EE40 Summer '04, Lab11

Introduction

In this lab we will use a NMOS for building a simple voltage amplifier. The component that we will be using is the BS170. Please read the data sheet (.pdf) before starting your lab. The configuration that we are going to use is the common source amplifier shown in figure 1.


Figure 1

Here is a summary of the specifications that you need to know:

• Q1:
  • K = 0.82
  • VT = 2.9 V
• Vdd = 10 V
• Av = 20
• vi:
  • Sinusoidal
  • Amplitude 100mv
  • frequency 5KHz

You have to compute the values for all components (see pre-lab)

Pre-lab

Q1) Compute the voltage gain of the common source amplifier of figure 1 (Av= vo/vi).
Q2) What is the relation between gm, Rd and the gain Av?
Q3) Write the KVL for the loop Vdd->VRd->Vds (the output loop)
Q4) Imposing VDS = VDD/2, find a relation between Rd and IDsat.
Q5) Using the relations in Q2 and Q4 and also the fact that in saturation IDsat = K(VGS - Vt)^2, find the values of IDsat, VGS and Rd.
Q6) Compute R1 and R2 to set VGS as in Q5

Lab

Build the circuit. Don't connect the function generator to the input yet. Connect the DC power supply with a value of 10 V. Verify that the operating point is as expected. In particular verify that:

• VDS = VDD/2
• VGS is as computed in Q5
• ID is as computed in Q5

If the operating point is not the one that we want, change the components values until it is exactly what you computed.

Report

Write the usual summary of your experiments and answer to the following questions.
Q1)Qualitatively, describe what happens if Rd increases.
Q2)Qualitatively, describe what happens if R2 decreases.