Bad Circuit Design 2 - Biasing with a Voltage Source
A simple way to ruin an otherwise good analog design is to bias with a voltage source. Donít do this
in a simulation and, especially, donít do this when you are testing a circuit!
An example bad design is seen below. A 3.6 V source is connected to a pad (say VDD = 5 V) and used to
generate Vbiasp. So why is this bad? Letís assume the PMOS has a bp of 400 uA/V2. We can then calculate
the PMOSís drain current, assuming a 900 mV threshold voltage, as (400uA/2)*(5 Ė 3.6 Ė 0.9)2 or 50 uA.
Now letís assume that there is noise on ground or VDD. This noise will appear directly across the MOSFETís S to G,
VSG , and change the drain current. So, if there is 100 mV (peak to peak) of noise on VDD or Vbiasp then the bias
current will vary from 40.5 uA to 60.5 uA (this is bad!)
Next consider using a resistor in place of the battery. The resistorís value is selected to set the bias current at 50 uA.
Again, suppose we have a 100 mV of noise on ground or VDD. Now this noise is dropped (mostly) across the resistor.
The change in current becomes 100mV/72k or 1.4 uA (better than 20 uA.)
The best way to generate Vbiasp is to use a current source (current reference see the BMR in Figs. 20.15 and 20.22).
Power supply or ground noise appears across the current source and thus neither VSG or the current in the MOSFET
(or any other MOSFETs connected to Vbiasp) vary.
Again, donít use voltage sources for biasing in simulations either. Use the bias circuits seen in Figs. 20.43 and 20.47.
If you need to adjust the bias point of your circuit change the resistor in the BMR. Using voltage sources results in the biasing
not changing with temperature, correctly with VDD, the way one bias source should relative to another, or the way it
should with process changes. This is important. A current source to a gate-drain device (the last figure above) is an infinitely
better way to provide biasing than using a voltage source.
Remember this, the three most important things in analog design are biasing, biasing, and biasing. Biasing sets the speed
(Eqs. [9.36] and [9.55]), matching behavior (Eq. [20.8]), operating range (Eq. [20.48] among many others), tolerance to power
supply and ground noise, and static power dissipation in an analog circuit.
Notice how, in the book, the biasing is detailed in every schematic. Itís not left to the imagination of the reader because itís