Data Sheet

LABCENTER ELECTRONICS LTD.
238
Now you can simulate the graph by right clicking on it and selecting the simulate option. Then,
by maximising the graph, you can see that the bias level reduces as the resistance of the bias
chain increases. By 5MW it is significantly altered.
Of course, altering these resistors will also have an effect on the frequency response. We could
have done an AC Sweep analysis at say 50Hz in order to see the effect on low frequencies.
Noise Analysis
The final form of analysis available is Noise analysis. In this form of analysis the simulator will
consider the amount of thermal noise that each component will generate. All these noise
contributions are then summed (having been squared) at each probed point in the circuit. The
results are plotted against the noise bandwidth.
There are some important peculiarities to noise analysis:
The simulation time is directly proportional to the number of voltage probes (and
generators) in the circuit, since each one will be considered.
Current probes have no meaning in noise analysis, and are ignored.
A great deal of information is presented in the simulation log file.
PROSPICE computes both input and output noise. To do the former, an input reference
must be defined - this is done by dragging a generator onto the graph, as with a
frequency reference. The input noise plot then shows the equivalent noise at the input for
each output point probed.
To perform a noise analysis on our circuit, we must first restore R1 and R2 back to 470kW. Do
this now. Then select a Noise graph type, and place a new graph on an unused area of the