Chapter 9 Flashcards
Cascode structure
Stacking a transistor atop another
Results in a high output impedance
Extreme case of source or emitter degeneration
Voltage gain of an amplifier can be expressed as
-GmRout
Gm
Short circuit transconductance of the amplifier
-GmRout
Indicates that the gain of amplifiers can be maximized by maximizing their output impedance
With its high output impedance, a cascode stage can operates as
A high gain amplifier
Load of a cascode stage is also realized as a cascode circuit so as to approach
An ideal current source
Resistive dividers tied to the base or gate of transistors result in
Supply and temperature dependent currents
If Vbe or Vgs are well defined, then
Ic and Id are not
Current mirrors
Can “copy” a well-defined reference current numerous times for various blocks in an analog system
Rarely used in discrete design as their accuracy depends on matching between transistors
Use bandgap current source (golden current), 20+ components
Cascode
Ideal current source, gain amplifier
PNP connected to
VCC
NPN connected to
GND
Self-impedance
Z=V/I
Transfer Impedance for two port network
Z=V/I
Cascode as a current source
Transistor loaded with a current source, gain increases
Use two back to back transistors, one as an amp, and other as current source
Gain still limited by ro
Cascading amp
Vin
To form a cascode,
Both transistors must be of the same type (both PNP or NPN)
Gm
Ic/Vt
Rpi
Beta/gm
Ro bjt
Va/Ic
Transconductance
Gm = Iout/Vin when Vout=0
Signifies strength of circuit in converting input voltage to output voltage
Bipolar cascode amplifier
Rc replaced with current source to increase gain
Current source consisting of a single PNP will reduce gain
Current mirror diode connected
Base currents going in opposite directions
To suppress error in current mirrors
Emitter Follower Circuit
PNP current mirror
Iref going to ground instead of being connected to VCC
Q1 has to be grounded so that Vx is the same for both
