1. Logistics

  • Install the latest LTspice version on all machines in GEM 166.

  • Install and use the new LTspice keyboard shortcuts and colors. see ltspice.

2. Lab goals

bjt rc re
Figure 1. Simple BJT circuit

Match the results between:

  1. Hand analysis using the simple “no base current” model, i.e. the day02 tables.

  2. Simulation using CircuitLab

  3. Simulation using LTspice

  4. Measurements of a physical circuit

Skill objectives

  • Practice identifying transistor operating modes (cutoff, active, saturation) from circuit waveforms.

  • Quickly building and measuring small transistor circuits with minimal wiring errors.

Table 1. Homework hardware requirements
Equipment Parts

(AD2 is A-OK)

  • 5 V power supply

  • 0-5 V variable voltage source

  • voltmeter

(all in your APK, or available in the lab room)

  • NPN transistor (e.g. 2N3904)

  • ≤10 Ω resistor (to approximate RC = 0)

  • 1 kΩ resistor

  • 2.2 kΩ resistor

3. Tips

You are measuring current (IC and IE) — remember how you make this “measurement” for your hand calculations.[1] Do the same for your physical circuit (don’t use an ammeter to detect the current, but leverage Ohm’s law and a tiny bit of mathematics).

Use the custom LTspice settings, they are worth it.

What DC Sweep increment is best to use such that you get simulation input values (VB) that match the table?

4. Procedure

Create a spreadsheet similar to the day02 tables to hold the results of each analysis method (Hand, LTspice, CircuitLab, Lab) for each Case (A, B, C). There are 12 combinations. Spend a small amount of time considering how you want to structure this data.

Finish your hand calculation tables and enter into your spreadsheet. Alternatively, cast your hand calculations and decisions (cutoff / active / saturation mode) into spreadsheet formulas.[2] Write a small program in Matlab or Python to generate the hand analysis computations?

Create a CircuitLab circuit similar to the Simulating the DC solution of a common-emitter bipolar transistor circuit video. Use the DC Sweep results and the plot cursors to round the results to the nearest 0.01 mA and 0.01 V.

Do the same set of simulations in LTspice.

→ did you set the BJT model to the same in both simulators?

Build the circuit and apply the range of base voltages, recording your measurements to the spreadsheet. Prefer to use the benchtop multimeters for measuring the voltages.

Add a resistor in series with and physically close to the base of the transistor, 100 Ω or so. This has little effect on the DC solution but will reduce the effect of series inductance from the wiring to the base — such inductance, combined with parasitic capacitances will tend to make the transistor oscillate (at 50+ MHz).

The results should be quite close between all four sets of data. If not, figure out where the mistake happened. The entire point of this activity is to get all four to match!

How much difference (“error”) is reasonable between the hand calculations and the other results? How do you know what is too much? What change(s) can you make for your hand calculation assumptions that will make the calculations much closer to reality without complicating the analysis math?

Repeat the simulation for Case B (RC = 1 kΩ) with a smaller simulation step.

Plot the circuit variables (all three transistor currents and transistor terminal node voltages).

  • Think long and hard about how the plots relate to the different operation modes of the BJT.

5. Submission

As stated in § 2, the most important outcome of this lab is for you to see the relationship between cutoff, active, and saturation modes and what these “look like” in hand calculations, simulation, and reality. You need high confidence about this in order for the rest of the class to make any sense at all.

Your deliverable from this activity is part of DBM 1, refer to that page for submission requirements.

1. Ohm’s law! Voltage across the resistor and its value.
2. Either way, you are responsible for documenting how you arrived at the hand calculation results.