Ibias representation in a circuit.

[yefj]

Hello, Why do we need this Ibias.i actual real life its not a source but just another transistor.
The only source we have is the Vdc source.
So why we need this NMOS at the buttom be in a role of Idc?
Thanks.

 

[srizbf]

Ibias is CCS for the long tailed pair.

 

[yefj]

But its not a source its just a transistor in fact in reality

 

[BradtheRad]

But its not a source its just a transistor in fact in reality

True, the current source is a handy tool in simulation, to apply the right voltage in order to achieve a desired current level.
Automatically the transistors receive the correct bias current, which means the right bias voltage.
Automatically the bottom supply rail is adjusted to the correct voltage.

In hardware there is the classic current limiter which is made from a PNP and NPN. It's sort of like a current source.

I've played with many simulations of a long-tail pair. It's possible to omit the current source. Put a resistor at the tail, or transistor with adjustable bias.

As for the bottom supply rail:

* It can be 0V ground, but then I have to finagle with the bias network to make the transistors conduct. This involves applying various levels of voltage & current & signal.

* Or make the bottom supply rail negative, adjustable. Then the bias can be 0V. The input signal can be symmetrical AC, or miniscule DC.

Either way, it requires a lot of experimentation when building a real circuit.

 

[sutapanaki]

But its not a source its just a transistor in fact in reality

It is a current source although it is practically realized by transistors. BTW, the voltage sources are also most of the time made with transistors, if they are not just a battery. So, yes, the current source is a transistor that behaves as a current source. We use it in the diff pair to bias the transistors. For example, what we care for in a diff pair amplifier is mostly gm and gm depends on the dc bias current. The use of the tail current source keeps the current in the diff pair transistors independent of the input common-mode voltage, of course within some rather broad range. If we didn't have that current source and connected the sources of the input transistors directly to gnd, then the current through them will be a function of the common-mode voltage and so will be the gm. That's not what we usually want.

 

[yefj]

Hello, How adding tail transistor removes this current dependancy on Vout?
current is volatage divided by the sum of the Rds of the transsistor.

It is a current source although it is practically realized by transistors. BTW, the voltage sources are also most of the time made with transistors, if they are not just a battery. So, yes, the current source is a transistor that behaves as a current source. We use it in the diff pair to bias the transistors. For example, what we care for in a diff pair amplifier is mostly gm and gm depends on the dc bias current. The use of the tail current source keeps the current in the diff pair transistors independent of the input common-mode voltage, of course within some rather broad range. If we didn't have that current source and connected the sources of the input transistors directly to gnd, then the current through them will be a function of the common-mode voltage and so will be the gm. That's not what we usually want.

 

[sutapanaki]

Transistor behaves as a resistor only in triode region. In saturation, which is the intended region of operation in the diff amp, transistors are much more than a resistor. They are current sources with some rather large output resistance. If you imagine the Id-Vds characteristic for a transistor, it has some slope when transistor is in saturation region. If you change Vds, you get some change in Id, but since the characteristic is pretty flat in that region, the variation is small. This is what we model by gds or ro in the small signal model of the transistors. Bottom line, changing the drain voltage does change a bit the drain current, but not as much as if the transistor were a resistor only.

If you are talking about the tail current dependency on Vout then it is similar to what a cascode does in that respect. Suppose you have the two input transistor gates connected to the common mode voltage and you move Vout up or down. In this situation you have your tail transistor (which in the figure represented as ideal current source) and on top of it the right hand side input transistor. This is a cascode configuration and has very high output resistance. Moving Vout divided by that high resistance results in negligible current variation. From another point of view, the input transistors keep the tail node voltage more or less constant, that is they shield it from the variation of Vout, so the tail current source doesn't see all that Vout change. And if the tail current source generates pretty constant current, that current reaches the output and it follows that the variation in Vout doesn't affect the current that much. Pay attention that we talk here about common-mode and biasing, not signal.

 

[BradtheRad]

Long-tail pair with resistor tail.

Notice input signals are AC centered around 0V. Their low amplitude is not sufficient to bias transistors On.
Transistors are turned on by applying negative voltage. This setup isn't all that different from transistor with fixed bias.
It's a differential amplifier whose outputs are sine PWM, 5V amplitude referenced to 0V ground.