## MOSFET AMPLIFIER EXPERIMENT

This demonstration is to develop your intuition about amplifiers and biasing, and to have fun with music! There are no responses that need to be checked.

In the lab below, you will find:

- A circuit schematic of the MOSFET amplifier. You can use the sliders to the left of the circuit to control various parameters of the MOSFET and the amplifier.

- A plot (as a function of time) of selected voltages from the amplifier circuit. You can select the input waveform (e.g., sine wave, square wave, various types of music) by using the \(v_\mathrm{IN}\) drop-down menu and the associated sliders. (The parameter \(V_\mathrm{MAX}\) sets the maximum range on the plots.)

- The "Play" button which lets you listen to the selected voltage waveform as sound. Try it out!

Listen to:

- vIN
- vOUT
- vR

Graph:

- vIN
- vOUT
- vR

vIN:

## Experiment 1: Distorted output

Begin by selecting a sine wave input in the drop-down menu for \(v_\mathrm{IN}\). Then, adjust the sliders to the baseline (default) setting shown below.

Baseline setting of sliders:

- Peak to peak voltage: \(v_\mathrm{IN}=3~\mathrm{V}\),
- Frequency: \(f = 1000~\mathrm{Hz}\),
- Supply voltage: \(V_\mathrm{S}=1.6~\mathrm{V}\),
- Input bias voltage: \(V_\mathrm{BIAS}=2.5~\mathrm{V}\),
- Pull-up resistor: \(R = 10~\mathrm{k}\Omega\),
- MOSFET parameter: \(K=\frac{1~\mathrm{mA}}{\mathrm{V}^2}\),
- MOSFET threshold voltage: \(V_\mathrm{T} = 1~\mathrm{V}\),
- Vertical plot range: \(V_\mathrm{MAX} = 2~\mathrm{V}\).

You should observe in the plot that with the baseline settings, the amplifier produces a distorted sine wave signal for \(v_{OUT}\). Next, go ahead and select one of the music signals as the input and listen to each of \(v_{IN}\) and \(v_{OUT}\), and confirm for yourself that the output sounds degraded at the chosen slider settings. You will notice that the graph now plots the music signal waveforms. Think about the reasons why the amplifier is producing a distorted output.

## Experiment 2: Linear regime

We now study the amplifier's small signal behavior. Select a sine wave as the input signal. To study the small signal behavior, reduce the value of \(v_{IN}\) to 0.1V (peak-to-peak) by using the \(v_{IN}\) slider. Keeping the rest of the parameters at their baseline settings, derive an appropriate value of \(V_{BIAS}\) that will ensure saturation region operation for the MOSFET for the 0.1V peak-to-peak swing for \(v_{IN}\). Make sure to think about both positive and negative excursions of the signals.

Next, use the \(V_{BIAS}\) slider to choose your computed value for \(V_{BIAS}\) and see if the observed plot of \(v_{OUT}\) is more-or-less distortion free. If your calculation was right, then the output will indeed be distortion free.

Next, select one of the music signals as the input and listen to each of \(v_{IN}\) and \(v_{OUT}\), and confirm for yourself that the output sounds much better than in Experiment 1. Also, based on sound volume, convince yourself that \(v_{OUT}\) is an amplified version of \(v_{IN}\).

## Experiment 3: Your settings

Now go ahead and experiment with various other settings while listening to the music signal at \(v_{OUT}\). Observe the plots and listen to \(v_{OUT}\) as you change, for example, the bias voltage \(V_{BIAS}\). You will notice that the amplifier distorts the input signal when \(V_{BIAS}\) becomes too small, or when it becomes too large. You can also experiment with various values of \(v_{IN}\), \(R_{L}\), etc., and see how they affect the amplification and distortion.