Analysis of Common-Drain (CD) or Source Follower Amplifier

Analysis of Common-Drain (CD) or Source Follower Amplifier

• Fig. 7.5.1 shows the common-drain amplifier circuit. It is also known as grounded drain amplifier.

• In this amplifier circuit, drain is used as a signal ground and hence RD is not needed.

• The input signal is coupled via CC1 to the MOSFET gate and the output signal at the MOSFET source is coupled via CC2 to a load resistance RL.

• Since RL is in effect connected in series with the source terminal of the MOSFET, it is more convenient to use the MOSFET's T model for the analysis. This is shown in Fig. 7.5.2.

Note : Current source I acts as an open circuit as far as signals are concerned.

Analysis

• From Fig. 7.5.2, it can be seen that the load resistance RL is in parallel with ro and resistance l/gm m series with R_L || r_o.

• The input voltage V_i appears across the total resistance and hence by applying the voltage divider rule, we have

• The open circuit voltage gain A_vo (R_L = ∞) is given as

A_vo =  r_o / 1 / gm + r_o .... (7.5.5)

• Since r_o >> 1/gm' the open circuit voltage gain tends to unity ; however, it is always less than unity.

• Usually, R_L << r_o and hence the voltage gain given by equation (7.5.4) becomes

• The overall voltage gain of the CD amplifier is given by

• The output resistance Ro is given by

Ex. 7.5.1 For the common drain amplifier, if gm = 1.2 mAFV, RG = 6.8 MΩ, r0 =150 fcΩ, Rsi = 500 K, RL = 10 K, calculate Ri,Avo,Av,Gv and Ro.

Sol. :

Review Question

1. Explain the Common Drain MOSFET amplifier and derive its input impedance, output impedance and voltage gain.

2. Draw the circuit diagram of a common drain MOSFET amplifier. Derive the expression for its voltage gain, input resistance and output resistance.