_s (1 + g_{m4} R + R/r_{ds4}) = v_s 1/r_{ds4}$$
$$R_{out} = v_s/i_s = r_{ds4} + g_{m4} R r_{ds4} + R = r_{ds4} + g_{m4} r_{ds2} r_{ds4} + r_{ds2} \approx g_{m4} r_{ds2} r_{ds4}$$
With your data I'm getting \$43M\Omega\$ with the exact formula.
I think what you did wrong is to think in the wrong way about what is an input. The input is the generator of the current that gets mirrored, but instead you shorted the gates and sources of all transistors, despite there not being any input voltage source linking them, and thus having no reason to do so.
Best Answer
Your question mixes several concepts and needs to be clearer to be sure of being answered well. But:
I will comment on current sources, then current mirrors, then MOS devices.
An ideal current source has an infinite output impedance. This means that the current "just flows" regardless of how large or small the load resistance is and the voltage adjusts accordingly. For example, if you had an ideal 3 amp current source, then if you loaded it with 10 ohms the output voltage would be V = IR = 3A x 10 = 30V. If you loaded it with 1000 ohms the output voltage would be V = IR = 3 x 1000 = 3000 volts. If you loaded it with nothing but a voltmeter with 1 million ohms input resistance then the current source's output voltage would be 3A x 1000000 ohms = 3,000,000 Volts. You probably wouldn't want to do that :-).
In practice the ability of a current source to drice an arbitrarily high resistance is limited by the available voltage (as least), so it can be nearly ideal only for a limited range of load resistances.
Even within its avaiable voltage range a current source will not produce a perfectly steady constant current. eg a 0.100A ccs may produce 1 Volt across 10 ohms (I = V/R = 1.000 / 10.000 = 100.00 mA, but produce 2.002 Volt across 2.000 ohms. In the latter case the voltage is higher than it should be so the current is higher than it should be so the current source has a large but finite (ie non infinite) resistance. As the current has DECREASED as load INCREASES the dynamic resistance is positive but effectively has a negative slope (which just means it gets smaller as load resistance gets bigger).
A current mirror is essentially a current source which is driven by a current input.
eg 1 mA into one "leg" of a current mirror should produce 1 mA in the other "leg" (or some contant vale of this). BUT in practice the current mirror will not be perfect and eg 1mA in may produce 1.002 mA or 0.978 mA or some other value in the other "leg". This will be the equivalent of a non ideal current source in which there is some apparent and non-infinite output resistance.
MOS = "Metal Oxide Semiconductor" is a form of semiconductor manufacturing technology. CMOS ICs use it and MOSFETs. Current sources and current mirrors may be made using it BUT the two are independent.
A vast array of MOS current mirror circuits and some other stuff can be found using this Google trick. Look at the page heading and general environment. All images are hot linked to web pages.
Wikipedia on Current Mirros
Wikipedia on active loads - relevant to my comments - read and understand.
Photo below - Simple MOSFET based current mirror from here - worth reading.
Look familiar?
From here - good
And
Language usage : i.e. and e.g.
The following is not directly electronics but assist in asking and answering good questions.
Note that you say "ie a MOS current mirror".
"ie" means "that is" = specifically and only.
That may be what you were interested in but usually "eg" is used in this context which meams "one example is" or "here is an example of what I mean".