Bipolar Junction Transistors as Switches

One of the simplest transistor “logic” circuits used in computer circuitry is the so-called inverter gate. Its logic diagram symbol is this:

Inverters are often bundled six to a “DIP” (Dual-Inline Package) module, where all gates share the same two power supply connections (“V_cc” and “Ground”), like this:

When constructed with bipolar junction transistors, the internal schematic diagram of a single inverter gate looks something like this:

Being a digital logic circuit, it only “understands” two states: on and off. As you can see, a SPDT switch provides signal input into the logic gate, either full supply voltage (V_CC) or no voltage at all (Gnd).

Determine the voltage across the load resistance for each of these switch states. Based on your analysis, what is the logical function of an “inverter” gate?

There is a problem somewhere in this electronic ignition circuit. The “coil” does not output high voltage as it should when the reluctor spins. A mechanic already changed the coil and replaced it with a new one, but this did not fix the problem.

You are then asked to look at the circuit to see if you can figure out what is wrong. Using your multimeter, you measure voltage between TP1 and ground (12 volts DC) and also between TP2 and ground (12 volts DC). These voltage readings do not change at all as the reluctor spins.

From this information, identify two possible faults that could account for the problem and all measured values in this circuit, and also identify two circuit elements that could not possibly be to blame (i.e. two things that you know must be functioning properly, no matter what else may be faulted). The circuit elements you identify as either possibly faulted or properly functioning can be wires, traces, and connections as well as components. Be as specific as you can in your answers, identifying both the circuit element and the type of fault.

Circuit elements that are possibly faulted

1.

2.

Circuit elements that must be functioning properly

1.

2.

There is a problem somewhere in this electronic ignition circuit. The “coil” does not output high voltage as it should when the reluctor spins. A mechanic already changed the coil and replaced it with a new one, but this did not fix the problem.

You are then asked to look at the circuit to see if you can figure out what is wrong. Using your multimeter, you measure voltage between TP1 and ground (12 volts DC) and also between TP2 and ground (0 volts DC). These voltage readings do not change at all as the reluctor spins.

From this information, identify two possible faults that could account for the problem and all measured values in this circuit, and also identify two circuit elements that could not possibly be to blame (i.e. two things that you know must be functioning properly, no matter what else may be faulted). The circuit elements you identify as either possibly faulted or properly functioning can be wires, traces, and connections as well as components. Be as specific as you can in your answers, identifying both the circuit element and the type of fault.

Circuit elements that are possibly faulted

1.

2.

Circuit elements that must be functioning properly

1.

2.