Test: MOSFET Amplifier with CD Configuration - Electronics and Communication Engineering (ECE) MCQ

M₂ behaves as a Source follower i.e. it is being used in a C.D. configuration. M₁ provides a output resistance of r_o1 but M₂ provides an impedance of (1/g_m2||r_o2). Thus, from the voltage gain of a follower stage, we conclude the overall voltage gain is r_o1 / {(1/g_m1 || r_o2) + r_o1}.

The voltage gain at node X is due to M₁ being used in the CD configuration. Since M₁ offers channel length modulation but not M₂, we note that the impedance looking into the source of M₂ is 1/g_m2 but for M1, it’s (1/g_m2 || r_o2). From the voltage gain of a follower stage, we conclude that the voltage gain is (1/g_m2 || r_o2) / {1/g_m1 + ( 1/g_m1 || r_o1)}.

The output impedance of the follower is a function of the transconductance. But it’s also a function of the channel length modulation. But the resistance offered due to channel length modulation is much greater than the inverse of the transconductance. It can be concluded that the output of the follower depends on the transconductance and it’s low.

The voltage gain is given by R_L/(1/g_m + R_L) where R_L is the 50 Ω load. Now, we see that if the voltage gain is .25, gm is 1/175. Now, gm is  where (W/L) is the aspect ratio, Id is the drain current. We have all the values and the aspect ratio becomes 32.6.

This is a cascade of a follower stage follower by a CS stage which precedes another follower stage. The gain due to each stage gets multiplied until we reach the output. For the first stage, the gain is R/ ((1/g || r)+ R) since M₁ has channel length modulation. The second stage has a gain of g * R while the final stage has a gain of R/ (1/g + R). After multiplying the gains, we get R/ ((1/g || r )+ R) * g *R * R/ (1/g + R).

The source of M₁ and M₂ are connected at node X. The impedance looking into the source of a MOSFET is (1/gm || ro). If we look at node X, M₁ and M₂ are connected from Node X to ground. Hence, they are parallel to each other and the overall impedance becomes (1/g_m2 || r_o2 || 1/g_m1 || r_o1).

The input to the follower stage is applied to the gate of the MOSFET. The gate of the MOSFET is made of SiO₂, which is a dielectric and provides very high impedance to the input.

The C.D. configuration offers high input impedance to the input signal and low output impedance while sensing the output signal. Thus, it is used as a buffer stage.

Since the transconductance is .02, it’s inverse is 50. From the general expression of voltage gain of a source follower, we conclude that the load should be 50 Ω since the inverse of the transconductance is the impedance looking into the source of the MOSFET. For maximum power transfer, we have to match both the impedance.

This is a cascade of a follower stage follower by a CS stage which precedes another follower stage. The gain due to each stage gets multiplied until we reach the output. For the first stage, the gain is R / ((1/g+ R). The second stage has a gain of g * (R || r) while the final stage has a gain of R/ ((1/g || r) + R)- the effect of r is simply due to channel length modulation. After multiplying the gains, we get R/ ((1/g + R) * g * (R || r) * R/ ((1/g || r) + R).

The gain of the follower stage is given by R_s/(1/g_m + R_s). Hence, we readily conclude that if the transconductance (g_m) increases, the gain will increase.

The input impedance is extremely high for a follower stage because the input is applied to the gate of the MOSFET. The SiO₂ layer is the primary reason for such a high impedance value.

This is a cascade of a follower stage follower by a CS stage which precedes another follower stage. The gain due to each stage gets multiplied until we reach the output. For the first stage, the gain is R/ ((1/g || r) + R). The second stage has a gain of g * R while the final stage has a gain of R/ ((1/g || r) + R). The presence of r is due to channel length modulation. After multiplying the gains, we get R/ ((1/g || r) + R) * g * R * R / ((1/gm || r)).

The square root of the aspect ratio is directly proportional to the transconductance of the MOSFET. If it increases, the transconductance increases and hence according to the expression of the voltage gain of a follower stage gain increases.

The output impedance of a follower stage is (1/g_m || R_d). If the transconductance increases, the output impedance will decrease, as can be seen from the formulae.