Title:

MOSFET

Objective:

To demonstrate the concept of large- and small-signal models, to determine the DC biasing, and to determine the small-signal parameters at low frequency, such as the voltage gain, input resistance, and output resistance for different circuit configurations.

Equipment and components:

1 MOSFET (IRF 820); 2 Resistors (1 kW); oscilloscope; function generators; DC power supply.

Note:

Analyze the circuit first, which includes hand calculations or computer simulation. Compare your experiment measurement with your analysis predictions.

1. Set up the circuit shown below on a solderless breadboard.

1. Increase V_G from 0 V to 5 V in step of 0.1 V, measure current I_D and output voltage V_O. Plot V_GS vs. I_D. Determine K and V_t.

[K can be obtained using the PolyFit function in Matlab. Plot the polynomial using the constants you obtained and compare it with the plot from your data. K will be the second order constant when the two plots fit closely. Use the K found to calculate V_t. Verify your V_t from the transistor data book.]

2. Adjust V_G so that I_D = 1 mA. Record V_GS, V_DS, and I_D. Calculate g_m.
3. Add v_g = 0.4 sin 2p ft with f = 1kHz in series with V_G. Measure the small-signal output voltage on the oscilloscope and determine the voltage gain.

2. Set up the circuit shown below on a solderless board.

1. With v_g = 0 V, adjust V_G so that I_D = 1 mA. Record V_GS, V_DS, and I_D.
2. Let v_g = 0.4 sin 2p ft with f = 1kHz. Calculate the ac input resistance r_in.
3. Calculate the ac output resistance r₀ looking into the output voltage v_o.
4. Measure the small-signal output voltage on the oscilloscope and calculate the voltage gain.