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An active load MOSFET amplifier is a MOSFET amplifier which uses an active load for a specific purpose. For one main reason, the active load can increase the gain of an amplifier instead of using a passive load that only consumes energy. This active element that is used as a load is known as an active load.

Active Load MOSFET Amplifier

What is active load?

An active load is an element or component or circuit that acts as a current-stable nonlinear resistor.

For a better understanding, observe the common MOSFET amplifier below with a passive load (a resistor).

active load mosfet amplifier 1

Compared to what we want to learn, an active load MOSFET amplifier below.

active load mosfet amplifier 2

The output of our active load MOSFET amplifier will be amplified further with a calculation.

Why do We Need to Use an Active Load for MOSFET Amplifier

Modern amplifiers are used for an integrated circuit.

Even if we have the option to choose between BJTs or MOSFETs to build an integrated circuit amplifier, MOSFET amplifiers are more efficient for building an integrated circuit. This integrated circuit amplifier has some advantages and disadvantages such as:

Advantages:

  • Circuit can be small and inexpensive
  • The current source is easy to get and use

Disadvantages

  • The circuit will be quite complex
  • Bias strategy will be more difficult because DC blocking capacitor is not possible since we can’t make large capacitor
  • It is hard to make resistors in an integrated circuit so we use a transistor to act as a load (active load)

MOSFET Amplifier with Active Load or Passive Load?

In order to fully understand why we use an active load in a MOSFET amplifier, we need to understand it first with a passive load. Observe a common source MOSFET amplifier with a resistor, RD below.

active load mosfet amplifier 3

As shown above, the overall gain of a common source NMOS amplifier above can be calculated with

    \begin{align*}|A_V|=g_m\times(R_D||r_o)\end{align*}

Where:
AV = voltage gain
gm = transconductance
RD = drain resistance
ro = output resistance

From the simple equation above, to increase the gain we need to increase either RD or gm. But increasing RD will result in decreased drain voltage thus MOSFET may operate in the saturation region.

On the other hand, increasing the transconductance (gm) by increasing the drain current (ID). But increasing the drain current will lead to bigger power dissipation. Not only that, the voltage drop across the RD will increase. This result on the MOSFET stopped working at the cutoff region.

Using a passive load gives us a little problem to maintain the MOSFET to operate in the active region as an amplifier. Of course we can increase the power supply voltage to produce amplification to some extent. But this approach is inefficient for modern applications such as integrated circuits where we need to make everything small and smaller.

Active Load MOSFET Amplifier with a Current Source

Observe an NMOS amplifier with an ideal current source below.

active load mosfet amplifier 4

Like before, we will still use an NMOS with a current source to replace the drain resistance (RD).

This way, the amplifier is biased by a current source. Of course since the current source generates energy, it is an active element.

Consider that the current source is ideal, we can get few advantages such as:

  • Bias current is stable and does not get affected by the circuit parameter since it will make sure the current is delivered with desired value regardless of the load.
  • A current source has infinite output impedance.

Since we use an ideal current source, we need to rewrite the voltage gain as

    \begin{align*}|A_V|=g_m\times r_o\end{align*}

Where:
gm = transconductance
ro = output impedance

We don’t use RD since we replace it with a current source. Just for another information, we can also replace the current source with an open circuit since it has infinite impedance.

Why do We Use a Current Source for an Amplifier as an Active Load

The maximum gain we get from an amplifier depends on its configuration. If we use a current source, its gain will increase significantly. This current source is used for biasing in an integrated circuit. Basically, we can also use the current mirror circuit but its impedance will reduce the gain further.

Since there is no “ideal” condition in the practical field, our current source also has finite impedance or resistance as shown below where ro is the output resistance.

active load mosfet amplifier 5

We can replace the current source with its output resistance. Thus our circuit will be simplified to

active load mosfet amplifier 6

From the circuit above, the voltage gain can be calculated by

    \begin{align*}|A_V|=g_m\times (r_{o1}||r_{o2})\end{align*}

Since we have the output resistance from the current source (ro2), the gain is further decreased. In order to increase the gain, we need to increase the current source resistance (ro2) needs to be increased.

We can use a cascode amplifier with a cascode current source to produce larger voltage gain.

The current source is not a “true current source” like a voltage source. We can use a current mirror circuit consisting of three transistors to act together as a current source. Observe the circuit below where the Q2, Q3, and Q4 act together as a current mirror circuit to provide current bias.

active load mosfet amplifier 7

We will talk about this later in the current mirror transistor circuit.

Frequently Asked Question

What is active load?

An active load is an element or component or circuit that acts as a current-stable nonlinear resistor.

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