CHAPTER 6
INTRODUCTION TO SMALL SIGNAL
MODEL
6.1
References
1. Section 4.4, B. Razavi, Fundamentals of Microelectronics, Wiley, 2008.
6.2
Objectives
After completing this lab, you will be able to
1. Obtain small signal parameters from DC simulation.
2. Perform transient simulation of an amplifier.
3. Perform gain calculation using small signal analysis.
Electronics I Laboratory Manual, First Edition.
c 2014 ,J. Ou.
Copyright 33
34
6.3
INTRODUCTION TO SMALL SIGNAL MODEL
Analysis
1. The schematic of a simple bipolar junction transistor is shown in Figure 6.1.
We assume in this experiment that VB is 0.5626 V and a 2n3904 transistor is
used for the bipolar junction transistor.
34.6kΩ
RC
vout (t)
C
B
vs (t)
E
+ V
− CC = 1.8V
VB +
−
Figure 6.1: A simple bipolar junction transistor amplifier.
2. Run a DC simulation in Cadence to obtain values for the following small signal
parameters.
(a) gm (gm)
(b) β (betadc)
(c) rpi (rpi)
(d) ro (ro)
3. Perform the small signal analysis to determine the gain of the amplifier.
(a) Replace the DC voltage source with a small signal ground.
(b) Replace the transistor by its small signal model.
(c) The gain of the circuti is defined as vout,m /vs,m . vout,m and vs,m are the
magnitude of the small signal source and the output voltage respectively.
4. Run a transient simulation in Cadence to determine the gain of the circuit.
6.4
Submission Checklist
1. What the values for the small signal parameters? Please summarize the results
in a table. Your summary should include gm , ro , rpi , β. (4 points)
SUBMISSION CHECKLIST
35
2. Please submit the Cadence schematic. (Please set the background to white to
conserve toner) (2 points)
3. Please submit the waveforms at the base and the collector. (2 points)
4. What is the gain determined from Cadence simulation? What is the gain determine from the small signal analysis? (4 points)
CHAPTER 7
CALCULATION OF DC OPERATING
POINT
7.1
Parts
One 2n3904 npn transistor (Mouser 512-2N3904TA)
One 100 kΩ resistor
One 1 kΩ resistor
7.2
References
1. Section 5.1-5.2, B. Razavi, Fundamentals of Microelectronics, Wiley, 2008.
7.3
Bias Calculation via Iteration
1. We start the iterations by assuming that VBE = 0.65V .
2. The base current is
IB =
Electronics I Laboratory Manual, First Edition.
c 2014 ,J. Ou.
Copyright VCC − VBE
RB
(7.1)
37
38
CALCULATION OF DC OPERATING POINT
RC
RB
1 kΩ
VCC +
− 2.5V
100 kΩ
Q1
2N 3904
Figure 7.1: Fixed Base Bias
3. Assuming that β = 125, the collector current is IC = βIB .
4. Assuming that IS = 2 × 10−17 A and VT = 25mV, the base-emitter voltage is
VBE = VT ln
IC
IS
(7.2)
Eq. 7.2 can be used to iterate for the base current using Eq. 7.1.
5. What are VBE and IC after a few iterations?
6. What are gm , rπ , and ro ? (You may assume that the early voltage is 25 V.)
7. Can you draw the small signal model for the circuit in Figure 7.1.
7.4
Cadence Simulation
1. Simulate the circuit shown in Figure 7.1 using Cadence.
2. Determine VBE , IC , β, gm , rπ and ro from simulation.
7.5
Measurement
1. Build the circuit shown in Figure 7.1.
2. Measure VBE . How are the measured base-emitter voltage compared to the
values determined via simulation and iteration?
3. Determine IC and IB by using Ohm’s law (i.e. measure the voltage across RB
and RC ) What is β? How is it compared to the value determined via simulation?
SUBMISSION CHECKLIST
7.6
39
Submission Checklist
1. Please answer all of the questions in this chapter.
2. Make a table to summarize VBE and IC obtained via iteration, simulation and
measurement.
3. Make a table to compare the small signal parameters obtained via simulation
and iteration.