Transistor Used as a Resistor

Thread Starter

Faisal Ikhlaq

Joined Sep 13, 2013
4
Dear All,

I have studied in Circuit and Systems Design course that transistor can be used as a resistor in IC technology because transistor has high resistance and it takes less space on chip than resistor.

Now i want to know how and why transistor has high resistance so that for same resistance value we prefer transistor in IC technology rather than normal resistor????
 

edmundopt

Joined May 4, 2011
60
Can you be more specific ? Because if you check the datasheet of an IC, you still can see resistors in the internal schematic.
Maybe you can post a IC schematic where that specific transistor is used as a resistor
 

studiot

Joined Nov 9, 2007
4,998
The name transistor is a contraction of "transfer resistor".

Of course the transistor can be used as a resistor that is exactly what it is - a controllable resistance.

The resistance is between the collector and emitter - we control it by applying suitable signals to the base.

However there is no requirement for that resistance to be high or low, it is up to us to set its value. And yess transistors can be cheaper to fabricate on chip than resistors.

One question for you

The transistor has two regions of operation the active region and the saturated region, which do you think applies in this case?
 

edmundopt

Joined May 4, 2011
60
Did not know that a fixed value resistor(2pins) is more expensive to produce than a transistor(3 pin) inside a IC, learning every day! I still see a lot of resistors inside IC.
 

MrChips

Joined Oct 2, 2009
23,849
You don't see resistors in an IC. What you are looking at is the circuit schematic.
The physical resistor structure is fabricated using the similar structure that is used to fabricate transistors. The resistor is not constructed from a 3-pin transistor. It is made from the conduction channel similar to that of a FET.
 

studiot

Joined Nov 9, 2007
4,998
Actually fabricating very high value resistors on chip is difficult. It is even difficult in discrete components.

A transistor can be connected in a mode known as an active load. This appears to other parts of the circuitry as a very high impedance.

Faisal, did you think aboput my question?

Here is another.

Why whould we want a very high impedance load?
 

Thread Starter

Faisal Ikhlaq

Joined Sep 13, 2013
4
We want high load so that maximum voltage drop should be across the load.

Actually question was related to Differential MOS pair where we prefer to use current source for biasing of the amplifier other than resistor on the Emitter.

So there was written that Current Source has high resistance as compared to resistor. So that's why i asked about transistor because current source is made up of transistor.
 

MrChips

Joined Oct 2, 2009
23,849
Hah! Now we are in a completely different universe.

You are not talking about IC transistor fabrication.

You are talking about replacing a resistor current source with an active current source.
Why did you not say that in the first place?

In a differential long tail amplifier, there is a current source on the tails of the amplifier. A single resistor is a cheap way of creating a current source.

You can create an active current source using a transistor. An ideal constant current source has infinite impedance.

This has nothing to do with replacing a resistor with a transistor.
 

Thread Starter

Faisal Ikhlaq

Joined Sep 13, 2013
4
Thanks for explaining.

Actually this is my actual question that why an ideal constant current source has infinite impedance.

What is the basic concept and reason behind that.
 

MrChips

Joined Oct 2, 2009
23,849
Firstly, think about a non-ideal voltage source connected to an unknown load of resistance R.
A non-ideal voltage source has internal resistance Rs.

What is the voltage across the load R if the voltage source is supplying a voltage of Vs?

What do you need Rs to be for an ideal voltage source?

Now do the same analysis for a non-ideal current source.
 

Thread Starter

Faisal Ikhlaq

Joined Sep 13, 2013
4
Firstly, think about a non-ideal voltage source connected to an unknown load of resistance R.
A non-ideal voltage source has internal resistance Rs.

What is the voltage across the load R if the voltage source is supplying a voltage of Vs?

What do you need Rs to be for an ideal voltage source?

Now do the same analysis for a non-ideal current source.
Thanks alot.... :)
 

PaulEngineer

Joined Dec 21, 2016
172
Can you be more specific ? Because if you check the datasheet of an IC, you still can see resistors in the internal schematic.
Maybe you can post a IC schematic where that specific transistor is used as a resistor
Dear

These datasheets hold an internal circuit diagram to demonstrate the working principle of that specific IC if it was made with ordinary components! In fact inside an ic there is only transistors! Nothing else! And maybe diodes too, although i doubt for that! You see, in order to make a 1M resistor with a size of, say 14 nm, is very difficult! And also expensive! There is no material found yet that can provide such a high resistance over that small size. Secondly, even if such technology existed, it is pointless to add a resistor in the IC, and that is because of a very high voltage drop, high current consumption, and in many cases... Overheating! You wouldn't place a 1Ohm of resistor directly to the voltage because either the resistor would burn, or it would face an overheating issue! So you understand that transistors is more useful at this applications, because even if it will work at saturation, which means very low impedance, it doesn't burn, due to the fact that transistor are nonlinear devices! Hope that helps
 
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