Lab #5 – MOSFET Amplifiers II
· 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.
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,
Kn = 20 mA/V2 gm = 30 mA/V or 30 mS VTN = 1.7 V
We will design our amplifier so that the drain current IDQ through both transistors will be close to 5mA and VDSQ2 will be close to 7V. Finally, make the total of R1 and R2 close to 300kΩ and choose a value for RS1 about one-fourth of RD1.
2. Using standard values for your design components, calculate the actual expected values for IDQ1, IDQ2, VDSQ1, and VDSQ2. Also, calculate the expected voltage gain AV, the input resistance Ri, and the output resistance Ro. 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 CC1 so that 1/ωCC1 < Ri/10
Choose CC2 so that 1/ωCC2 < Ro/10
Choose CS so that 1/ωCS < (1/gm2)/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 Ri, Ro, and AV. You can measure Ri 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 Ri. 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 Ro. Measure the voltage gain AV 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.