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[SOLVED] Reason for Active loads in ICs?

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iVenky

iVenky

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I am just wondering why we use active loads in ICs instead of resistors.

Is that because the size of the transistors is less than that of the resistors?


Thanks in advance.
 

Hey Gentleman!
Hi
consider a simple ce amplifier. its AV is :( RC in parallel with Rl)/(RE+re)
if you want to obtain high gains , you should increase the RC . and if you increase the RC(and RE) , the IC will decrease, and the re will increase and the AV will not be high. ok you can by pass the RE but re is high , (yet). you should use a high resistance like the resistance of current source and then the current can be high , but RC can be about infinite.
ok the out put impedance of transistor called 1/hoe =ro =Va/Ic . we can select the transistors with high value of va to obtain large ro . then we can obtain high value of gains.
Best Wishes
Goldsmith
 
Last edited:

And see below circuits that i designed them , please:
active.JPG
active 2.JPG
Best wishes
Goldsmith
 

I am just wondering why we use active loads in ICs instead of resistors.
Click to expand...
Review an analog IC design textbook of your choice, e.g. Razavi.

You'll notice, that the basic amplifier configuration is introduced with resistive load. At some point, active loads are added. There should be an explanation, why.
Razavi tells in 3.2.2:
In many CMOS technologies, it is difficult to fabricate resistors with tightly-controlled values or reasonable physical size. Consequently, it is desirable to replace Rd ... with a MOS transistor.
Click to expand...
Similar considerations apply to bipolar ICs. You'll find however resistive loads in a number of classical OPs, particularly high frequency designs.
 

FvM said:
Hey Gentleman!
Hi
consider a simple ce amplifier. its AV is :( RC in parallel with Rl)/(RE+re)
if you want to obtain high gains , you should increase the RC . and if you increase the RC(and RE) , the IC will decrease, and the re will increase and the AV will not be high. ok you can by pass the RE but re is high , (yet). you should use a high resistance like the resistance of current source and then the current can be high , but RC can be about infinite.
ok the out put impedance of transistor called 1/hoe =ro =Va/Ic . we can select the transistors with high value of va to obtain large ro . then we can obtain high value of gains.
Best Wishes
Goldsmith
Click to expand...
You said if I increase Rc then re increases but doesn't that happen when I have a active load?

---------- Post added at 17:51 ---------- Previous post was at 17:50 ----------
FvM said:
Review an analog IC design textbook of your choice, e.g. Razavi.

You'll notice, that the basic amplifier configuration is introduced with resistive load. At some point, active loads are added. There should be an explanation, why.
Razavi tells in 3.2.2:

Similar considerations apply to bipolar ICs. You'll find however resistive loads in a number of classical OPs, particularly high frequency designs.
Click to expand...

As I said before. The main reason is because of the size. I read the same in Adel Sedra book too.

---------- Post added at 18:30 ---------- Previous post was at 17:51 ----------
Razavi tells in 3.2.2:
In many CMOS technologies, it is difficult to fabricate resistors with tightly-controlled values or reasonable physical size. Consequently, it is desirable to replace Rd ... with a MOS transistor.
Click to expand...

Wow. Even we use Razavi. Looks like it's the best book for Analog throughout the world. Anyway thanks.
 
Last edited:

Dear iVenky
Again Hi
"You said if I increase Rc then re increases but doesn't that happen when I have a active load?"
Yes . if you use the current source , the current will be constant . and its out put resistance will be high . and its impedance will reflect to the collector of the transistor of your CE amplifier . thus the current is constant (approx) and the collector resistance is very high . and thus the gain could considered infinite . remember that re=vt/ic(or IE) and vt is:temperature (in kelvin )/ 11600 . and thus vt at 27 degree is about 26 mv.
Best Lucks
Goldsmith
 
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    iVenky

    iVenky

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goldsmith said:
Dear iVenky
Again Hi
"You said if I increase Rc then re increases but doesn't that happen when I have a active load?"
Yes . if you use the current source , the current will be constant . and its out put resistance will be high . and its impedance will reflect to the collector of the transistor of your CE amplifier . thus the current is constant (approx) and the collector resistance is very high . and thus the gain could considered infinite . remember that re=vt/ic(or IE) and vt is:temperature (in kelvin )/ 11600 . and thus vt at 27 degree is about 26 mv.
Best Lucks
Goldsmith
Click to expand...

Thanks man. You have a great deal of knowledge in analog.

---------- Post added at 23:03 ---------- Previous post was at 23:00 ----------

So the main use in using a current source is because it gives you constant current so that re always remains constant (in addition it has high output impedence). Whereas in a normal resistor when you increase the resistance Rc the current through it decreases i.e. ic decreases and hence re increases and gain don't increase that much. Am I right?
 

Dear iVenky
Hi
Yes . and it means that with current source we can increase equal resistance of collector (as RC ) without decreasing the collector current.
Best Wishes
Goldsmith
 

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