E-MOSFET Amplifier: Solution with Drain Feedback Bias

The IRF540N E-MOSFET is a popular choice for switching application due to its low drain to source resistance ON value and due to its high voltage and current handling capabilities. While using E-MOSFET as a switch it is biased in the ohmic region where it effectively behave like a gate operated switch which is simply turned on or off. E-MOSFET cannot be used as amplifier if it is biased in the ohmic region because then it exhibits linear behavior. To use E-MOSFET are an amplifier it has to be biased in the saturation region where it exhibits non-linear behavior. To bias E-MOSFET in the saturation region we can either use gate bias method, drain feedback method or voltage divider bias method. Here it is described how one can build an amplifier using IRF540N N-channel E-MOSFET biased with drain feedback method.

This drain feedback biased E-MOSFET amplifier uses feedback current from the drain terminal to the gate terminal, which helps to establish gate voltage greater than the gate to source threshold voltage and also stabilize the operating point of the MOSFET and improve linearity. An input signal of lower amplitude can then be amplified to higher level because of the non-linearity. The picture below shows IRF540N enhancement MOSFET amplifier with drain feedback bias implemented on a breadboard and the signal waveform of the input signal and the output signal from the amplifier on an oscilloscope.

E-MOSFET Amplifier drain feedback bias on breadboard

E-MOSFET Amplifier input output waveform

The input signal sine wave signal was generated from PC using waveform generator software and the frequency is in the audio frequency range. The tutorial Collector-Emitter Feedback Bias BJT Amplifier Test with PC Oscilloscope shows how to use the free PC waveform generator software.

The basic circuit diagram for a drain feedback bias circuit using the IRF540N E-MOSFET is shown below:

E-MOSFET Amplifier with Drain Feedback Bias Circuit diagram

The key components in this circuit are the feedback resistor (RF) and the bias resistor (RD). The feedback resistor(RF) puts the gate voltage above the gate to source threshold voltage(Vgs(th)) so that the IRF540 MOSFET operates in the saturation region. The drain resistor(RD) sets up the drain current at the bias point location and the drain to source voltage(Vds) at the Q-point bias location. The required component values including the coupling capacitors can be directly computed using the online E-MOSFET drain feedback bias Amplifier design calculator. The following shows the calculator usage.

E-MOSFET drain feedback bias Amplifier design calculator

One important thing to note is that the drain feedback bias circuit is only effective when the MOSFET is operating in the saturation region. This means that it is most useful for high power applications, such as audio amplifiers or power supplies.

In addition to E-MOSFET inherent high input impedance, the drain feedback bias circuit also helps to reduce output impedance and increase power efficiency. This makes it an ideal solution for high power applications where performance and efficiency are key concerns.

The following video demonstrates the performance of IRF540N based E-MOSFET amplifier biased with drain feedback Bias method.

Overall, the drain feedback bias circuit is a valuable tool for improving the performance of the IRF540N E-MOSFET amplifier. By using a small amount of feedback current to stabilize the operating point of the MOSFET, this circuit can help to reduce distortion and improve overall performance in high power applications.

If you are interested in building amplifier using E-MOSFET such as IRF540N then see IRF540N E-MOSFET as Signal Amplifier where voltage divider bias method is used to bias the MOSFET. Voltage divider bias provides greater stability then the drain feedback bias method. For application purpose, see the tutorial Playing Sound with Arduino and MOSFET Amplifier in which the E-MOSFET drain feedback amplifier is employed.


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