She knows Conservation of Angular momentum .

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Common source Amplifier with source degeneration (i.e with Rs)

Hello all, this is the common source amplifier with source degeneration . As you can see the circuit has a source resistance. This resistor Rs has a big role to play.
The Rs decides the gain of the amplifier,stability etc. This is the most stable circuit than the common source amplifier.

Let's see the Gain of this amplifier.

The small signal model of common source amplifier with source degeneration is as shown above

Gain=Vout/vin

where

      vin=v1+ (gm*v1*Rs)

    vout=-(gm*v1*RD)

The gain of the amplifier is

The value of gmRs>>1 . So,the gain of the amplifier becomes

Vout/vin=-(RD/Rs)

So if RD=8k and Rs=2k

The gain of the common source amplifier will be 4.

Note: The gain of this amplifier depends totally on RD and Rs values. As long as they are constant ,the amplifier gain doesn't get affected. The amplifier is very stable and doesn't change its gain depending on the temperature as it's independent of gm. Thus Source degeneration influences the gain,stability etc.

For any Queries :Contact Hobby Projects.  

CMOS Amplifiers-Common Source Amplifier

Hello all! We are starting a new series on CMOS Amplifiers.
I am Sure you all will have a good time reading these posts.


Q) First let us understand WHAT IS AN AMPLIFIER?

Ans:An amplifier is an active device which gives  POWER GAIN.
where Power=voltage*current.
  Suppose you have 0.2 v sine wave as input and amplifier  gain is 10. Your amplifier output will be 0.2*10=2v(sine wave).

Note:LAW OF CONSERVATION OF ENERGY is not violated ,because the amplifier is an ACTIVE DEVICE. The active device doesn't create nor destroy the energy.The power gain is achieved by using the raw DC power supply(Which is usually a power supply to the amplifier). So, the amplifier being the active device uses the dc power supply to increase the power of the input signal. 


 summary

• An active device is one which converts the dc power into ac power. 
• Amplifier is an active device. It needs a power supply(DC) to amplify the input signals. It cannot amplify more than the DC supply voltage level.
• Amplifier amplifies small signal voltage levels(low power) to high power signals(Without modifying the signal) 

Thought for a day
 Is transformer an amplifier?? 
Yes 
No

Having learnt what is an amplifier.Let's go ahead with CMOS Amplifiers. 

CS Amplifier(Common emitter version of MOS)

Power gain is achieved in the above circuit with the help of DC supply(VDD). The amplification of small signals is facilitated by the amplifier(which uses VDD). 

The question is By what amount the the signals are amplified?
And: That is determined by the gm and RD.where gm=trans conductance of the amplifier
gm=iout/vin
Iout=Id(In MOS)
Vin=vgs(In MOS)
RD=Drain resistance.

In order to calculate the gain,the small signal model is used. The small signal model is as shown in the above picture.
There are some rules to be followed while drawing the small signal model(prerequisite){You can comment here if you want to know about small signal model}

So after the small signal model is done.

vout= -(gm*v1*RD)
Gain=vout/vin=-gm*RD. 

By this amount, the input signal gets amplified. Make sure that M1 is in saturation region.Triode has a very low gain!
i.e VDS>=VGS-VTH.

In the next part: we show the pros and cons of increasing RD to increase the gain(Does increasing RD i really increases the gain of the amplifier?)