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

The impedance looking into the source of M₁ is 1/g_m. V_in experiences a potential divider before entering the source of M₁. The voltage drop across R_s is typically the voltage at the source and hence, it is equal to V_in * R_s / (R_s + 1/g_m).

The voltage gain from the source of M₁ to the drain of M₁ can be found out from a small signal analysis and it’ll turn out to be g_m * R_d. But the input voltage is a result of the potential divider between R_s and 1/g_m. Hence, the overall voltage gain is g_m * R_d * {R_s/ (R_s + 1/g_m)}.

In absence of channel length modulation, r_o = 0. The gain is simply gm * R_d. It should be noted that the CG stage doesn’t invert and hence the voltage gain is not -g_m * R_d.

The above circuit is quite similar to the CS stage with degeneration. But the CG stage doesn’t invert and after performing a small signal analysis, the voltage gain comes out to be R_d/(1/g_m + R_s).

The input to a CS stage is at the source of M₁. This current simply flows into the channel and flows out of M₁. Approximately, we can say that the overall current gain is unity since the gain contributes very low current.

This is a cascade of a CG stage preceding a CS stage. For the CG stage, the voltage gain is g_m * R_d. But after this stage, the signal gets amplified by the CS stage with a factor of –(g_m2* R_d1). Hence, the overall voltage gain is the product of both the factors ie –(g_m1 * R_d * g_m2 * R_d1). Note that since channel length modulation is absent, r_o is not present in the expression of gain.

This is a cascade of a CG stage preceding a CS stage. For the CG stage, the voltage gain is g_m * R_d. But after this stage, the signal gets amplified by a degenerated CS stage with a factor of -Rd / (1/g_m2 + R_s). Hence, the overall voltage gain is the product of both the factors i.e. – {g_m1 * R_d * R_d / (1/g_m2 + R_s)}. Note that the input impedance looking into M₂ is infinite ad hence the voltage gain of the C.G. stage remains unaffected by the C.S. stage (for low frequency operations only).

The impedance looking into the source of M₁ is 1/g_m1. Hence, the product of this times the transconductance of M₂ will be the overall voltage gain i.e. g_m2/g_m1. Note that the gain is function of only the intrinsic parameters of the MOSFET and independent of other parameters.

The input impedance can be calculated by performing a small signal analysis at the input node. We need to set V_g and V_dd to 0V and apply a small input voltage at the source. Now, the impedance looking into the source is 1/g_m and R_s is parallel to this impedance. Hence, the total input impedance is R_s || 1/gm.

The input to the CG stage is placed at the source of M₁. The impedance looking into the node of M₁ is simply 1/g_m. Hence, the input impedance is 1/g_m. R_d || r_o is the output impedance if channel length modulation is present.

In presence of channel length modulation, r_o appears to be in parallel to R_d. Hence, the total output impedance becomes R_d || r_o. In the absence of channel length modulation, the output impedance is R_d only.

The impedance looking into the drain of M₁ would be similar to that of a CS stage with source degeneration. That implies the impedance is R_s*(1+g_m * r_o) + r_o. But this impedance is parallel to R_d. Hence, the overall output impedance becomes R_d || {R_s*(1+g_m* r_o) + r_o}.

The voltage gain of a follower is always less than that of a CG stage. This can be proven by a small signal analysis that the voltage gain for a follower is R_s/(1/g_m + R_s) while that of the CG stage is g_m * R_d. Hence the above statement is true.

This is a cascade of a CG stage preceding a CS stage. For the CG stage, the voltage gain is g_m * (R_d || r_o1). But after this stage, the signal gets amplified by the CS stage with a factor of – {g_m2 * (R_d1 || r_o2)}. Hence, the overall voltage gain is the product of both the factors i.e. – {g_m1 * (R_d || r_o1) * g_m2 * (R_d1 || r_o2)}. Note that the input impedance looking into M₂ is infinite ad hence the voltage gain of the CG stage is typically unaffected by the CS stage (for low frequency operations only).

This is a cascade of CS stage preceding a CG stage. The voltage gain of the first stage is – (g_m1 * R_d || 1/g_m2). This is because the drain of M₁ is connected to the source of M₂ and the impedance looking into the source of M₂ is 1/g_m2. Now, the source of the CG stage is connected to R_d and hence the voltage gain due to this stage is affected by source degeneration and the voltage gain is R_d1/ (1/g_m2 + R_d). The overall voltage gain is – {g_m1 * (R_d || 1/g_m2) * (R_d1/ (1/g_m2 + R_d))}.