GATE EC 2023 Set 1 — Question 21

Let's analyze the input and output stages of the given circuit:Input Stage:
The input signal $V_{IN}$ is applied to the gate of the first MOSFET. The source of the first MOSFET is connected to a current source (implied by $V_1$ and $R_1$ if it were a common source, but here it's a common gate configuration for the first transistor if we consider $V_1$ as a bias for the source). However, looking at the overall structure, $V_{IN}$ is applied to the source of the first transistor, and its gate is biased by $V_1$. This configuration is a common-gate (CG) amplifier stage. A common-gate amplifier has a low input impedance.Output Stage:
The output $V_{OUT}$ is taken from the drain of the second MOSFET. The gate of the second MOSFET is biased by $V_2$. The drain is connected to $V_{DD}$ through a resistor (or current source) and the source is connected to the output node. This configuration is a common-drain (CD) amplifier stage, also known as a source follower. However, the diagram shows $V_{OUT}$ at the drain of the second transistor, which is a common-source (CS) configuration if $V_2$ biases the gate and the source is grounded or connected to a current source. Let's re-examine the connections.Let's trace the signal path:Let's re-evaluate the input and output impedances based on the overall circuit behavior.Input Impedance ($Z_{in}$):
The input is applied to the source of the first MOSFET, which is configured as a common-gate stage (gate is biased at $V_1$). A common-gate stage has a very low input impedance, typically $Z_{in} \approx 1/g_m$, where $g_m$ is the transconductance of the MOSFET. Thus, the circuit has a low input impedance.Output Impedance ($Z_{out}$):
The output is taken from the drain of the second MOSFET, which is configured as a common-source stage (gate is driven by the first stage, source is biased at $V_2$). The output impedance of a common-source stage with a resistive load $R_L$ (here, $R_1$ in the output part) is approximately $R_L$ in parallel with the MOSFET's output resistance $r_o$. If the load is a current source, the output impedance would be high. Given the resistor $R_1$ at the output, the output impedance is high. More precisely, for a common-source stage with an active load or current source load, the output impedance is very high. The diagram shows $R_1$ connected to $V_{DD}$ at the output, which acts as a load. The output impedance of a CS stage with a current source load (or high resistance load) is high.Let's consider the overall behavior based on input and output impedances:

• Low Input Impedance implies it is a current-controlled device (it 'accepts' current easily).
• High Output Impedance implies it is a current source (it 'delivers' current).

Therefore, the circuit behaves as a current-controlled current source.Let's verify the options:
(A) Voltage controlled voltage source: High input impedance, low output impedance.
(B) Voltage controlled current source: High input impedance, high output impedance.
(C) Current controlled voltage source: Low input impedance, low output impedance.
(D) Current controlled current source: Low input impedance, high output impedance.Based on the analysis, the circuit has low input impedance (common-gate input) and high output impedance (common-source output with a load that makes it behave like a current source). This matches the characteristics of a current-controlled current source.The final answer is $\boxed{D}$.