Draw the circuit of a MOSFET-based current-mirror-loaded differential amplifier

Draw the circuit of a MOSFET-based current-mirror-loaded differential amplifier. Explain the difference(s) between the circuit and a discrete common-source amplifier. Furthermore, explain the current-mirror-loaded spec of the circuit.

The correct answer and explanation is :

Circuit Diagram:

Here is the basic circuit diagram of a MOSFET-based current-mirror-loaded differential amplifier:

           VDD
            |
           M3
           |
    Drain of M1 -----------+----------- Drain of M2
         |                |                 |
         |                |                 |
        M1               M2                M4
         |                |                 |
         +-------|        |        |--------+
                 |        |        |
                Source   Source   Drain
                 of       of      of M4
                M1       M2
                 |        |
                 +--------+
                      |
                    Current
                    Source
                      I_tail
                      |
                     GND

• M1 & M2: Differential pair transistors
• M3 & M4: Current mirror load
• I_tail: Bias current source for differential pair

Explanation (300+ words):

The MOSFET-based current-mirror-loaded differential amplifier is a widely used analog circuit in integrated circuits. It consists of a differential pair of transistors (M1 and M2) with their sources connected together and biased by a tail current source (I_tail). The drains of M1 and M2 are connected to a current mirror (M3 and M4), which acts as an active load.

Working Principle:

• The differential input is applied at the gates of M1 and M2.
• The tail current I_tail is split between M1 and M2 depending on the input differential voltage.
• The current through M1 generates a mirrored current through M4 (assuming M3 and M4 are identical and properly biased).
• The voltage at the drain of M2 changes depending on the difference in current between M2 and the mirrored current from M4, producing an amplified differential output.

Comparison to Discrete Common-Source Amplifier:

A common-source amplifier is a single-transistor amplifier where the input is applied at the gate, and the output is taken from the drain. It uses a passive resistor or active load at the drain. Its gain is limited by the load resistance and has poor common-mode rejection.

Key Differences:

• The differential amplifier uses two transistors vs. one in the common-source.
• It amplifies the difference between two inputs, making it suitable for rejecting common-mode signals (e.g., noise).
• It employs active loads (current mirrors) for higher gain and better performance in ICs.
• The differential amplifier provides higher common-mode rejection ratio (CMRR).

Current-Mirror-Loaded Spec:

The term “current-mirror-loaded” refers to the use of a current mirror (M3 and M4) instead of resistive loads. This provides several advantages:

• Higher gain: Because the output resistance of a current mirror is much higher than a resistor.
• Better matching: In ICs, MOSFETs can be closely matched.
• Improved linearity and performance in analog front-ends.