Chapter 10 Flashcards
Voltages measured with respect to ground
Single-ended signals
Consists of two single-ended signals carries over two wires, with two components beginning from the same dc (common-mode) level and changing by equal and opposite amounts
Differential signal
More immune to common-mode noise
A differential pair consists of:
Two identical transistors, a tail current, two identical loads
P acts as virtual ground
Active mode - making sure input is in signal range and not too large
The circuit “rejects” input CM changes
Transistor currents in differential pair remain constant as the input CM level changes
The transistor currents change in opposite directions if
A differential input is applied
For small, differential changes at the input, the tail node voltage of a differential pair
Remains constant - considered a virtual ground node
Bipolar differential pairs exhibit
A hyperbolic tangent input/output characteristic
Tail current can be mostly sorted to one side with a differential input of 4Vt
Small signal operation of differential pair
Input differential swing of a bipolar differential pair must remain below roughly Vt
Can be decomposed into two half circuits, each of which is simply a common-emitter stage
MOS differential pairs can steer the tail current with a differential input equal to
Sqrt(2) times larger than the equilibrium overdrive of each transistor
MOS differential pairs can provide
Linear characteristics depending on the choice of the device dimensions
In the presence of asymmetries and a finite tail current source impedance,
A fraction of the input CM change appears as a differential component at the output, corrupting the desired signal
Gain seen by the CM change normalized to the gain seen by the desired signal is
Common-mode rejection ratio
Differential pair with active load
Replace loads of a differential pair with a current mirror so as to provide a single-ended output while maintaining the original gain
Single ended amps
Amplify errors - ripples in VCC
Differential mode
2 times the input
Differential amp gain
Has differential mode gain and common mode gain
We want max differential mode gain and zero common mode gain to reject common mode signal
Vin1»Vin2
Q2 almost off (Ve > Vb so current decreases)
All current steers towards Q1
Vout2 = VCC
Tail current
Guarantees constant collector currents and rejection of the input CM level
Telescopic cascode
Cascode DX amp loaded with cascode current source
Common mode rejection
First non-ideality is due to the finite impedance of tail current source (causes change to output)
Short output terminals
2gm
Vp increases
Ree = infinity
Noise would not show up on outputs
Ree < infinity
Each input sees noise but differential one doesn’t
Noise will not impact differential output as long as circuit is symmetrical
Single ended output affected
Difference in Rd (or Rc) makes CM gain visible in differential output if
Ree/Rss = infinity
In order for output to be corrupted
Tail current source must have finite impedance
Resistor mismatch
