Physics 536 - Spring 2008

Electronic Techniques for Research

Still Another Example of Using Berkeley SPICE with nanoHUB

Example of temperature dependence

Consider the circuit shown below:
In which the components have the following values:
VCC 10 V
VEE 10 V
R1 29.6 kΩ
R2 4.64 kΩ
RE 200 Ω
Q1 2N3904
VC 0 V
The voltage source VC is used to measure the current into the collector of the transistor. Since it has a voltage of zero volts, it does not change the behavior of the circuit in any way. It only provides a way to measure the current in one part of the circuit. Because the sign of the current is positive when it flows into a voltage source, the current into VC will be positive when connected in this way.

SPICE netlist

The SPICE netlist, including the model for the transistor is as follows:

The output indicates that the current into the collector is 8.79 mA.

But, by increasing R2 to 5.3 kΩ, the current is increased to 10.008 mA, without significantly changing the current through the bias network.

Simulation with different temperatures

The temperature of the circuit can be specified, in degrees Celcius, using the line
          .OPTION TNOM=
For example, to simulate the circuit at 100 degrees C with the adjusted value for R2, the netlist would look like this:

and the resulting output would be

demonstrating that the temperature increase from the default of 27 degrees C to 100 degrees C results in the current decreasing by 1 mA, due to the temperature dependence of the PN junction in the transistor.