Small Signal Model PNP Transistor Section ,4.6
Schedule 92/11TuesdayPhysics of a BJT L2/11TuesdayMeasure Beta of a transistor 102/13ThursdayPNP /18TuesdayBJT in saturation mode4.5 L2/18Tuesday BJT in saturation/BJT implementation of an NAND gate 122/20ThursdaySmall Signal model [homework: small eq. circuit, (PNP)] 4.4,4.6
Overview
Review
Small Signal Model Section 4.4
Schematic of an Audio Amplifier Microphone produces a small signal. How does the amplifier circuit respond to a small change in the input signal? How is the analysis performed? Small signal model
Small Signal Analysis (For a Circuit You have not Seen Before) 1.Replace each ideal DC voltage source with a small signal ground. 2.Replace each ideal DC current source with an open circuit. 3.Replace each transistor by its small signal model 4.Analyze the small signal equivalent circuit.
Small Signal Analysis (For a Circuit You have not Seen Before) 1.Analyze the Circuit by Inspection
Voltage Source DC Voltage Source in Small Signal Analysis R S should be 0 for a good battery!
Current Source R S should be infinity for a good battery!
Small Signal Model (NPN)(PNP) Statements that are always true for both NPN and PNP. 1.r π is between B and E. 2.the direction of the dependent current source always points from the collector to emitter. 3. r o is always between B and C.
Question Replace Q1 and Q2 by their small equivalent circuit.
Answer
Question
Answer
Derivation of the Small Signal Model
Change in the Collector Current Due to a Small Change in Base-Emitter Voltage If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current ?
Derivation of Transconductance If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current ? Small signal model of Q1
But there is something else…. A change in V BE creates a change in base current! Small signal model
Example 4.10 Signal Generated By a microphone Small Signal Equivalent Circuit V BE =800 mV β=100 I S,Q1 =3 x A Question: If a microphone generates a 1 mV signal, how much change is observed in the collector and base current ?
A Simple Amplifier Determine the output signal level if the microphone produces a 1 mV signal.
AC Ground The voltage produced by a voltage source is constant. The small signal model is concerned only with changes in quantities. Therefore, a DC voltage source must be replaced with a ground in small signal analysis.
Example Small Signal Model
Summary
Output Resistance Due to Early Effect A larger reverse bias voltage leads to a larger BC depletion region. The effective base width (WB) is reduced. The slope of the electron profile increases. I C increases as VCE is increased.
Early Effect
James M. Early
Modeling of Early Effect
What Doesn’t Change with Early Effect ?
Modification of the Small Signal Model
Slides to Cover During the Lab
A Simple Cadence Example
Assumption Assume that 1.The DC at Vout is 0.9 V 2.g m =1 mS Gain is approximately equal to –g m R C. Bias current is I C =g m V t R=(1.8V-0.9V)/26uA=34.6 Kohms Gain is
DC Bias of the Amplifier
Sweep the Base Voltage to Get the IC=26 uA
Display the Transconductance
Display Transconductance
Verify Transconductance (1)
Verify Transconductance (2)
Transconductance VBIC mV25.64 uA mV26.64 uA ∆VBE1 mV ∆IC1 uA gm=∆VBE/∆IC1 mS
Introduce a Small Signal
Calculate Peak to Peak Voltage
Peak to Peak Voltage=67.78 mV mV/2=33.9