Chapter 11 Flashcards
Why there is a lower voltage swing at the output
Capacitor on output steals some of the signal current and shunts it to ground
Inductance
Ls
Capacitance
1/Cs
If s=0, then H(s) is
DC gain or Ao
Narrow bandwidth
Sluggish time response
Noise/fuzziness since it cannot rapidly jump from low to high and spends time in intermediate levels
Bode’s Rules
As w passes each pole frequency, the slope decreases by 20dB/dec
As w passes each zero frequency, the slope increases by 20dB/dec
Miller multiplication
Capacitor tied between input and output of an inverting amplifier with a gain of Ao raises the input capacitance by (1+Ao)C
Capacitors reduce gain when
High frequencies - Low pass
Low frequencies - high pass
Cb
To change the collector current, must change the base charge profile by injecting or removing electrons or holes
- non-uniform charge profile so that charge flow from BE to CB junction by diffusion (similar to charging and discharging of capacitor)
Cje
BE junction
Cmu
BC junction
Ccs
Collector-substrate
Cpi
Total BE junction
Prominent high frequency capacitor in MOSFETs
Due to oxide layer (capacitance between gate and channel Cgs + Cgd)
Between S and Bulk (Csb)
Between D and bulk (Cdb)
MOSFET overlap capacitance
Cgs
Cgd
Transit frequency
Internal H.F. Capacitors limit the operation model speed of transistors
At high operating speed, H.F. Caps are dominant
Measure of this speed is transit frequency
Ft
The frequency at which small signal current gain falls to unity
To calculate transit frequency
Inject sinusoidal current into base/gate
Measure resulting collector/drain current
Repeat while frequency increases
Red flag is circuits operating frequency greater than transit frequency
Steps to find transfer function
- Add internal capacitance to schematic
- Assume band pass response
- Low frequency
- open H.F. Caps, leave L.F. Caps as is
- if signal path method applies, list these poles
- if not, use KVL - High Frequency
- short L.F. Caps, include H.F. Caps
- May have to use miller transformation to remove feedback caps - Mid-band or pass-band gain
- open H.F. caps
- short L.F. caps
- apply Biasing techniques to find DC gain per circuit configuration - Form the transfer function
Low pass Characteristics
DC gain
Band-pass Characteristics
Passband gain
DC gain
Found by opening all caps
Passband gain
Open H.F. caps, short L.F. caps
High frequency response on cb/Cgd stages
Low input impedance (1/gm)
Does not suffer from miller effect
Frequency response of cascode
CE/CS has high input impedance but suffers from miller transformation
CB/CG has no feedback caps but the impedance is low
Cascodes obtain high input impedance with little or no effect from miller transformation
