What is Active Load?
In MOSFET amplifier circuits, instead of a passive resistor, the active component like a MOSFET or MOSFET circuit (Current Mirror) is used to increase the gain of the amplifier. This active component or the active circuit is known as the Active Load.
Fig.1 MOSFET Amplifier with Active Load
Why Active Load is used in the circuit design?
The Active Load is usually used in the integrated circuits where the size and power consumption are major constrain. Also, in the integrated circuits, fabricating a resistor requires a lot of space. And therefore, instead of a resistor, the active load is used. The active load also helps in increasing the voltage gain of the amplifier. To understand this point, let’s take the example of a simple common source amplifier with the passive load.
Fig. 2 Common Source amplifier with Passive Load
As shown in Fig.2, the gain of the common source amplifier is |Av| = gm*(RD || ro).
Where, ro is the output resistance of MOSFET. If ro >> RD then |Av| ≈ gm*RD
To increase the gain, either RD or gm needs to be increased. But as RD increases, the voltage drop across RD also increases and hence, the available voltage at the drain terminal reduces. At one stage, the MOSFET may come output of the saturation.
Similarly, by increasing the drain current ID, the transconductance (gm) can be increased. But as the drain current increases, the power dissipation in the circuit increases. Also, with the increase in the drain current, the voltage drop across the drain resistor will increase. And at one point, MOSFET may come out of the saturation.
Of course, by increasing the power supply voltage, the MOSFET can be kept in the saturation region and it is possible to increase the gain by some extent. But that is not an option in the modern integrated circuits where the supply voltage range is shrinking day by day. All these problems can be eliminated using the active load.
Ideal Current Source as an Active Load
Fig.3 Biasing the MOSFET with Current Source (Current Source as an active load)
The amplifier can be biased using the constant current source. The current source is the example of active load. If the current source is ideal then there are couple of advantages.
- Bias current remains stable irrespective of the changes in the external circuit parameters like temperature
- The ideal current source has infinite output impedance.
In the AC equivalent circuit, the ideal current source can be replaced by the open circuit. And the voltage gain of the amplifier |Av| = gm*ro, where ro is output impedance of the MOSFET.
gm*ro is known as the intrinsic gain of the amplifier. It is a maximum obtainable gain for the given amplifier configuration. Using the current source as a load, there is a significant improvement in the voltage gain. Typically, with the current process technology, it is possible to obtain the gain in between 20 to 50. Typically, in the integrated circuits, the current source for the biasing is generated using the current mirror circuit. And since it is a non-ideal current source, it has some finite output impedance. Because of its finite output impedance, the gain further reduces.
Fig. 4 Actual Current Source (with finite output resistance)
In the small-signal equivalent circuit, the actual current source can be replaced with its output impedance. (As shown in Fig. 5)
Fig.5 Current Source will get replaced by its output impedance in the AC equivalent circuit
And the voltage gain |Av| = gm*(ro1 || ro2). Hence, because of the finite output resistance of the actual current source, the voltage gain reduces. To increase the voltage gain, the output resistance of the current source needs to be increased. Typically, to achieve a large voltage gain, the cascode amplifier along with the cascode current source is used. In the upcoming articles, the cascode amplifier and current mirror circuits will be discussed.
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