Hello friends, I hope you all are doing great. In today’s tutorial, we will have a look at Common-Drain FET Amplifiers. The common drain amplifier configuration is associated to the common collector amplifier configuration of BJT.  We can also say that the common amplifier which we discussed in the previous tutorial is the follower of common emitter amplifier configuration. Like this common drain is a follower of source amplifier that we discussed in the last tutorial.

In this configuration value of the voltage at source is similar to the input or gate voltage and has also same phase. In simple words source, voltage is a follower of input voltage at gate. In today’s post, we will have a detailed look at its circuit, working, and some related parameters. So let’s get started with Common-Drain FET Amplifiers.

Common-Drain FET Amplifiers

• The common drain junction field effect transistor amplifier is such an amplifier in which input is given to the gate and output is taken out from the source to make drain terminal as common for both gate and source.

• As drain is common so there is no need of drain resistance. In below figure you can see the common drain JFET amplifier configuration.
• The common drain configuration is also known as source follower.
• In this circuitry, self-biasing is used. The input voltage is given to the gate with the use of a bypass capacitor denoted as C1 and output is coupled with the load resistance through capacitor C2.

Common-Drain FET Voltage Gain

• Like other amplifier for this amplifier configuration, the voltage gain is Av=Vout/Vin.
• For source follower configuration  Vout is
  IdRs and Vin is Vgs +IdRs it shown in the below figure.

• So gate to source voltage gain will be IdRs/(Vgs +IdRs).
• Putting Id =gmVgs into above equation we have resultant expression as.

Av=gmVgsRs/(Vgs + gmVgsRs)

• So we have.

Av=gmRs/(1+gmRs)

• Note that gain value is always less than one. If gmRs>>1 the good approximation is Av=1.

Common-Drain FET Input Resistance

• Since input voltage is given to the gate the input resistance observed by the input signal source is very large like common source amplifier.
• The gate resistance RG is in parallel with the input resistance observing at the gate the total input resistance will be.

Rin=RGΙΙRIN(gate)

• Here RIN(gate) = VGS/IGSS.

So friends that is a detailed post about Common-Drain FET Amplifiers if you have any query ask in the comments. Thanks for reading. Have a good day.