**Lab #5 – MOSFET Amplifiers II**

**Objectives:**

· To study the operation of a typical MOSFET amplifier circuit.

· To see how the dc bias can set the Q point around which we amplify the ac input.

· To better understand the advantages of a multitransistor amplifier.

**Procedures:**

1. Our goal is to design a two-transistor amplifier of the form shown in Figure 4.53 in the text. We will use BS170 transistors and assign parameter values that are similar to those we determined for our transistors two weeks ago. Specifically,

K_{n}
= 20 mA/V^{2} g_{m}
= 30 mA/V or 30 mS V_{TN}
= 1.7 V

We will design our amplifier so that the drain current I_{DQ}
through both transistors will be close to 5mA and V_{DSQ2} will be
close to 7V. Finally, make the total of
R_{1} and R_{2} close to 300kΩ and choose a value for R_{S1}
about one-fourth of R_{D1}.

2. Using standard
values for your design components, calculate the actual expected values for I_{DQ1},
I_{DQ2}, V_{DSQ1}, and V_{DSQ2}. Also, calculate the expected voltage gain A_{V},
the input resistance R_{i}, and the output resistance R_{o}. Start a table with these values as the
calculated ones.

3. Build your circuit. For the capacitors choose values that will make their impedance at f = 1kHz less than one tenth of the associated resistance that they are working with. Specifically,

Choose C_{C1} so that
1/ωC_{C1} < R_{i}/10

Choose C_{C2} so that
1/ωC_{C2} < R_{o}/10

Choose C_{S} so that
1/ωC_{S} < (1/g_{m2})/10

4. Measure the dc bias drain currents and the drain source voltages and put them in your table as the actual measured values. Compare them with the calculated values and comment on their agreement.

5. Use the Wavetek
function generator to generate the 1kHz sine wave for your input signal. Choose an amplitude that will not overdrive
the output and cause distortion. Measure
R_{i}, R_{o}, and A_{V}. You can measure R_{i} by adding a
resistor between the function generator and the circuit input. When this resistor causes the output to drop
by one half, it must be equal to R_{i}.
Similarly, you can add a load resistor and vary its value until the
output drops by one half, and again this value must be equal to R_{o}. Measure the voltage gain A_{V} with
no load resistance connected. Add these
values to your table and again comment on the agreement with the calculated
values.

6. Summarize your experience with the two-transistor amplifier circuit.